Salt Analysis

U N IT - 7

YSTEM M ATI ATIC C Q UALITATIVE S YSTE A NALYSIS

A

always does not mean breaking of substance into its ultimate constituents. Finding out the nature of substance and identity of it s constit constit u ent s is also analysis and is known as qualit ative analysis. Qualit ative analysis analysis of inorganic inorganic salts m eans th e ident ident ification of cations cations and anions present present in t he salt or a mi xt u re of salts. In orga organi ni c salts salts may be obtain ed by compl compl ete or or part ial neut ralisation of acid acid wit h base or vice-ve vice-versa. rsa. In t he formati on of of a salt, salt, t he part part co cont nt ri but ed by th e ac acid id is called called anion and t he part part cont co nt rib ut ed by t he base base is calle called d cation. For example, example, in th e salts salts Cu SO 4 an d – NaCl, Cu 2+ and Na + ions are cati cati ons and SO 2– and Cl ions are anions. Qualit ative 4 analysis is carr carr ied out on variou s scale scales. s. Amou nt of subst ance emp loye loyed d in th ese is different different . In macro analysis, 0.1 to 0.5 g of subst subst ance and about 20 m L of solut solut ion is u se sed. d. For semi semi mi cro analysis, 0.05 g subst ance and 1 m L solu solu ti on is neede needed d wh ile for for micr o analysis analysis amoun t req requi ui red is very very small. Qu alit ative analysis is carri ed out t hr ough th e reac reacti ti ons which are easily percepti percepti ble to our senses se nses su su ch as sight and sm ell. Such reac reacti ti ons in volve volve:: NALYSIS

(a) Fo Form rm atio ation n of a pre precipitate cipitate (b) Cha Change nge in colour colour (c) Evo Evolut lut ion of gas gas etc. etc. Systemat Sys temat ic analysis of an i norganic salt in vo volve lves s th e followin g steps: steps: (i) Preliminary examination examination of so solid salt and it s solution. solution. (ii) Determin Determin ation of anions by reac reacti ti ons carri carri ed out in so solu lu tion (wet (wet t ests) and confi confi rm atory tests. tests. (iii) Determin Determin ation of cations cations by reac reactions tions carr carr ie ied d out i n soluti on (we (wett t ests) and confi confi rm atory tests. tests. Prelimi nary examin examin ation of a salt salt often often fu rn ishes im portant inform ation, which simpl ifies furt her course of of analysis. Alth ough th ese tests are not co conclu nclu sive bu t someti some ti mes th ey give give qui t e im port ant clu es for th e pr ese sence nce of of cert cert ain an ions or cations. These These tests can can be perform perform ed wit hi n 10- 15 m in u tes. These These in vo volve lve noti ng th e ge general appearance appearance and and physical pr ope opert rt ies ies,, su ch as colour colour , smell, solubi lit y et c. of t he salt . These are named as dr y tests. Heating of dry salt, blow pipe test, flame tests, borax bead test, sodium carbonat e bead bead t est, ch arcoal cavity t est etc. come un der dr y t ests. Some of of t hese tests tes ts are give iven n lat er in th is un it .

L ABORATORY M A N U A L C HEMISTRY

Solu bili ty of a salt Solu salt in wate waterr an d th e pH of aqueo aqueous us solut ions give give im portant in formati on abou abou t t he natu re of of ions prese present in t he salt. salt. If a solu solu ti on of of th e salt is acidic or or basic in n atu re, this m eans that it is being being hydr olys olyse ed in wat er. If t he solu so lu ti on is basic in n atu re th en salt may be some some ca carbonate rbonate or sulph ide etc. etc. If th e so solu lu ti on shows acidic acidic n atu re th en i t m ay be an acid salt or salt of weak weak base and strong acid. In this case it is best to neutralise the solution with sodium carbonate before before testin g it for ani ons. Gases Gase s evolve evolved d in t he prelim in ary t ests wit h di l. H 2SO4 / dil . HCl and con con c. H 2 SO4 also give good indication about the presence of acid radicals (See Tables 7.1 and 7.3). Preliminary tests should always be performed before starting the confir co nfir matory t ests for t he ions. EXPER XPERIIM ENT 7 . 1

A im To dete detect ct one ca cati ti on and one anion in th e give given n salt fr om th e followin g ions: 2+ 2+ 3+ 3+ 3+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ Cat io i on s - Pb , Cu , As A s , Al , Fe , M n , Ni , Zn Z n , Co , Ca , Sr , B a , 2+

+

Mg , NH 4 2–

Anions - CO2– , S , SO2– , 3 3

2–

–

–

–

–

–

3–

2–

SO 4 , NO 2 , NO 3 , Cl , Br , I , PO 4 , C2 O4 ,

–

CH3 COO . (In solubl e salts t o be exclu ded)

Theor The or y Two basic basic pr in cipl es of great great u se in t he analysis are: (i) the So Solubility pr oduct; and (ii) th e Co Comm mm on ion effe effect. ct. When Whe n ionic pr oduct of a salt exce exceeds it s solu solu bilit y pr oduct, pr ecipitati on tak es place. Ionic product of salt is controlled by making use of common ion effect whi ch you have stu died in t he textbook textbook of chemist chemist ry.

M at er ia iall Requ Requir ir ed • • • • • • • •

50

B oi l i n g t u b e Test t u b es M easu ri r i n g cy l i n de der Test t u b e st an d Test t u b e h ol d er D el i ver y t u b e Cor k s Fi l t er p ap er

: : : : : : : :

As p er As p er On e On e On e On e As p er As p er

n eed r equ i r em en t • Reagen t s

n eed n eed

:

As p er n eed


Solu bili ty of a salt Solu salt in wate waterr an d th e pH of aqueo aqueous us solut ions give give im portant in formati on abou abou t t he natu re of of ions prese present in t he salt. salt. If a solu solu ti on of of th e salt is acidic or or basic in n atu re, this m eans that it is being being hydr olys olyse ed in wat er. If t he solu so lu ti on is basic in n atu re th en salt may be some some ca carbonate rbonate or sulph ide etc. etc. If th e so solu lu ti on shows acidic acidic n atu re th en i t m ay be an acid salt or salt of weak weak base and strong acid. In this case it is best to neutralise the solution with sodium carbonate before before testin g it for ani ons. Gases Gase s evolve evolved d in t he prelim in ary t ests wit h di l. H 2SO4 / dil . HCl and con con c. H 2 SO4 also give good indication about the presence of acid radicals (See Tables 7.1 and 7.3). Preliminary tests should always be performed before starting the confir co nfir matory t ests for t he ions. EXPER XPERIIM ENT 7 . 1

A im To dete detect ct one ca cati ti on and one anion in th e give given n salt fr om th e followin g ions: 2+ 2+ 3+ 3+ 3+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ Cat io i on s - Pb , Cu , As A s , Al , Fe , M n , Ni , Zn Z n , Co , Ca , Sr , B a , 2+

+

Mg , NH 4 2–

Anions - CO2– , S , SO2– , 3 3

2–

–

–

–

–

–

3–

2–

SO 4 , NO 2 , NO 3 , Cl , Br , I , PO 4 , C2 O4 ,

–

CH3 COO . (In solubl e salts t o be exclu ded)

Theor The or y Two basic basic pr in cipl es of great great u se in t he analysis are: (i) the So Solubility pr oduct; and (ii) th e Co Comm mm on ion effe effect. ct. When Whe n ionic pr oduct of a salt exce exceeds it s solu solu bilit y pr oduct, pr ecipitati on tak es place. Ionic product of salt is controlled by making use of common ion effect whi ch you have stu died in t he textbook textbook of chemist chemist ry.

M at er ia iall Requ Requir ir ed • • • • • • • •

50

B oi l i n g t u b e Test t u b es M easu ri r i n g cy l i n de der Test t u b e st an d Test t u b e h ol d er D el i ver y t u b e Cor k s Fi l t er p ap er

: : : : : : : :

As p er As p er On e On e On e On e As p er As p er

n eed r equ i r em en t • Reagen t s

n eed n eed

:

As p er n eed

SYSTEMATIC Q UALITATIVE A NALYSIS

S YSTEM ATIC A NALYSIS

O F A NIONS

St ep - I : Pr elimina liminarr y Test Test w it h Dilut e Sul Sulphu phurr ic Acid In th is test t he action action of dilut e sul sul phu ric acid (proce (procedur e is given given below) be low) on th e salt is note n oted d at r oo oom m t emperatu re and on warmi ng. Carbonate ( CO 2– ), su su lph ide (S (S 2– ), sulph it e ( SO2– ), nit rit e ( N O – ) 3 3 2 –

and ace acett ate (CH (CH 3 COO ) rea react ct wit h di lu te sul sul phu ri c acid acid t o evo evolve lve diffe diff er ent ga gase ses. s. Stu dy of t he characterist char acterist ics of the t he gase gases s evolve evolved d give ives s inform ation about th e anions. anions. Summ ary of characteristic characteristic pr ope opert rt ies of of gases gases is given in Table 7.1 below.

Procedure (a)

Take 0.1 Take 0.1 g of the salt salt in a test test tu be and add add 1–2 1–2 mL of of dil u te su su lph u ri c acid. acid. Observe Observe th e change change,, if any, at room temperature. If no gas is evolved, warm the cont en t of t h e test tu be be.. If gas gas is evo volve lved d test test i t by u sing th e apparatus apparatus shown in Fig Fig.7.1 .7.1 and identi fy t he gas gas evolved (See (See Table 7. 1).

Fig. 7.1 : Te Testin stin g a Gas

Table 7.1 : Preliminary test with dilute sulphuric acid Inference

Observations

Gas E v o l v ed A colourless, odourless gas is evolved with brisk effervescence, which turns lime water milky.

CO 2

Colourless gas with the smell of rotten eggs is evolved which turns lead acetate paper black.

H 2S

Colour les Colour less s gas gas with a pu nge ngent nt sme smell, ll, li ke burning sulphur which turns acidified potassium dichromate solution green.

SO2

Brown fumes which turn acidified potassium iodide solution containing starch solution blue.

NO 2

Colourless vapours with smell of vi n eg egar . Vap ou ou r s t u r n b lu lu e l it i t m u s r ed ed .

