Title: Oxidation-Reduction Titration Iodimetry Activity No. 5 Date: August August 22, 2!5 Names: "a. T#eresa ". $lasos and %essica "agayanes A&'TRA(T There are several types of redox titrations and two two of whic which h are are iodi iodime metr try y and and iodom iodomet etry ry.. Iodimetry is a redox titration which uses iodine as the titrant, an oxidizing agent that reacts with the analyte, a reducing agent. Iodometry is a redox redox reacti reaction on in which which the liber liberate ated d iodine iodine produced in the analyte of the sample is titrated with a standard solution of thiosulfate, Na 2S2!. The Na2S2! solution is the reducing agent while the liberated iodine in sample is the oxidizing agent in iodometry. In this experiment, iodometry was performed with starch as the indicator. Its reaction is given by" I2 # S2!2$
2I$ # S%&2$
In iodi iodime metr try y, iodi iodine ne is a mode modera rate tely ly stro strong ng oxid oxidiz izin ing g agen agentt and and can can be used used to titr titrat ate e reduci reducing ng agents agents.. Titr Titrati ations ons with with I2 are called called iodimetric methods. These methods are usually performed in neutral or mildly al4aline (p+ 0) to wea4ly acid solutions. If the p+ is too al4aline, I 2 will disproportionate to hypoiodate and iodide"2 I2 # 2+$ 7 I$ # I$ # +2 Iodi Iodine ne has has a low low solu solubi bili lity ty in wate waterr but but the the $ complex, I! , is very soluble. So iodine solutions are prepared by dissolving I2 in a concentrated solution of 8I" ! I2 # I$
I!$
The The ob'e ob'ect ctiv ives es of this this expe experi rime ment nt were were to prep prepar are e a stan standa darrd solut olutiion of sodi sodium um thiosulfate (Na 2S2!) and determine the strength of a bleac bleachin hing g agent agent by oxidat oxidation ion$re $reduc ducti tion on titration. The The amou amount nt of Na2S2!
∙
*+2 crystals crystals
needed to prepare a * m- of . / Na 2S2! solution was 2. % g. The average molarity of Na2S2! from from the the four four tria trials ls perf perfor orme med d was was .!0* g1mol. The 3l present in the sample was .02& given *. m- of Na2S2! and . g sample. In this experiment, the 3l content of the un4nown bleach was determined. 5ossible sources of error include oxidation and loss of iodine by vapor, insufficient acidity of standard iodine iodine soluti solution on which which can cause cause incomp incomplet lete e reduction of dichromate by iodide. INTROD)(TION
There are several types of redox titrations and two of which are iodimetry and iodometry. 6oth of these these titra titratio tions ns involv involve e the use of iodine iodine.. Iodine (I2) is an oxidizing agent that can be used to titrate fairly strong reducing agents. n the other other hand, hand, iodide iodide ion (I $) is a mild mild redu reduci cing ng agent and serves as the basis for determining strong oxidizing agents.
I!$ is ther theref efor ore e the the actu actual al spec specie ies s used used in titration. In iodometry, iodide iodide ion is a wea4 reducing agent and will reduce oxidizing agents. It is not used, how however ever,, as a tit titrant rant bec becaus ause of lac4 lac4 of convenient visual indicator system, as well as other factors such as speed of reaction. % 9hen an excess of iodide is added to a solution of an oxid oxidiz izin ing g agen agent, t, I2 is produc produced ed in an amount e:uivalent to the oxidizing agent. This I 2 can therefore be titrated with a reducing agent and the result will be the same as if the oxidizing agent were titrated directly. The titrating agent used is sodium thiosulfate. * Standard solution of sodium thiosulfate is one of the few reducing agents that is stable toward air oxidation. The The end end poin pointt for for iodo iodome metr tric ic titr titrat atio ions ns is detected detected with starch. starch. The disappeara disappearance nce of the blue starch$I starch$I2 colo colorr indi indica cate tes s the the end end of the the titra itrattion. ion. The star starch ch is not not adde added d at the begi beginn nnin ing g of the the titr titrat atio ion n when when the the iodi iodine ne concentration is high. Instead, it is added 'ust
before the endpoint when dilute iodine color becomes pale yellow. ;n example of this procedure is the determination of hypochlorite in bleaches. The reactions are 2S2!2$ # I2 → 2I$ # S%&2$ 2+# # 3l$ # 2I$→ I2# 3l$ # +2 In this experiment, sodium thiosulfate solution was standardized iodometrically against a pure oxidizing agent, the 823r 2< and the strength of a bleaching agent was determined by oxidation$ reduction titration. "*T+ODO$O ;. 5reparation of Starch Indicator ; .* g of starch was weighed and was dissolved with * m- of distilled water. The starch solution was added to a mboiling water and was boiled for another 2 minutes. 6. 5reparation and Standardization of . / Na2S2! solution The weight of Na2S2!