C H3COOH va vap ou rs rs

Po ssi b l e A n i o n

2–

Carbonate ( CO 3 )

2–

Sulphide (S )

2–

Sulphite ( S O 3 )

–

Nitrite ( NO 2 )

–

Ac et at at e, e, (C (CH 3 COO )

51

L ABORATORY M A N U A L C HEMISTRY 2-

2-

2 –

–

–

Confirmatory tests for CO3 S , SO3 , NO2 and CH 3 COO

Confi rm atory (wet) tests for ani ons are perfor med by usin g wat er ext ract when salt is soluble in water and by using sodium carbonate extract when salt is insolubl e in water. Confirm ation of CO 2– is done by using aqueous solution of 3 th e salt or by u sing solid salt as such because sodiu m carbonate extr act cont ains carbonate ions. Water extr act is m ade by dissolving salt in water. Preparati on of sodium carbonat e ext ract is given below.

Preparation of sodium carbonate extract Take 1 g of salt in a porcelain dish or boiling tube. Mix about 3 g of solid sodiu m carbonate and add 15 mL of distil led water to it . Sti r and boil t he content for about 10 mi nu tes. Cool, filter and collect t he filtr ate in a test tu be and label it as sodium carbonate ext ract. Confir matory t ests for acid radicals, which r eact with d ilu te sul phu ri c acid are given below in Table 7.2. 2–

2–

2–

–

–

Table 7.2 : Confirmatory tests for CO3 , S , SO3 , NO3 , CH3 COO Anion

Confirmatory Test

Carbonate ( CO

2

-

3

2 –

Sulphide (S )

)

Take 0.1 g of salt in a test tube, add dilute sulphuric acid. CO 2 gas is evolved wit h bri sk effervescence whi ch tu rn s lim e water mil ky. On passing t he gas for some more time, milk iness disappears. Take 1 mL of water extract and make it alkaline by adding ammonium hydroxide or sodium carbonate extract. Add a drop of sodium nitroprusside solution. Purple or violet colouration appears.

2–

*Sulphite ( SO3 )

(a )

(b)

–

Nitrite ( NO )

(a)

2

(b)

*

Ta k e 1 m L of wa t er ex t r a ct o r sod i u m c ar b on a t e extract in a test tube and add barium chloride solu tion. A whit e precipit ate is formed wh ich d issolves in dilute hydrochloric acid and sulphur dioxide gas is also evolved. Take the precipitate of step (a) in a test tube and add a few drops of potassium perman ganat e solut ion a c i d i f i e d w i t h d i l . H 2 S O4 . C o l o u r o f p o t a s s i u m permanganate solution gets discharged. Take 1 mL of water extract in a test tube. Add a few drops of potassium iodide solut ion and a few drops of starch solution, acidify with acetic acid. Blue colour appears. Acidify 1 mL of water extract with acetic acid. Add 2-3 drops of sulphanilic acid solution followed by 2-3 drops of 1-naphthylamine reagent. Appearance of red colour indicates the presence of nitrite ion.

Like CO 2 sulphur dioxide also turns lime water milky. But CO 2 is odourless gas a char acteristi c smell.

52

and SO 2 h as


–

(a)

Acetate (CH 3COO )

(b)

Take 0.1 g of salt in a china dish. Add 1 mL of ethanol and 0.2 mL conc. H 2 SO4 a n d h e a t . F r u i t y o d o u r confirms the presence of acetate ion. Take 0.1 g of salt in a test tu be, add 1-2 mL distilled water, shake well filter if necessary. Add 1 to 2 mL neutral** ferric chloride solution to the filtrate. Deep red colour appears which disappears on boiling and a brown-red precipitate is formed.

** Prepareation of neutral Ferric Chloride : Add di lut e NaOH solution to ferric chlorid e solution dr op by drop w ith shak ing until a small bu t permanent precipitat e of ferric hyd roxide is obtained. Filter the precipitat e and use the filtrate for analy sis.

Chemi str y of Confir mat ory Tests 2 –

1. Test for Carbonate ion [CO3 ] If t here is effervescence wit h th e evolut ion of a colour less and odou rl ess gas on adding dil. H 2 SO4 to th e solid salt, t hi s ind icates the presence of carbonat e ion. The gas tur ns li me water mi lk y du e to t he form ation of CaCO 3 (Fig. 7.1 ) Na 2 CO3 + H 2 SO4 ⎯→ Na2 SO 4 + H 2O +CO2 Ca(OH)2 + CO 2 ⎯→ CaCO 3 + H 2O If CO 2 gas is passed in excess through lime water, the milkiness produced disappears due to th e formati on of calciu m h ydrogen carbonate which is soluble in water. CaCO3 + CO2 + H 2 O ⎯→ Ca (HCO3 )2

Hydrogen sulphide

2–

2. Test for Sulphide ion [S ] (a)

Wi th war m di lu t e H 2 SO4 a sul phi de gives hydr ogen sul ph ide gas which smells like rotten eggs. A piece of filter paper dipped in lead acetate solution turns black on exposure to the gas due to the formation of lead sul phi de whi ch is black in colour . Na 2 S + H2 SO4

⎯→

(CH 3COO)2 Pb + H2 S ⎯→

(b)

Na 2 SO4 + H2 S PbS + Lead sulph ide Black precipit ate

2CH 3 COOH

If the salt is soluble in water, take th e solut ion of salt in water m ake it alkaline with ammonium hydroxide and add sodium nitroprusside solu ti on. If it i s insoluble in water tak e sodiu m carbonate extr act and add a few drops of sodium nitroprusside solution. Purple or violet

53


colour ation du e to th e format ion of complex compoun d Na 4 [Fe(CN)5 NOS] confir ms th e pr esence of sul phi de ion in th e salt. Na2 S +

Na 2 [Fe(CN)5 NO] ⎯→ Na 4 [Fe(CN)5 NOS] Sodiu m nit ropru sside Compl ex of Pur ple colour 2 –

3. Test for Sulphite ion [SO3 ] (a)

On treating sulphite with warm dil. H2 SO 4, SO 2 gas is evolved whi ch i s suffocatin g with t he smell of bur ning sulphu r. Na2 SO 3 + H 2SO 4 ⎯→ Na 2 SO4 + H 2 O + SO2 The gas tu rn s potassium dichr omate paper acidified wit h dil. H 2 SO4 , green.

Barium compounds

Potassium permanganate

K2 Cr 2 O7 + H 2SO 4 + 3SO2 ⎯→

(b)

K 2 SO4

+

Cr 2 (SO4 )3 + H 2 O Chromium sul ph ate (green)

An aqueous solut ion or sodium carbonate extract of the salt produces a whit e precipit ate of bariu m sulphite on addition of bariu m chloride solution. Na2 SO 3 + BaCl 2 ⎯→ 2NaCl + BaSO 3

(i )

This p recipit ate gives followin g tests. T h i s p r ec i pi t a t e on t r ea t m en t w i t h d i l u t e H C l , di s sol v es d u e t o decompositi on of sulp hi te by dil u te HCl. Evolved SO 2 gas can be tested. BaSO3 + 2HCl ⎯→ BaCl 2 + H 2 O + SO2

(ii)

Precipitate of sulph ite decolourises acidified potassium perm anganate solution. BaSO3 + H 2SO 4 ⎯→ BaSO 4 + H 2 O + SO2 2KMnO4 + 3H 2 SO4 ⎯→ K 2 SO4 + 2MnSO4 + 3H 2 O + 5 [O] SO2 + H 2 O + [O] ⎯→ H 2SO 4 –

4. Test for Nitrite ion [NO2 ] (a)

On treating a solid nitrite with dil. H 2 SO4 an d warming , reddish brown fu mes of NO2 gas are evolved. Addit ion of potassiu m i odide solut ion t o the salt solution followed by freshly prepared starch solution and acidification wit h aceti c acid produces blu e colour . Alternat ively, a filt er paper m oistened with potassium iodide and starch solu ti on and a few drops of acetic acid turns blue on exposure to the gas due to the int eraction of liberated iodine with starch. (i)

2NaNO 2 + H 2 SO 4 ⎯→ Na2SO 4 + 2HNO2 3HNO2 ⎯→ HNO3 + 2NO + H 2 O 2NO + O2 ⎯→ 2NO 2 Br own gas

54


(ii)

–

–

NO 2 + CH 3 COOH ⎯→ HNO2 + CH 3 COO

2HNO 2 + 2KI + 2CH 3 COOH ⎯→ 2CH 3 COOK + 2H 2O + 2NO + I 2 I 2 + Starch ⎯→ Bl u e complex (b)

Sulph anilic acid — 1-naphthylamine reagent test (Griss-Ilosvay test) On adding sulph anili c acid and 1- napht hylami ne reagent to th e water extr act or acidifi ed wit h aceti c acid, sul phan ili c acid is diazoti sed in t he reaction by nitrous acid formed. Diazotised acid couples with 1-naph th ylamin e to form a red azo-dye. NO – + CH 3 COOH ⎯→ HNO 2 + CH 3COO – 2

The test solution should be very dilute. In concentrated solutions reaction d oes not pr oceed beyond diazoti sation . –

5. Test for Acetate ion [CH 3 COO ] (a)

If the salt smells like vinegar on tr eatment with dil. H 2SO4 , thi s indicates th e pr esence of acetat e ions. Take 0.1 g of salt i n a chin a dish and add 1 mL of eth anol. Then add about 0.2 m L of conc. H 2 SO4 and h eat. – Fru it y odou r of eth yl acetat e in dicates the presence of CH 3COO ion. 2 CH 3COONa + H 2 SO4 ⎯→ Na 2 SO4 + 2 CH 3COOH H

+

CH 3 COOH + C 2H 5 OH ⎯→ CH 3COOC 2 H5 + H 2O Ethylacetate (Fruity odour)

55


(b)

Acetate gives deep red colour on reaction with n eut ral ferr ic chloride solution due to the formation of complex ion which decomposes on heatin g t o give Ir on (III ) dih ydr oxyacetat e as brown r ed pr ecipit ate. –

3+

6 CH 3 COO + 3Fe

+

+ 2H2 O ⎯→ [Fe3 (OH)2 (CH3 COO) 6] + 2H

+

[Fe3 (OH)2 (CH 3COO) 6]+ + 4H 2O ⎯→ 3[Fe (OH)2 (CH3 COO)] + 3CH 3COOH + H + Iron(III)dihydroxyacetate (Brown-red precipitate)

St ep-II : Preliminary Test w it h Concent r at ed Suphur ic Acid If no posit ive resul t i s obtain ed from di l. H 2 SO4 t est, t ake 0.1 g of salt in a test tu be and 3- 4 dr ops of conc. H 2 SO4 . Observe the change in th e reaction m ixt ur e in cold an d t hen war m it . Ident ify t he gas evolved on h eati ng (see Table 7.3). Table 7.3 : Preliminary examination with concentrat ed sulphuric acid

Observations

Gas/ Vap ou r s Ev ol v ed

Po ssi bl e An i on s

A colourless gas with pungent smell, which gives dense white fumes when a rod dipped in ammonium hydroxide is brought n ear th e mout h of the test t ube.