°
*+2 crystals
needed to prepare a * m- of . / Na2S2! solution was calculated. The crystals were dissolved in a bea4er with m- distilled water (pre$boiled). It was diluted to ma4e a * m- solution. ; .2 g of Na23! was added to the Na 2S2! solution. ; . = .* g of pure, dry 8 23r 2< was weighed into each of three 2* m>rlenmeyer flas4s. It was dissolved by adding * m- distilled water (pre$boiled). ; % m- of "2 + 2S% was added to the solution. ; *.g of 8I was weighed and was dissolved by adding water. ; * m- of the 8I solution was added to each >rlenmeyer flas4 and was covered with watch glass. The analyte was allowed to stand by for ! minutes. The analyte should have an initial brown color.
;fter ! minutes, the analyte was diluted with * m- of distilled water (pre$boiled) and was titrated with thiosulfate solution until brown color of iodine had disappeared. ;fter the first change in color, a * m- of starch solution was added and the titration continued until the last drop of the titrant removes the blue color of the starch$iodine complex gives a clear emerald green solution. The molarity of the Na 2S! solution was calculated. The average deviation should be about $!ppt. 3. ;nalysis of the ?n4nown ; 2. m- of tap water was pipette into each of the three >rlenmeyer flas4s. ; * m- of distilled water (pre$boiled), ! m- of 8I, 0 m- of "& + 2S% and ! drops of ! ammonium molybdate (optional) were added. >ach flas4 was covered with a watch glass and was allowed to stand for ! minutes to allow the reaction to be completed. The analyte was titrated with standard thiosulfate solution until brown color of iodine had disappeared. ;fter a change in color, a * m- of starch indicator was added and the titration continued until the disappearance of the blue color. The 3l was calculated in each product assuming that the density of li:uid is . g1m-. R*')$T' AND DI'()''ION A. re/aration o0 Na 2'2O1 solution ;mount of Na 2S2!
∙
*+ 2 crystals needed"
!2. ! g * m-, . / Na2S2! (.* -)(. mol1 -)(2%0.& g Na 2S2! *+2) 3 !2.4 g 3 !2. ! g &. 'tandardiation o0 Na2'2O1
∙
Trial
6t. 72(r 2O8 9g
;ol. Na2'2O1 9m$
"olarity Na2'2O1 9g
!
.!*
0.%
.2*@!
2
.%0
&.&
.!2!0
1
.2*
&.@
.!*@
Average
.
<.&
.2@0 .!0*
'am/le (om/utation: Trial !: " Na 2'2O1 3 .25=1 (.!* g 3r 2<2)( mol 3r 2<21 2@%. 2 g 3r 2<2) (! mol I21 mol 3r 2<2)(2 mol S 2!21 mol I2) 7 .2*@! g1mol Na2S2! Trial 2: " Na 2'2O1 3 .1214 2 <
2 <
2 <
(.%0 g 3r 2 )( mol 3r 2 1 2@%. 2 g 3r 2 ) (! mol I21 mol 3r 2<2)(2 mol S 2!21 mol I2) 7 .2*@! g1mol Na2S2! 7 .!2!0 g1mol Trial 1: " Na 2'2O1 3 .15=! (.2* g 3r 2<2)( mol 3r 2<21 2@%. 2 g 3r 2<2) (! mol I21 mol 3r 2<2)(2 mol S 2!21 mol I2) 7 .2*@! g1mol Na2S2! 7 .!*@ g1mol Trial : " Na 2'2O1 3 .2=!4 (. g 3r 2<2)( mol 3r 2<21 2@%. 2 g 3r 2<2) (! mol I21 mol 3r 2<2)(2 mol S 2!21 mol I2) 7 .2*@! g1mol Na2S2! 7 .2@0 g1mol Average "olarity o0 Na2'2O1 3 .2*@! # .!2!0 # .!*@ # .2@01 % 3 . 145 ger Trial
!
;ol. Na2'2O1 9m$ *.
6t. sam/le 9g . g
? (l in t#e 'x. .02&
(.* -)(.!0* mol1-Na2S2!)( mol I212 mol S2!)( mol 3l$1 mol I2)(!*.%* g1 mol 3l $) A . g sample B 7 .42@ ? (l In the standardization, the molarity of the thiosulfate solution was obtained by dividing the weight of the dried dichromate by the molecular mass of potassium dichromate. The :uotient was then stoichiometrically converted to moles of S2!2$, and then divided by the volume of sodium thiosulfate obtained through titration. The molarity per trial exceeded the . / of the standard sodium thiosulfate solution. The volume of the titrant consumed is so small that it produced such molarities. The possible sources of this are from () the expired 8I added to the sample and (2) the additional +2S% to the sample for it did not yield an initial brown color before titration. The iodine solution was so diluted that it did not yield the brown color. In the determination of the bleaching power of the un4nown, the 3l was obtained by determining the moles of S 2! and converting it to g of 3l $. The answer was divided by .g sample and multiplied to . The 3l in the sample was .02& . (ON($)'ION In this experiment, the 3l content of the un4nown bleach was determined. 5ossible sources of error include oxidation and loss of iodine by vapor, insufficient acidity of standard iodine solution which can cause incomplete reduction of dichromate by iodide. R**R*N(*'
'am/le (om/utation: S4oog, et al. Fundamentals Chemistry . @th edition
of Analytical
Cary 3hristian, Analytical Chemistry ,