H Cl

Chloride, (Cl )

Reddish brown gas with a pungent odour i s evolved. In tensit y of reddish gas increases on heating the reaction mixture after addition of solid MnO2 t o the reaction mixture. Solution also acquires red colour.

Br 2 vapours

Bromide, (Br )

I 2 vapours

Iodid e, (I )

Br own fu mes evolve whi ch become dense upon heating the reaction mixture after addition of copper turnings and the solution acquires blue colour.

NO 2

Nitrate, ( NO )

Colourless, odourless gas is evolved which turns lime water milky and the gas comin g out of lime water bu rn s with a blue flame, if ignited.

CO and CO 2

Oxalate, ( C2O4 )

Violet vapours, which turn starch paper blue and a layer of violet sublimate is formed on the sides of the tube. Fumes become dense on adding MnO2 to the reaction mixture.

56

Inference

–

–

–

– 3

2–


Confir matory t ests for t he anions which react wit h concent rat ed sulph ur ic acid are given in Table 7.4.

–

–

–

2 –

Table 7.4 : Confirmatory tests for Cl , Br , I , NO 3 – and C2 O 4 Anion –

Chloride (Cl )

Confirmatory Test (a )

(b)

(c)

–

Bromide (Br )

(a)

(b )

(c)

–

Iodide ( I )

(a)

(b)

T ak e 0 . 1 g of sa l t i n a t es t t u b e, ad d a p i n c h of manganese dioxide and 3-4 drops of conc. sulphuric acid. Heat the reaction mixture. Greenish yellow chl orin e gas is evolved whi ch is detected by it s pu ngent odour and bleaching action. Take 1 mL of sodium carbonate extract in a test tube, acidfy it with di l . HNO3 or take water extract and add silver nitrate solution. A curdy white precipitate is obtained which is solu ble in ammoniu m hydroxide solu ti on . Ta k e 0 . 1 g s al t a n d a p i n ch o f sol i d p ot a ss iu m dichromate in a test tube, add conc. H2 SO4 , heat and pass the gas evolved through sodium hydroxide solution. It becomes yellow. Divide the solution into two parts. Acidify one part with acetic acid and add lead acetate solution. A yellow precipitate is formed. Acidify the second part with dilute sulphuric acid and add 1 mL of amyl alcohol followed by 1 mL of 10% hydrogen peroxide. After gentle shaking the organic layer turns blue. Take 0.1 g of salt and a pinch of MnO2 in a test tube. Add 3-4 drops conc.sulphuric acid and heat. Intense brown fumes are evolved. Neu t r a l i se 1 m L of sod i u m c ar b on a t e ex t r a ct w i t h hydrochloric acid (or take the water extract). Add 1 mL carbon tetrachloride (CCl4)/ chloroform (CHCl 3)/ carbon disulphide. Now add an excess of chlorine water dropwise and shake the test tube. A brown colour ation i n th e organic layer confir ms t he presence of bromide ion. Aci d i fy 1 m L of sod i u m c ar b on a t e ex t r ac t wi t h d i l . HNO 3 (or take 1 mL water extract) and add silver nit rate soluti on. A pale yellow pr ecipitat e soluble with difficulty in ammonium hydroxide solut ion is obtained. Take 1 mL of salt solution neutralised with HCl and add 1 mL chloroform / carbon tetrachloride/ carbon disulphide. Now add an excess of chlorine water drop wise and shake the test tube. A violet colour appears in the organic layer. Take 1 mL of sodium carbonate extract acidify it with dil. HNO3 (or take water extract). Add, silver nitrate solution. A yellow precipitate insoluble in NH4OH solution is obtained.

57


*Nitrate ( NO – ) 3

2–

Oxalate ( C O ) 2 4

Take 1 mL of salt solu tion in wat er i n a test tu be. Add 2 mL conc. of H 2 SO4 and mix thoroughly. Cool the mixtu re under the tap. Add freshly prepared ferrous sulphate along the sides of the test tube without shaking. A dark brown ring is formed at the junction of the two solutions. (a )

(b)

Ta k e 1 m L of wa t er ex t r a ct or s od i u m c ar b on a t e extract acidified with acetic acid and add calcium chloride solution. A white precipitate insoluble in ammonium oxalate and oxali c acid solution but soluble in dilute hydrochloric acid and dilute nitric acid is formed. Take the precipitate from test (a) and dissolve it in dilute H 2SO4. Add very dilute solution of KMnO 4 an d warm. Colour of KMnO 4 solution is discharged. Pass th e gas comin g out th rough l ime water. The lim e water turns milky.

Chemi str y of Confir mat ory Tests –

1. Test for Chloride ion [Cl ] (a)

Manganese dioxide

I f on t r eat m en t w i t h war m c on c. H 2 SO4 the salt gives a colourless gas wit h p un gent smell or and i f t he gas which gives dense white fum es with ammonia solution, then the salt may contain Cl – ions and the following reacti on occu rs. NaCl + H 2 SO4 ⎯→ NaHSO4 + HCl Sodiu m Hydr ogen chlor ide hydr ogen sulph ate Colourless gas HCl + NH 3 ⎯→ NH 4 Cl Ammoniu m chloride Whit e fum es

Silver nitrate

(b)

If a salt gives effervescence on heatin g with conc. H 2 SO4 and MnO 2 an d a light greeni sh yellow pu ngent gas is evolved, t hi s indicat es th e pr esence of Cl – ions. MnO 2 + 2NaCl + 2H 2SO 4 ⎯→ Na 2SO4 + MnSO4 + 2H 2O + Cl 2

(c)

Salt solution acidified with dilute HNO 3 on addition of silver nitrate solu ti on gives a cur dy whit e precipit ate solu ble in ammoniu m h ydroxide – solu ti on. This ind icates t he pr esence of Cl ions in t he salt. NaCl + AgNO 3 ⎯→

AgCl + 2NH 4OH ⎯→

NaNO 3

+

Ag Cl Silver chloride (White precipitate)

[Ag(NH 3 )2 ]Cl Diam mi ne silver (I) chloride

+ 2H 2 O

* This test can also be performed by a ddi ng first ferrous sulphate solution and then conc. H 2 SO 4 .

58


(d)

Mix a little amount of salt and an equal amount of solid potassium dich rom ate (K 2 Cr 2 O7 ) in a test tu be and add conc. H 2 SO4 to it. Heat th e test tu be and pass th e evolved gas th rou gh sodiu m h ydroxide solu ti on. If a yellow solu tion is obtained, divide th e solut ion in to t wo parts. Acidify the first part with acetic acid and then add lead acetate solution. Form ation of a yellow pr ecipit ate of lead chr omate confi rm s th e presence of chl oride ions in th e salt. Thi s test is called c h r o m y l c h l o r i d e t e st .* 4NaCl + K 2 Cr 2 O7 + 6H 2SO 4 ⎯→ 2KHSO4 + 2CrO 2 Cl 2 + 4NaHSO 4 + 3H2 O (Chromyl chloride)

Chromyl chloride

Cr O 2 Cl 2 + 4NaOH ⎯→ Na 2Cr O4 + 2NaCl + 2H 2 O (CH 3COO)2 Pb + Na2 Cr O 4 ⎯→ PbCrO4 + 2CH 3 COONa Sodi um Lead ch rom at e chromate (Yellow precipitate)

Lead chromate

Acidify t he second part with dilu te sul phu ric acid and add small amoun ts of amyl alcohol and t hen 1 m L of 10% hydrogen p eroxide solu ti on. On gent le shak in g organic layer turns blue. CrO 24 – ion formed in the reaction of chromyl chloride wit h sodium hydr oxide reacts with hydr ogen peroxide to form chr omiu m pent oxide (CrO 5 ) (See str u ctu re) whi ch di ssolves in amyl al cohol to give blu e colou r. 2–

Cr O 4

+

+ 2H + 2H 2 O 2 ⎯→

Cr O5 + 3H 2 O Chromium pentoxide

–

2. Test for Bromide ion (Br ) If on heating the salt with conc. H 2 SO 4 reddish brown fumes of bromine are evolved in excess, th is in dicat es th e pr esence of Br – ions. The fum es get in tensified on addition of MnO 2. Bromi ne vapour s tu rn starch paper yellow. 2NaBr + 2H 2 SO4 ⎯→ Br 2 + SO2 + Na2 SO 4 + 2H 2 O 2NaBr + Mn O 2 + 2H 2 SO4 ⎯→ Na 2SO 4 + MnSO4 + 2H 2O + Br 2 (a)

Bromine very toxic by inhalation corrosive

Add 1 mL of carbon tetrachloride (CCl 4 )/ chloroform (CHCl 3 )** and excess of freshly pr epared chlorin e water dropwise to the salt solut ion in water or sodiu m carbonate extr act neutr alised wit h di lu te HCl. Shak e th e test tu be vigorou sly. The appearance of an orange brown colour ation i n t he organic layer due to the dissolution of bromine in it, confirms the pr esence of br omid e ions. 2NaBr + Cl 2 ⎯→ 2NaCl + Br 2 3+

* Chromyl chloride test should be performed w ith minimum a mount of substance to avoid pollution by Cr ions. ** In pl ace of carbon t etrachloride or chloroform, carbon di sulphi de or di chloromethane (CH 2 Cl 2 ) can al so be used .

59


(b)

Acidify th e sodium carbonate extract of the salt with dil. HNO3 . Add silver nitrate (AgNO3 ) solution and shake the test tube. A pale yellow precipitat e is obt ained which dissolves in amm oniu m h ydroxide with difficulty. NaBr + AgNO3 ⎯→

NaNO 3 +

AgBr Silver br omide Pale yell ow precipit ate

–

3. Test for Iodide ion ( I ) (a)

Iodine,

If on heating the salt with conc. H 2 SO4 , deep violet vapours with a pu ngent smell are evolved. These t u rn s starch paper blu e and a violet subl im ate is form ed on t he sides of th e test t ub e, it in dicates th e presence – of I ions. Some HI, sul phu r di oxide, hydr ogen su lph ide, and sulph ur are also form ed du e to t he followin g reactions. 2NaI + 2H 2SO 4 ⎯→ Na 2 SO4 + SO2 + 2H 2 O + I 2

harmful by inhalation and contact with skin

I 2 + Starch ⎯→ Blue colour solution NaI + H 2 SO4 ⎯→ NaHSO 4 + HI

Chlorine, toxic by inhalation

2HI + H2 SO 4 ⎯→ 2H 2 O + I2 + SO 2 6NaI + 4H2 SO4 ⎯→ 3I 2 + 4H 2 O +S + 3Na 2 SO4

Chloroform, harmful and toxic by

8NaI + 5 H2 SO4 ⎯→ 4I 2 + H 2 S + 4Na2 SO4 + 4H 2 O

inhalation

On adding MnO2 to the reaction mixture, the violet vapours become dense. 2NaI + Mn O2 + 2H 2 SO4 ⎯→ I 2 + MnSO4 + Na2 SO 4 + 2H 2 O (b )

Add 1 m L of CH Cl3 or CCl 4 and chlorine water in excess to the salt solu ti on in wat er or sodium carbonate extr act n eut ralised wit h di l.HCl and sh ake th e test t ub e vigorously. Presence of violet colour ation in th e organi c layer confi rm s th e pr esence of iodide ions. 2NaI + Cl2 ⎯→ 2NaCl + I2 Iodine dissolves in th e organic solvent and t he solu ti on becomes violet.

(c)

Acidify sodium carbonate extract of the salt with dil.HNO 3 and add AgNO3 solu ti on. Appearan ce of a yell ow precipit ate insolu ble in excess of NH4 OH confi rm s th e pr esence of iodide ions. NaI + AgNO3 ⎯→

AgI + silver iodide (Yellow pr ecipi tat e)

60

NaNO 3

SYSTEMATIC Q UALITATIVE A NALYSIS –

4. Test for Nitrate ion [NO3 ] (a)

If on heating the salt with conc. H 2 SO4 li ght b rown fu mes are evolved th en heat a small quant it y of the given salt with few copper tu rn ings or chips and conc. H 2SO 4. Evolu ti on of excess of br own fum es in dicat es th e presence of nitr ate ions. The solu ti on tu rn s blue due to th e format ion of copper su lph ate. NaNO3 + H 2 SO4 ⎯→ NaHSO 4 + HNO 3

Copper sulphate

4HNO 3 ⎯→ 4NO 2 + O2 + 2H 2 O 2NaNO3 + 4H 2SO4 + 3Cu ⎯→

3 CuSO 4 + Na2SO4 + 4H 2 O + 2NO Copper su lphat e (Blue)

Nitric acid

2NO + O 2 ⎯→ 2NO 2 (Br own fu mes) (b)

Take 1 mL of an aqueous solut ion of the salt and add 2 mL conc. H 2SO 4 slowly. Mix th e solu ti ons thoroughly and cool t he test t ube un der t he tap. Now, add freshly pr epared ferr ous sulp hat e solut ion along th e sides of the test t ube dropwise so that i t form s a layer on th e top of the liquid already present in the test tube. A dark brown ring is formed at the ju nct ion of t he t wo solu t ions du e t o t h e form at ion of nit r oso fer r ou s sulph ate (Fig. 7.2). Altern atively fir st ferrou s sulph ate is added and th en concent rated sulph ur ic acid is added.

Oxalates

Fig. 7.2 : Formation of brown ring

NaNO3 + H 2 SO4 ⎯→ Na HSO 4 + HNO 3 6 FeSO4 + 3H 2 SO4 + 2HNO 3 ⎯→ 3Fe 2 (SO 4 )3 + 4H 2 O + 2NO FeSO 4 + NO ⎯→ [Fe(NO)]SO4 Nitr oso ferr ous sul phate (Brown) 2 –

5. Test for Oxalate ion [C 2O 4 ] If carbon dioxide gas along with carbon monoxide gas is evolved in the prelim inary examination with concent rated sulphu ric acid, th is gives indication about t he presence of oxalate ion. (COONa)2 + Con c. H 2SO 4 ⎯→ Na 2 SO4 + H 2O + CO 2 ↑ + CO ↑

61


Oxalate is confir med by th e followin g tests: (a)

Acidify sodium carbonate extract wit h acetic acid and add calcium chloride solu tion. A wh ite precipit ate of calciu m oxalate, in solu ble in amm onium oxalate and oxali c acid solution in dicat es th e pr esence of oxalat e ion. CaCl2 + Na 2 C2 O 4 ⎯→

(b)

CaC 2O 4 + 2NaCl Calciu m oxalate (Whit e precipit ate)

KMnO 4 test Filt er t he precipit ate from test (a). Add dil. H 2 SO4 to it followed by dilu te KMnO4 solu ti on and warm . Pin k colour of KMn O 4 is di scharged: CaC2 O4 + H 2SO 4 ⎯→

CaSO 4 + H2 C2 O4 Cal ci u m su l ph at e Ox al ic aci d

2 KMnO4 + 3H2 SO4 + 5H2 C2 O4 ⎯→ 2MnSO4 + K2 SO4 + 8H2 O + 10CO 2 Pass th e gas evolved th rou gh lim e water. A wh it e pr ecipi tat e is form ed wh ich dissolves on passing t he gas for some more tim e.

St ep-III : Test for Sulphat e and Phosphat e If n o posit ive test is obtain ed in Steps-I an d II, t hen t ests for t he presence of sul phat e and ph osph ate ions are perform ed. These tests are su mm ari sed in Table 7.5.

Table 7.5 : Confirmatory tests for Sulphate and Pho sphate I on

Confirmatory Test 2–

Sulphate ( SO 4 )

(a)

(b )

Take 1 mL water extract of the salt in water or sodium carbonate and after acidifying with dilute h y d r o c h l o r i c a c i d a d d B a C l2 s o l u t i o n . W h i t e precipitate insoluble in conc. HCl or conc. HNO3 i s obtained. Ac id i f y t h e a qu eou s sol u t i o n or s od i u m ca r b on a t e extr act wit h acetic acid and add lead acetate solu tion. Appearance of white precipitate confirms the presence of SO 2– ion. 4

3–

Phosphate ( PO 4 )

62

(a)

Acidify sodium carbonate extract or the solution of th e salt in water wit h conc. HNO3 and add ammoniu m molybdate solution and heat to boiling. A canary yellow precipitate is formed.


Chemi str y of Confir mat ory Tests 2 –

1. Test of Sulphate ions [SO4 ] (a)

Aqueous solut ion or sodium carbonate extract of the salt acidified with acetic acid on addition of barium chloride gives a white precipitate of bariu m su lph ate insolubl e in conc. HCl or conc. HNO3 . Na 2 SO4 + BaCl 2 ⎯→ BaSO4 + Barium sulphate (White precipitate)

(b)

2NaCl

Sulph ate ions give whit e precipitate of lead sulph ate when aqueous solution or sodium carbonate extract neutralised with acetic acid is tr eated with l ead acetat e solu ti on. Na 2 SO4 + (CH 3COO)2 Pb ⎯→

PbSO 4 + 2CH 3 COONa Lead sulph ate (White precipitate)

3 –

2. Test for Phosphate ion [PO4 ] (a)

Add con c. HNO 3 and ammoniu m m olybdate solut ion to the test solu tion contain in g ph osph ate ions and boil. A yellow colour ation in solut ion or a canary yellow precipitate of ammonium-phosphomolybdate, (NH 4 )3 [P (Mo3 O1 0 )4 ] is formed. Each oxygen of phosphate has been replaced by Mo 3O 1 0 group.

Na2 HPO4 + 12 (NH 4)2 MoO 4 + 23 HNO 3 ⎯→ (NH4 )3[P (Mo3 O10 )4] + 2NaNO 3 + 21NH 4 NO3 + 12H 2 O Canary yellow precipitate

S YSTEM ATIC A NALYSIS O F C ATIONS

The tests for cation s may be carr ied ou t accordin g to t he foll owing scheme.

St ep - I : Pr eliminar y Examinat ion of t he Salt for Ident ificat ion of Cat ion 1. Colour Test Observe th e colour of the salt carefull y, which m ay provide useful in format ion about t he cati ons. Table 7.6 gives th e charact eri sti c colou rs of the salt s of some cations.

63


Table 7.6 : Characteristic colours of some metal ions Col o u r

Cat i on s I n d i cat ed

Ligh t gr een , Yellow, Br own

Fe2+, Fe3+

Blu e Br i gh t gr een Blu e, Red, Violet , Pin k

Cu2+ Ni2+ Co2+

Li gh t pin k

Mn 2+

2. Dry Heat in g Test (i) (ii)

Take about 0.1 g of the dry salt in a clean and dry test t ube. Heat th e above test t ube for about one minut e and observe the colour of t he residu e wh en i t i s hot an d also when it becomes cold. Observati on of changes gives in dications about th e presence of cations, wh ich may n ot be t ak en as conclu sive evidence (see Table 7.7 ).

Table 7.7 : Inferences from the colour of the salt in cold and on heating Colour when cold

Col ou r wh en h ot

I n f er en c e

Blu e

Wh i t e

Cu2+

Gr een

Dir t y wh it e or yellow

Fe2+

Wh i t e

Yellow

Zn 2+

Pi n k

Blu e

Co2+

3. Flame Test The chl orides of several metals im part characteristi c colour to t he flame because th ey are volati le in n on-lu mi nou s flame. This test is performed with th e help of a platin um wir e as follows : (i) Make a tiny loop at one end of a platinum wire. (ii) To clean the loop dip it into concentr ated hydrochloric acid and hold it i n a non-l um in ous flame (Fig. 7.3). (iii) Repeat step (ii) un til t he wire impart s no colour to the flame. (iv) Put 2-3 drops of concentr ated hydrochloric acid on a clean watch glass and m ake a paste of a small quant it y of the salt i n i t. (v) Dip th e clean loop of the platinu m wire in t his paste and intr oduce th e loop in t he non-lu mi nou s (oxidisi ng) flam e (Fig. 7.3). (vi) Observe the colour of th e flame first wit h t he naked eye and then thr ough a blu e glass and identify t he metal ion wi th th e help of Table 7.8.

64


Fig.7.3 : Performing flame test

Table 7.8 : Inference from t he flame t est Colour of t he flame observed by naked eye

Colour of the flame observed through blue glass

Inference

Gr een flam e with blu e cen t r e

Sam e colou r as obser ved wi t h ou t glass

Cu2+

Cr i m son r ed

Pu r p l e

Sr 2+

Apple gr een

Bl u ish gr een

B a2+

Br ick r ed

Gr een

Ca2+

4. Borax Bead Test This test is emp loyed only for colour ed salts because borax r eacts with metal salts to form metal borates or metals, which have characteristic colou rs. (i) To perform this test make a loop at the end of the platinu m wire and h eat it in a flame till it is red hot. (b) (a) (ii) Dip the hot loop into borax powder and heat it again until borax forms a colourless transparent bead on the loop. Fig. 7.4 : Borax bead test Before dippin g the borax bead in th e test salt or mi xtu re, (a) Heating in reducing confir m t hat t he bead is tr ansparent and colour less. If it flame (b) Heating in oxidising flame is coloured this m eans that, t he platin um wire is not clean. Then mak e a fresh bead after cleanin g the wir e. (iii) Dip the bead in a small quantit y of the dry salt and again hold it in th e flame. (iv) Observe the colour im part ed to th e bead in th e non - lu minous flame as well as in th e lu mi nous flam e whi le it is hot an d when it is cold (Fig. 7.4). (v) To remove the bead from t he platinu m wir e, heat it t o redness and tap the platin um wir e with your finger. (Fig. 7.5).

65


On h eati ng, borax loses it s water of cryst alli sation and decomp oses to give sodiu m metaborate and boric anh ydride. Na2 B 4 O7 .10H 2 O ⎯→ Na 2 B4 O 7 + 10H 2 O Borax Na2 B 4 O7 ⎯→

2NaBO 2 + B 2 O3 Sodiu m metaborate Boric anhydr ide

On tr eatm ent wit h m etal salt, boric anh ydri de forms metaborate of th e metal which gives different colours in oxidising and reducing flame. For example, in th e case of copper sul phat e, followin g reactions occur . CuSO4 + B 2 O 3

Non-luminous flam e

Cu(BO 2 )2 + Cupr ic metaborate Blue-green

SO3

Two react ions may t ake place in th e reduci ng flame: (i) The blu e Cu (BO2 )2 is reduced to colou rl ess cu pr ous metaborat e as foll ows: Luminous flame

2Cu(BO2 )2 + 2NaBO2 + C

2CuBO 2 + Na2 B4 O7 + CO

or (ii) Cu pr ic m etaborat e may be reduced t o metalli c copper and t he bead appears red and opaqu e. 2Cu(BO2 )2 + 4NaBO 2 + 2C

Luminous flame

2Cu + 2Na 2B 4O 7 + 2CO

The prelim inar y identi fication of m etal ion can be made from Table 7.9.

F i g . 7 . 5 : Removing bora x bead

Table 7.9 : I nference from t he borax bead test Heatin g in oxidi sing (non-luminous) fl ame Col ou r of t h e sal t bead

66

Heating in reducing (lum in ous) flame Col ou r of t h e sal t bead

Inference

I n col d

I n h ot

I n col d

I n h ot

Blu e

Gr een

Red opaqu e

Col ou r l ess

Cu2+

Reddish br own

Vi ol et

Gr ey

Gr ey

Ni2+

Ligh t violet

Ligh t violet

Col ou r l ess

Col ou r l ess

Mn2+

Yellow

Yellowish br own

Gr een

Gr een

Fe3+


5. Charcoal Cavit y T est Metallic carbonate when heated in a charcoal cavity decomposes to give corresponding oxide. The oxide appears as a coloured residue in the cavity. Someti mes oxid e may be reduced t o met al by t he carb on of th e char coal cavit y. The test may be perform ed as follows: (i) Make a small cavity in a charcoal block wit h t he help of a charcoal borer. Do not appl y pressu r e oth erw ise it wil l crack [Fi g.7.6 (a)]. (ii) Fill the cavity with about 0.2 g of the salt and about 0.5 g of anhydrous sodiu m carbonate.

(b)

(a)

F i g . 7 . 6 : (a) Making charcoal cavity (b) Heating sal t in the cavity

(iii) (iv)

(v)

Moisten the salt in th e cavity with one or two drops of water, otherwise salt/ mixt ur e will blow away. Use a blowpipe to heat th e salt in a lu minous (reducing) flame and observe the colour of oxide/ metalli c bead form ed in th e cavit y both wh en hot and cold [ Fi g. (7.6 b)]. Obtain oxidising and redu cing flame as shown in Fig. 7.7 a and b. Always bore a fresh cavity for t esting the new salt.

(a)

(b)

F i g . 7 . 7 : Obtaining oxidising and reducing flame (a) Oxidis ing flam e; and (b) Redu cing fla me

Note : • To obtain oxidising flame hold the nozzle of the blowpipe about one third w ithin the flame. • To obtain reducing flame place nozzle of the blowpi pe just outsid e the flame.

67


When test is perform ed wit h CuSO4 , t he following change occur s. CuSO 4 + Na 2CO3 Heat

CuCO 3 Cu O + C

Heat

CuCO3 + Na 2 SO 4

CuO +CO 2

Heat

Cu

+ CO

Redc olour

In case of ZnSO4 : ZnSO 4 + Na2 CO 3 ZnCO 3

Heat

Heat

ZnCO3 + Na 2 SO 4

ZnO

+

CO 2

Yellow wh en h ot, White when cold The metal i on can be in ferr ed fr om Table 7.10. Table 7.10 : Inference from the charcoal cavity test Observations

Inference

Yellow r esidu e wh en h ot an d gr ey m et al wh en cold

Pb2+

Wh it e r esidu e wit h t h e odou r of gar lic

As3+

Br own r esi du e

Cd2+

Yellow r esidu e wh en h ot an d wh it e wh en cold

Zn2+

6. Cobalt Nit rat e Test If t he residue in th e charcoal cavity is wh it e, cobalt nit rate test is perform ed. (i) Treat the residue with t wo or th ree drops of cobalt nitr ate solut ion. (ii) Heat it strongly in non-luminous flame with t he help of a blow pipe and observe th e colour of th e residu e. On heating, cobalt nitrate decomposes into cobalt (II) oxide, which gives a characterist ic colour with metal oxide present in th e cavit y. Thus, with ZnO, Al 2 O3 and MgO, th e followin g reactions occu r. 2Co (NO3 )2

Heat

2CoO + 4NO 2 + O2

CoO + ZnO ⎯→ CoO.ZnO Green CoO + MgO ⎯→ CoO. MgO Pink CoO + Al 2 O3 ⎯→ CoO. Al2 O3 Blue

68


St ep-II : W et Test s for Ident ificat ion of Cat ions The cations indicated by the preliminary tests given above are confirmed by systemat ic an alysis given below. The fir st essent ial step is to pr epare a clear and tr ansparent solu ti on of th e salt. Thi s is call ed original solut ion . It is pr epared as foll ows: Preparation of Original Solution (O.S.) To pr epare th e origin al solut ion, foll owing steps are followed one aft er t he oth er in a systemat ic order. In case th e salt does not dissolve in a part icu lar solvent even on h eatin g, try t he next solvent . The followin g solvent s are tr ied: 1.

2.

3. 4. 5.

Take a litt le amoun t of th e salt in a clean boiling tube and add a few mL of disti lled water an d shake it. If t he salt does not dissolved, heat t he content of the boil in g tu be ti ll t he salt compl etely dissolves. If the salt is insoluble in water as detailed above, take fresh salt i n a clean boiling tu be and add a few mL of dil.HCl to it. If th e salt is insolubl e in cold, h eat t he boiling t ube ti ll th e salt is completely dissolved. If the salt does not dissolve eith er in water or in dilu te HCl even on heating, tr y to dissolve it in a few m L of conc. HCl by heati ng. If salt does not dissolve in conc. HCl, then dissolve it in dilu te nit ric acid. If salt does not dissolve even in nit ric acid th en a mixtu re of conc. HCl and conc. HNO 3 in th e rat io 3:1 is tr ied. This m ixt ur e is called aqua regia. A salt not solubl e in aqua regia is considered t o be an in solubl e salt.

Gr oup Analysis (I) Analysi s of Zero group cat i on (NH 4+ i o n )

(a ) Take 0.1 g of salt in a test tu be and add 1-2 mL of NaOH solut ion t o it and h eat. If th ere is a smell of ammoni a, thi s indi cates the presence of amm oniu m i ons. Brin g a glass rod dipped in h ydrochloric acid near th e mou th of th e test tu be. Whit e fu mes are observed. (b) Pass the gas th rough Nessler’s reagent . Brown precipit ate is obtain ed. +

Chemist ry of Confirm atory Test s for NH 4 i on (a)

Ammonia gas evolved by the action of sodium hydroxide on amm onium salts reacts with hydr ochlori c acid to give ammoni um chloride, which is visibl e as dense whi te fu me. (NH 4) 2 SO4 + 2NaOH ⎯→ Na 2SO 4 + 2NH 3 + 2H2 O NH 3 + HCl → NH4 Cl

69


On passing the gas through Nessler’s reagent, a brown colouration or a pr ecipi tat e of basic mercu ry(II) ami do-iodin e is form ed. 2K 2 HgI 4 + NH 3 + 3KOH ⎯→

Mercury Salts

HgO.Hg(NH 2 )I + Basic mercu ry (II) amido-iodine (Br own precipit ate)

7KI + 2H 2 O

For th e analysis of cat ion s belongin g to groups I-VI , th e cations are precipit ated from t he ori ginal solu ti on by using the grou p reagent s (see Table 7.11) accordin g to th e scheme shown in th e flow chart given below: The separat ion of all th e six grou ps is r epr esent ed as below:

Flow Char t * Original Solution Dil.HCl

Precipitate, Group I

Pass H 2 S gas

(Pb 2+ ) as chloride

Precipitate, Group II (Pb 2+ , Cu 2+, As 3+) as sulphides

Precipitate, Group III 3+

If no precipitate is formed

If no precipitate, take original solution Heat (O.S.) with conc. HNO 3 , cool and add solid NH 4 Cl + NH4 OH solution in excess

If no precipitate

3+

(Fe , Al ) as hydroxides

Precipitate, Group IV (Co2+ , Ni 2+, Mn 2+ , Zn 2+ ) as sulphides

Precipitate, Group V 2+

2+

2+

(Ba , Sr , Ca ) as carbonates

Pass H 2 S

If no precipitate, take original solution Add NH 4 OH and solid (NH 4 )2 CO3

If no precipitate, take original solution to test Group VI, M g2+

* This flow chart is for the detection of one cation only . For d etection of more than one cation modif ication w ill be required.

70


Table 7.11 : Group reagents for precipitating ions Gr o u p

Cat i on s*

Group Reagent

Gr ou p zer o

NH 4

Gr ou p-I

Pb

Gr ou p-II

Pb , Cu , As

Gr ou p-III

Al , Fe

Gr ou p-IV

Co , Ni , M n , Zn

Gr ou p-V

B a , Sr , Ca

Gr ou p-VI

Mg

+

None

2+

Dilute HCl

2+

3+

2+

3+

H 2S gas in presence of dil. HCl

3+

NH 4OH in presence of NH 4Cl

2+

2+

2+

2+

2+

2+

2+

2+

H 2S in presence of NH 4 OH (NH 4)2CO 3 in presence of NH4 OH None

(II ) Analysis of Group-I cat ion s Take a small amoun t of ori ginal solu ti on ( if prepared in hot conc. HCl) in a test tu be and add cold water to it and cool th e test t ube under tap water. If a white pr ecipit ate appears, t hi s in dicat es th e pr esence of Pb 2+ ions in group –I. On t he other hand, if t he ori ginal solut ion is prepared in water and on addit ion of dil. HCl, a wh it e pr ecipit ate appears, t hi s may also be Pb 2+. Confir matory t ests are descri bed below in Table 7.12. 2+

Table 7.12 : Confirmatory tests for Group-I cation (Pb ) Ex per i m en t

Obser vat i on

Dissolve the precipitate in hot water and divide the hot solution into three parts. 1. Ad d pot a ssi u m i od i de s ol u t i on t o t h e f ir s t part.

A yellow precipitate is obtained.

2.

To the second part add potassium chromate solution.

A yellow precipitate is obtained which is soluble in NaOH and insoluble in ammonium acetate solution.

3.

To the third part of the hot solution add few drops of alcohol and dilute sulphuric acid.

A white precipitate is obtained which is soluble in ammonium acetate solution.

Chemist ry of t he Confir m at ory Test s of Pb 2 + i o n s Lead is precipi tat ed as lead chlori de in th e fir st group. The precipi tat e is solu ble in h ot water. 1. On adding potassium iodide (KI) solut ion, a yellow precipitate of lead 2+ iodide is obtain ed wh ich confi rm s th e pr esence of Pb ions. PbCl 2 + (Hot solu t ion)

2KI

⎯→

PbI 2 + 2KCl Yellow pr ecipit at e

* Here only those cations are given w hich are in the syll abus.

71


2.

This yell ow pr ecipit ate (PbI 2) is solubl e in boili ng water and r eappears on coolin g as shin in g crystals. On addition of potassium chromate (K 2 CrO 4 ) solu ti on a yellow pr ecipit ate of lead chr omate is obtain ed. Thi s confi rm s th e pr esence of Pb 2+ ions. PbCl2 + K 2 Cr O 4 ⎯→ PbCrO4 + 2KCl (Hot solu t ion) Lead chr om at e (Yell ow pr ecipi tat e) The yellow pr ecipit ate (PbCrO 4 ) is solu ble in h ot NaOH solut ion. PbCrO4 + 4NaOH



Na 2 [Pb(OH)4 ]

+

Na2 Cr O 4

Sodium tetr a hydr oxoplu mbat e (II) 3.

Hydrogen sulphide

A white precipitate of lead sulphate (PbSO 4 ) is formed on addition of alcohol followed by dil . H 2 SO4 . PbCl2 + H 2 SO4 ⎯→ PbSO 4 + 2 HCl Lead sulph ate (Whit e precipit ate) Lead sulph ate is solu ble in amm oniu m acetat e solu ti on due to th e format ion of tetraacetoplu mbat e(II) ions. This r eaction m ay be promot ed by addi ti on of few dr ops of aceti c acid. PbSO4 + 4 CH3 COONH 4 ⎯→ (NH 4 )2 [Pb(CH 3COO)4 ] + (NH 4) 2SO 4 Ammonium tetraacetoplumbate(II) (III) Analysis of Group–II cations If group- I is absent , add excess of water to th e same t est t u be. Warm t he solu ti on and p ass H2 S gas for 1-2 min ut es (Fig. 7.6). Shak e t he test t u be. If a precipit ate appears, this indicates the presence of group-II cations. Pass more H 2 S gas th rough t he solut ion to ensur e compl ete precipit ation an d separate the precipi tat e. If t he colour of the precipit ate is black, it in dicat es t he pr esence of Cu 2+ or Pb2+ ions. If it is yellow in colour, th en pr esence of As 3+ ions is ind icated. Take the precipitate of group-II in a test tube and add excess of yellow ammoniu m sul phide solu tion to it. Shake th e test t ube. If th e precipit ate is insoluble, group II-A (copper group) is present. If th e precipit ate is solu ble, th is indi cates th e presence of group-II B (arsenic group). Confir matory t ests for th e groups II A and II B are given in Table 7.13.

F i g . 7 . 8 : Kipp’s apparat us for preparat ion of H 2 S gas

72


Table 7.13 : Confirmatory tests for Group-II A and II B cations Black precipitate of Group II A ions (Pb , Cu ) insoluble in yellow ammonium sulphide is formed.

If a yellow precipitate soluble in yellow ammonium sulphide is 3+ formed then As ion is present.

Boil the precipitate of Group II A with dilute nitric acid and add a few drops of alcohol and dil. H2SO4 .

Acidify this solution with dilute HCl. A yell ow pr ecipi tat e is form ed. Heat the precipitate with concentrated nitric acid and add ammonium molybdate solution. A canary yellow precipitate is formed.

2+

White precipitate confirms 2+ the presence of Pb ions. Dissolve the precipitate in ammonium acetate solution. Acidify with acetic acid and divide the solution into two parts. (i ) To t h e f i r st p ar t a dd p o t as si u m c h r o m a t e solution, a yellow precipitate is formed. (i i ) T o t h e s ec on d p a r t , add potassium iodide solution, a yellow precipitate is formed.

2+

If no precipitate is formed, add excess of ammonium hydroxide solution. A blue solution is obtained, acidify it with aceti c acid and add potassium ferrocyanide solution. A chocolate brown precipitate is formed.

Group-II A (Copper Group) Chemist ry of confir m at ory t est s of Group-II A cat ions

Alcohol

1. Test for Lead i on (Pb 2+) Lead sul phi de precipit ate dissolves in dilu te HNO 3. On adding dil. H 2 SO4 and a few dr ops of alcohol to th is solut ion a wh it e precipitat e of lead su lph ate appears. This i nd icates th e presence of lead ion s. 3PbS + 8HNO 3 ⎯→ 3Pb (NO3 )2 + 2NO + 4H 2 O + 3S Pb(NO 3) 2 + H 2 SO4 ⎯→ PbSO 4 + 2HNO 3 The white precipit ate dissolves in ammoni um acetat e solu ti on on boilin g. When t hi s solu ti on is acidified with aceti c acid and potassium chromat e solu ti on is added, a yell ow precipit ate of PbCrO 4 is formed. On addin g potassium iodide solut ion, a yell ow precipit ate of lead iodid e is form ed. PbSO 4 + 4CH 3 COONH 4 ⎯→ (NH4 )2 [Pb (CH 3 COO)4 ] + (NH 4)2 SO 4 Ammonium tetraacetoplumbate(II) ⎯→ PbCrO4 Pb2 + + CrO 2– 4 Lead chr om at e (Yellow pr ecipit at e)

Pb2 + + 2I – ⎯→ PbI 2 Lead iodide (Yellow pr ecipit at e)

73


2. Test for Copper i on (Cu 2+) (a)

Copper sul phide dissolves in ni tr ic acid due to the form ation of copper nitrate. 3CuS + 8HNO 3 ⎯→ 3Cu(NO 3 )2 + 2NO + 3S + 4H 2 O On heating the reaction mixture for long time, sulphur is oxidised to sulphate and copper sulphate is formed and the solution turns blue. A small amount of NH 4 OH precipit ates basic copper su lph ate which is solubl e in excess of ammonium hydroxide due to the formation of tetr aammi necopper (II) complex. S + 2HNO3 ⎯→ H 2 SO 4 + 2NO 2Cu 2+ + SO 24 +2NH 3 +2H 2 O ⎯→ Cu(OH) 2. CuSO4 + 2NH 4 Cu(OH)2 .CuSO4 + 8NH 3 ⎯→ 2 [Cu (NH 3 )4 ]SO4

–

2

+ 2OH + SO4

Tetr aamm in ecopper (II) sul ph ate (Deep bl u e) (b)

The blue solut ion on acidification with acetic acid and th en adding potassium ferr ocyanide [K 4 Fe(CN)6 ] solu t ion gives a chocolate colour ati on du e to th e form ati on of copper ferr ocyanid e i.e.Cu 2[Fe(CN)6 ]. [Cu(NH 3 )4 ] SO4 + 4CH3 COOH ⎯→ CuSO4 + 4CH 3 COONH4 2CuSO 4 + K 4 [Fe(CN)6] ⎯→ Pot assiu m hexacyanoferrat e (II)

Cu 2 [Fe(CN)6 ] + 2K 2 SO4 Copper hexacyanoferr ate (II) (Chocolat e br own pr ecipit ate)

Group-II B (Arsenic Group) If grou p- II pr ecipit ate dissolves in yellow amm oniu m su lph ide and t he colour of the solution is yellow, this indicates the presence of As 3+ ions. Ammonium th ioarsenid e form ed on d issolu ti on of As 2 S3 decomposes wit h dil. HCl, and a yellow precipit ate of arsenic (V) sul phide is form ed which dissolves in concentr ated nit ri c acid on h eatin g due to th e formati on of arsenic acid. On adding ammoniu m molybdate solution to the reaction mixture and heating, a canary yellow pr ecipit ate is formed. This confir ms t he presence of As 3+ ions. As 2S3

+ 3 (NH4 )2 S2 ⎯→ 2 (NH 4 )3 As S4 + S Yellow amm oniu m sulphide

2(NH 4 )3 AsS4 + 6HCl ⎯→ As2 S5 + 3H 2 S + 6NH 4 Cl 3As 2 S5 + 10HNO3 + 4H 2 O ⎯→ 6H 3 AsO 4 + 10NO + 15S Arsenic acid

74


H 3AsO 4 Ar sen ic acid

+ 12(NH 4 )2 MoO4 Am m oniu m m olybdat e

+ 21HNO 3 ⎯→ (NH4 )3 [As (Mo3 O10)4 ] + 21NH4 NO3 +12H 2 O Am m on iu m ar sin om olybdat e (yell ow pr ecipi tat e)

(IV) Analy sis of Group–II I cati ons If group-II i s absent , tak e original solut ion and add 2-3 drops of conc. HNO 3 t o oxidi se Fe2+ ions t o Fe3+ ions. Heat t he solu ti on for a few mi nu tes. After cooling add a small amount of solid ammoniu m chloride (NH 4Cl) and an excess of amm oniu m hydr oxide (NH4 OH) solu tion t ill it smells of ammonia. Shake th e test tu be. If a b rown or white precipitate is formed, this indicates the presence of group-III cations. Confi rm atory tests of group -III cati ons are su mm ari sed in Table 7.14. Observe th e colour and th e natu re of th e precipit ate. A gelatin ous whit e precipit ate indicates the presence of alu min iu m ion (A1 3+ ). If the pr ecipi tat e is 3+ br own in colour , th is in dicates th e pr esence of ferr ic ions (Fe ). Table 7.14 : Confirmatory test for Group-III cation s Brown precipit at e 3+ Fe Dissolve the precipitate in dilute HCl and divide the solution into two parts. (a) To the first part add potassium ferrocyanide solution [Potasium hexacyanoferrate (II)]. A blue precipitate/ colourati on appears. (b) To the second part add potassium thiocyanate solution. A blood red colouration appears.

Whit e precipit ate 3+ Al Dissolve the white precipitate in dilute HCl and divide into two parts. (a) To th e first part add sodiu m h ydroxide solution and warm. A white gelatinous precipitate soluble in excess of sodium hydroxide solution. (b) To the second part first add blue litmus solution and then ammonium hydr oxide solu tion dr op by drop along the sides of the test tube. A blue floating mass in the colourless solution is obtained.

Chemi st ry of confirm at ory t ests of Group-II I cat ions When ori ginal solut ion is heated with concent rated nit ri c acid, ferr ous ions are oxidised to ferr ic ions. 2FeCl 2 + 2HCl + [O] ⎯→ 2FeCl 3 + H 2 O Thir d group cati ons are precipit ated as th eir hydr oxides, which dissolve in dilu te hydr ochl oric acid due to t he form ation of corresponding chlorid es. 1. Test for Alum ini um ions (A1 3+ ) (a)

When th e solution containing alumin ium chloride is treated with sodium hydr oxide, a whit e gelatin ous precipit ate of alum in iu m hydr oxide is formed

75


whi ch is solubl e in excess of sodiu m h ydroxide solu ti on du e to th e format ion of sodium meta alu min ate. AlCl 3

+ 3NaOH ⎯→ Al(OH) 3 + 3NaCl

Al(OH)3 + NaOH ⎯→ NaAlO 2 + 2H 2 O Whit e gelat in ou s Sodiu m pr ecipit at e m et a alu m inat e (b)

In t he second test when blue litm us is added to the solut ion, a red colour ation is obtained due to th e acidic natu re of th e solu tion. On addition of NH 4 OH solution drop by drop the solution becomes alkaline and alum ini um h ydroxide is precipit ated. Alum ini um h ydroxide adsorbs blu e colour from t he solut ion and form s insoluble adsorpti on compl ex nam ed ‘lake’. Thu s a blu e mass floati ng in th e colour less solu ti on is obtain ed. The t est i s th er efore call ed lak e t est . 3+

2. Test for ferr ic ion s (Fe ) Reddish br own precipit ate of ferr ic hydr oxide dissolves in h ydrochl oric acid and ferr ic chlori de is formed. Fe(OH)3 + 3HCl ⎯→ FeCl3 + 3H2 O (a)

When the solut ion cont aining ferr ic chloride is tr eated with potassium ferr ocyanide solut ion a bl ue precipit ate/ colourat ion i s obtained. The colour of the precipitate is Prussian blue. It is ferric ferro-cyanide. The reaction t ak es place as foll ows: 4FeCl3 + 3K 4[Fe(CN)6 ] ⎯→ Fe 4[Fe(CN) 6] 3 + 12KCl Potassium Pru ssian blue ferrocyanide precipitate If pot assiu m h exacyanoferr ate (II) (i.e. potassiu m ferr ocyanide) is added in excess th en a pr oduct of composit ion KFe[Fe(CN)6] is formed. This tends to form a colloidal solut ion (‘solubl e Pru ssian blu e’) and cann ot be fil tered. FeCl 3 + K4 [Fe(CN)6 ]

(b)

⎯→ KFe[Fe(CN)6 ] + 3KCl (Solu ble pru ssian blu e)

To the second part of the solut ion, add potassium thi ocyanate (potassium sulphocyanide) solution. The appearance of a blood red colouration 3+ confir ms t he pr esence of Fe ions. Fe3+ + SCN – ⎯→ [Fe(SCN)] 2+ Bl ood r ed colour

(V) Analysis of group-IV cati ons If grou p-II I is absent , pass H 2S gas in the solution of group-III for a few mi nu tes. If a pr ecipit ate appears (whi te, black or flesh coloured), t his i ndi cates

76


th e presence of group -IV cations. Table 7.15 gives a sum mar y of confir mat ory tests of grou p-IV cations.

Table 7.15 : Confirmatory test for Group-IV cations White precipitate 2+ (Zn ) Dissolve the pr ecipitat e in dilut e HCl by boiling. Divide the solution into two parts. (a) To the first part add sodium hydroxide solution. A white precipitate soluble in excess of sodiu m hydroxide solution confirms the 2+ presence of Zn ions.

Flesh coloured precipitate 2+ (Mn ) Dissolve the precipitate in dilute HCl by boiling, then add sodium hydroxide solution in excess. A white precipitate is formed which turns brown on keeping.

(b ) N eu t r a l i se the second part with a m m o n i u m hydroxide solution and add potassium ferrocyanide solution. A bluish white precipitate appears.

Black precipitate 2+

2+

(Ni , Co ) Dissolve the precipitate in aqua regia. Heat the solution to dryness and cool. Dissolve th e residu e in water and divide the solution into two parts. (a) To the first part of the solut ion add ammoni u m hydroxide solut ion till it becomes alkaline. Add a few drops of dimethyl glyoxime and shak e the test tube. Formation of a bright red precipitate confirms the presence 2+ of Ni ions. (b) Neutralise the second part with ammonium hydroxide solution. Acidify it with dilute acetic acid and add solid potassium nitrite. A yellow precipitate confirms the presence 2+ of Co ions.

Chemi st ry of confi rm at ory t est s of Group–IV cat ions Four th group cations are precipit ated as th eir sulph ides. Observe the colour of the precipitate. A white colour of the precipitate indicates the presence of zinc ions, a flesh colour in dicat es th e pr esence of man ganese ions an d a black colou r 2+ 2+ in dicat es t he pr esence of Ni or Co ions. 1. Test for Zin c ion (Zn 2+ ) Zinc sul phide dissolves in hydr ochlori c acid t o form zinc chl oride. ZnS + 2HCl ⎯→ ZnCl 2 + H 2 S

77


(a)

On addition of sodium hydroxide solut ion it gives a whit e precipitate of zinc hydroxide, which is soluble in excess of NaOH solution on heating. This confi rm s th e presence of Zn 2+ ions. ZnCl2 + 2NaOH ⎯→ Zn(OH)2 + 2NaCl Zn(OH)2 + 2NaOH ⎯→

(b)

Na 2Zn O2 + Sodiu m zincate

2H 2 O

When potassium ferrocyanide K 4 Fe(CN)6 solu ti on is added t o the solu ti on after neut ralisation by NH 4 OH solu tion, a whit e or a bluish wh it e precipit ate of zinc f err ocyani de appears. 2 ZnCl2 + K4 [Fe(CN)6 ] ⎯→ Zn 2 [Fe(CN)6 ] + 4 KCl Zinc ferrocyanide

2. Test for Man ganese ion (Mn 2+ ) Manganese sulp hid e precipit ate dissolves in dil. HCl on b oiling. On addit ion of NaOH solution in excess, a white precipitate of manganese hydroxide is formed which tu rn s brown du e to atm ospheric oxidation in to hydr ated m anganese dioxide. Mn S + 2HCl ⎯→ Mn Cl 2 + H 2 S MnCl2 + 2NaOH ⎯→ Mn(OH)2 + 2NaCl (White precipitate) Mn (OH)2 + [O]

→

MnO(OH)2 Hydr ated manganese dioxid e (Brown colour )

3. Test f or Nick el i on (Ni 2+) The black p recipit ate of ni ckel sul phi de dissolves in aqu a regia and t he reaction t ak es place as follows: 3NiS + 2HNO 3 + 6HCI ⎯→ 3NiCl 2 + 2NO + 3S + 4H 2 O After t reatm ent wit h aqua regia nick el-chlori de is obtained whi ch is solu ble in water. When dim eth yl glyoxim e is added t o the aqueou s solu ti on of nick el chlor ide, made alk alin e, by addin g NH 4 OH solu ti on, a bri lliant red precipitate is obtained.

Complex of red colour (Stable form of complex)

78

SYSTEMATIC Q UALITATIVE A NALYSIS 2+

4. Test for Cobalt ion (Co ) Cobalt su lph ide dissolves in aqua regia in t he same man ner as nick el sul phi de. When th e aqueous solu ti on of th e residue obtain ed after tr eatm ent w it h aqua regia is tr eated wit h a str ong solut ion of potassiu m n itr ite after neutr alisation wit h ammoniu m hydroxide and t he solut ion is acidified wit h di l. aceti c acid, a yellow precipit ate of t he compl ex of cobalt nam ed potassium hexanit ri tocobaltat e (III) is form ed. CoS + HNO3 + 3HCl ⎯→ CoCl 2 + NOCl + S + 2H 2 O CoCl 2 + 7KNO2 + 2CH 3 COOH ⎯→ K3 [Co(NO2 )6 ] + 2KCl + 2CH 3COOK + NO + H 2 O Potassium hexanitritocobaltate(III) (Yell ow p recipit ate) (VI) Analysis of Group–V cations If grou p-IV is absent th en t ake ori gin al solu ti on and add a small amou nt of solid NH 4 Cl an d an excess of NH 4OH solu tion followed by solid amm onium carbonate (NH 4)2 CO3 . If a wh it e precipit ate appears, t hi s in dicates th e pr esence of group–V cations. Dissolve th e white precipit ate by boilin g with dilu te aceti c acid and divide 2+ 2+ 2+ th e solu ti on in to th ree parts one each for Ba , Sr and Ca ions. Preser ve a sm a l l a m o u n t o f t h e p r ec i p i t at e f o r f l a m e t est . Summ ary of confir matory tests is given i n Table 7.16. Table 7.16 : Confi rm atory t est for Group–V cati ons Dissolve th e precipit ate by boilin g wit h dil ut e acet ic acid and divide th e 2+ 2+ 2+ solut ion i nt o t hree part s one each for Ba , Sr and Ca i o n s Ba

2+

i on s

(a) To the first part add potassium chromate solution. A yellow precipitate appears. (b ) Per f o r m t h e f l a m e test with the preserved precipitate. A grassy green flame is obtained.

Sr

2+

i on s

(a ) I f b ar i u m i s ab sen t , t a k e second part of the solut ion and add ammonium sulphate solution. Heat and scratch t he sides of the test tube with a glass rod and cool. A white precipitate is formed. (b) Perform th e flame test with the preserved precipitate. A crimson-red flame confirms the presence of Sr 2+ ions.

2+

Ca

ions

(a) I f b ot h b a r i u m a n d strontium are absent, take the third part of the solution. Add ammonium oxalate solution and shake well. A white precipitate of calcium oxalate is obtained. (b) Perform th e flame test with the preserved precipitate. A brick red flame, which looks greenish-yellow through blue glass, confirms the 2+ presence of Ca ions.

79


Chemist ry of Confi rm at ory Test s of Group–V cat ions The Grou p–V cations are precipit ated as th eir carbonates which dissolve in aceti c acid du e to t he format ion of correspond in g acetat es. 1. Test for Bar iu m i on (Ba2+ ) (a)

Potassium chromate (K 2 Cr O4 ) solut ion gives a yellow pr ecipit ate of bar iu m chr omate when t he solut ion of fifth grou p precipitat e in aceti c acid is t reated with it. BaCO3 + 2CH 3COOH ⎯→ (CH 3 COO)2 Ba + H2 O + CO 2 (CH3 COO)2 Ba + K 2Cr O4 ⎯→

(b)

BaCrO4 + 2CH 3 COOK Barium chromate (yellow pr ecipit ate )

Flame test : Take a platin um wire and di p it in conc. HCl. Heat it strongly until the wire does not impart any colour to the non-luminous flame. Now dip t he wir e in t he paste of th e (Grou p–V) pr ecipit ate in conc. HCl. Heat it in t he flame. A grassy green colour of th e flam e confir ms th e pr esen ce of Ba 2+ ions.

2. Test for Str onti um ion (Sr 2+ ) (a)

Solut ion of V group pr ecipitate in acetic acid gives a white precipitate of strontium sulphate with ammonium sulphate, (NH 4 )2 SO4 , solution on heati ng and scratchi ng th e sides of the test tu be with a glass rod. SrCO3 + 2CH 3 COOH ⎯→ (CH3 COO)2 Sr + H 2O + CO 2 (CH3 COO)2 Sr + (NH 4 )2 SO4 ⎯→

SrSO 4 + 2CH 3COONH 4 Strontium sulphate (White precipitate)

(b)

Flame test : Perf orm t he flam e t est as given i n t he case of Ba 2+. A crimson r ed flam e confi r ms t he presence of Sr 2+ ions. 2+

3. Test for Calciu m i on (Ca ) (a)

Solut ion of the fifth group precipitate in acetic acid gives a whit e precipitate with ammonium oxalate solut ion. CaCO 3 + 2CH 3COOH ⎯→ (CH 3COO)2 Ca + H 2 O + CO 2 (CH3 COO)2Ca + (NH 4)2 C2 O4 ⎯→ (COO)2Ca + 2CH 3COONH 4 Ammonium Calcium oxalate oxalate (Whit e pr ecipi tat e)

(b)

80

Flame t est : Perform th e flame test as ment ioned above. Calcium im part s bri ck r ed colour to t he flame which l ooks greenish -yellow t hr ough blu e glass.


(VII ) Analy sis of Group–VI cati ons If group–V is absent th en perform th e test for Mg

2+

ion s as given below.

Chemist ry of Confir m at ory Test s of Group–VI cati ons 2+

Test for Magnesium ion ( Mg ) (a)

If group–V is absent the solut ion may contain m agnesium carbonate, which is soluble in water in the presence of ammonium salts because the equil ibr iu m is shifted towards th e ri ght hand side. NH 4 + + CO 23

    

NH 3 + HCO – 3

The concentration of carbonate ions required to produce a precipitate is not at tain ed. When disodium hydr ogenph osphate solu ti on is added and the inner walls of the test tube are scratched with a glass rod, a white crystalli ne precipit ate of magnesium ammoni um phosphate is formed which in dicates th e presence of M g2+ ions. Mg

2+

+ Na 2 HPO4 ⎯→ Mg (NH 4)PO4 + NH 4 OH + 2Na+ + H 2 O Magnesium ammonium phosphate (White precipitate)

Note down t he observat ions and t he inferences of th e quali tat ive analysis in tabu lar for m as given in t he specim en r ecord given in pages 114-115 .

Note : Some times precipitate of magnesium ammonium phosphate appears after some time. So warm the solution a nd scrach the sides of test tube after a d d i n g s o d i u m hydrogen phosphate solution.

P r ec a ut i on s (a) Always use an apron, an eye protector and hand gloves while work in g in t he chemist ry laborat ory. (b) Before using any reagent or a chemical, read the l a b e l o n t h e b o t t l e c a r ef u l l y . N ev er u s e u nl abelled r eagent . (c) D o n o t m i x c h em i c a l s a n d r ea gen t s u nn ecessari ly. Never t aste any chemical. (d) Be careful in smelling chemicals or vapour s. Always fan the vapours gently towards your n ose (Fig. 7.9 ). (e) Never add sodium metal to water or thr ow it in th e sink or dustbin. (f) Always pour acid int o water for dilut ion. Never add water to acid. (g) Be careful whi le heating th e test tu be. The test tube should never point towards yourself or towards your neighbours while heating or addin g a reagent .

Fig. 7.9 : How to smell a gas

81


(h) Be careful wh ile dealing with th e explosive compoun ds, infl ammable substances, poisonous gases, electr ic appl ian ces, glass wares, flame and t he hot su bst ances. (i) Keep your work ing surroun dings clean. Never t hrow papers and glass in t he sink . Always use dust bin for t hi s pur pose. (j) Always wash your hands after t he completion of the laboratory work . (k ) Always use th e reagent s in min imu m quant it y. Use of reagent s in excess, not only l eads to wastage of chemi cals bu t al so causes damage to t he envi ron ment .

Discussion Quest ions (i) What is th e difference between a qualit ative and a quant itat ive analysis? (ii) Can we use glass rod inst ead of platin um wir e for perform ing th e flame test? Explain your answer. (iii) Why is platin um metal preferr ed to other m etals for the flame test? (iv) Name the anions detected with t he help of dilute H 2SO 4 ? (v) Wh y i s d i lu t e H 2SO 4 preferr ed over dilu te HCl wh ile testi ng anions? (vi) Name the anions detected by conc. H 2 SO 4. (vii) How is sodium carbonate extr act pr epared ? (viii) What is lime water and what h appens on passing carbon dioxide gas th rough it ? (ix) Carbon dioxide gas and sulph ur dioxide gas both t ur n lim e water mi lky. How will you disti nguish between th e two? (x) How will you t est th e presence of carbonate ion? (xi) What is the composition of dark brown rin g which is form ed at the junct ion of two layers in t he ri ng test for nit rates? (xii) Name th e radical confirm ed by sodium nit roprusside test. (xiii) What is chromyl chloride test ? How do you ju stify that CrO 2 Cl2 is acidic in n atu re? (xiv) Why do bromid es and iodides not give tests simil ar to chromyl chlori de test? (xv) Descri be th e layer t est for bromide and iodide ions.

82


(xvi) Why is silver n itr ate solut ion stored in dark coloured bott les? (xvii) How do you test t he presence of sulph ide ion? (xviii) Why does iodine give a blue colour with starch solut ion? (xix) What is Nessler’s reagent ? (xx) Why is original solut ion for cations not pr epared in conc. HNO 3 or H 2 SO4 ? (xxi) Why cannot conc. HCl be used as a group r eagent in pl ace of dil. H Cl for t he precipit ation of Ist group cati ons? (xxii)

How can one prevent th e precipitation of Group–IV radicals, with th e second group radicals?

(xxiii)

Why is it essential to boil off H 2 S gas before precipi tat ion of r adicals of grou p–III ?

(xxiv)

Why is heating with conc. nit ric acid done before precipitation of group–III?

(xxv)

Can we use ammonium sulphate instead of ammonium chloride in group–III?

(xxvi )

Wh y i s NH 4 OH added before (NH 4 )2 CO3 solu ti on whi le precipitat in g group–V cati ons?

(xxvii)

Why do we sometim es get a whi te precipit ate in group–VI even if t he salt does not 2+ cont ain Mg radical?

(xxviii)

What is aqua regia?

(xxix)

Name a cation, which is not obtained from a metal.

(xxx)

How can you t est the presence of amm oniu m ion?

(xxxi)

Why are th e group–V radicals tested in t he order Ba 2+ , Sr 2+ and Ca 2+ ?

(xxxii)

Why does conc. HNO 3 kept in a bott le turn yellow in colour?

(xxxiii )

Why shoul d the solut ion be concent rat ed before proceedin g to group–V?

(xxxiv)

Why is the reagent bott le cont aining sodium hydroxide solut ion never stoppered?

(xxxv)

What do you u nderstand by the term common ion effect?

(xxxvi)

Why is zinc sulph ide not precipitated in group–II?

83


SPECIM EN RECORD

OF

S A LT A NALYSIS

Aim To analyse th e given salt for one ani on and one cati on pr esent in it .

M at er ial requir ed • Boiling tubes, test tu bes, test tu be holder, t est tu be stand, delivery t ube, corks, filt er papers, reagent s

Sl . No.

84

Ex p er i m en t

Obser v at i o n

I n f er en c e 2+

2+

2+

2+

1.

Noted the colour of the given salt.

White

Cu , Fe , Ni ,C o , 2+ Mn are absent.

2.

Not ed t h e smell of t h e salt .

No specific sm ell.

S , SO3 , CH 3 COO may be absent.

3.

Heated 0.5 g of the salt in a dry test tube and noted the colour of the gas evolved and change in the colour of the residue on heating and cooling.

(i) No gas was evolved.

(i ) CO 3 may be – – present, NO3 , NO2 , – Br may be absent. 2+ (i i) Zn may be absent.

4.

Prepared a paste of the salt with conc. HCl and p erform ed the flame test.

No distin ct colour of th e flame seen.

5.

Borax bead test was not performed as the salt was white in colour.

—

—

6.

Treated 0.1 g of salt wit h 1 mL dil.H 2 SO4 and warmed.

No effervescence and evolution of vapours.

CO 3 , SO 3 , S , NO 2 ,

7.

Heated 0.1 g of salt wit h 1 mL conc. H 2SO4.

No gas evolved.

Cl , B r , I , NO3 , C2 O4 are absent.

8.

Acidified 1mL of aqueous salt s o l u t i o n w i t h c o n c . H N O3 . Warmed the contents and then added 4-5 drops of ammonium molybdate solution.

No yel low pr ecipi tat e

PO4 absent.

(i i ) N o particu lar change in colour of the residue is observed when heated and when cooled.

2–

2–

–

2 –

2+

2+

2+

Ca , Sr , Ba may be absent.

2 –

2 –

2 –

Cu

2+

–

–

CH 3COO absent. –

3–

–

–

–

–

Salt Analysis

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