EXPERIMENT 6 Analysis of bleach and copper (ii) unknown
Abstract The objectives for this experiment are to standardize the sodium thiosulphate solution, to determine the percentage of NaClO in sample and to determine the percentage of copper in an unknown copper sample.firstly, get the standardization molarity of the sodium thiosulphate solution. Then, in part b, 10.0ml of the first bleach sample was pipette into the pre-weighed 100ml volumetric above. The flask was reweighed and recorded. This will give the density of the bleach solution. The sample was diluted to the 100ml mark with distilled water. Then, it was mixed well. The pipette was rinsed with distilled water followed with a little of the diluted bleach solution and a fresh 10.0ml aliquot was pipette into a 250ml conical flask. Next, 1.0 g of potassium iodide was added and it was swirled . then, 5.0ml of 6 M HCl was added to the mixture. The sample was titrated with the standardized sodium thiosulphate solution until the amber iodine colour begins to fade to light yellow. 2 ml of the starch solution was added and the titration was continued until the dark colour of the starch-iodine complex just disappears. The titration was repeated with two more 10.0ml aliquot of the diluted bleach solution. The analysis with the second branch of bleach sample. About 0.3 g of copper (II) unknown was weighed into a 250ml conical flask. The solid was dissolved with 35ml of 0.05 M sulphuric acid. 1.0 g of potassium iodide was added and it was swirled. Then, the mixture was titrated with the standardized sodium thiosulphate solution until the amber color of the triiodide begins to fade. At this point 2 ml of starch solution was added. the titration was continued until the grey starchiodine color disappears to produce a milky white-pinkish color that marks the end point. The titration was repeated with the two more samples. The percentage by mass of copper was calculated. The experiment is very important because it gives the idea of oxidation and reduction at the same time it views a clear about iodometric reaction. From the calculation, the percentage of the NaClO in Depex is 6.8% while in Clorox is 6.5 %. The percentage of copper in an unknown copper sample is 58.7%. this percentage shows that the unknown copper (II) is actually copper (II) chloride dehydrate.
Introduction Household bleach contains an oxidizing agent known as sodium hypochlorite, NaOCl (an alternative formula is NaClO). NaOCl is prepared by bubbling chlorine gas into a sodium hydroxide solution. Part of the chlorine is ox idized to the hypochlorite ion, OCl- , and some is reduced to the chloride ion, Cl- . The excess hydroxide keeps the resulting solution strongly basic. An equation describing this process is: -
-
-
Cl2(g) + 2 OH • ClO + Cl + H2O The ability of bleach to serve as an oxidizing agent is reported in terms of “available chlorine.” Chemically, this is incorrect because the true oxidizing agent in chlorine bleach is the ClO anion, not Cl2(aq). Because both Cl2(aq) and ClO react with a similar stoichiometry, the term“available chlorine” allows the hypochlorite concentration to be calculated as if Cl2(aq) wasactually present in the solution. “Chlorine free” bleaches, typically marketed as safe for use with colors generally contain other oxidizing agents such as sodium perborate (NaBO3·2 H2O or, alternatively,NaBO2·H2O2·H2O) which produce hydrogen peroxide, H2O2, when dissolved in water. These products are commonly referred to as oxygen bleaches. Clorox 2® is an example of an oxygen bleach. The mechanism of action of both chlorine and oxygen bleaches is a combination of chlorination and oxidation reactions. Since the active ingredients are oxidizing agents, they are reduced in the reactions which means they accept electrons from the compounds undergoing oxidation. The chromophore or “color center” in most organic dyes or stains involves multiple bonds such as C=C or N=N groups. These dyes or stains absorb light in a visible region of the electromagnetic spectrum and reflect other visible colors. A blue dye absorbs red and thereforeblue is reflected to our eyes. The purpose of using a bleach is to reduce these chromophores to single bonds through either oxidation or chlorination. If the stain or dye loses its ability to absorb a particular wavelength, then all wavel engths are reflected which gives rise to white as the observed color. When this occurs, light will be reflected by the material rather than absorbed and the material will seem to be both whiter and brighter.
One commonly used method for determining oxidizing agents in solution is termed an “iodometric titration”. Iodide ion is a strong enough a reducing agent that many oxidizing agents can react completely with the iodide ion resulting in many useful iodometric processes. The usual procedure involves the addition of an excess of iodide ion to the oxidizing agent analyte which produces iodine, which can be titrated with standard sodium thiosulfate solution. The iodine-thiosulfate reaction is quite fast and the equilibrium is far to the product side. In the hypochlorite titration, the following steps occur. Copper (II) unknown analysis
The percent by mass of copper in copper (II) unknown can be determined using similar procedure as for the bleach sample. The copper (II) unknown compound is first dissolved in dilute acid followed by reacting with excess potassium iodide. This reaction produces triiodide according to the following equation : 2+
2 Cu
-
+ 5I
2CuI(s) + I3
-
The triiodide produced in this reaction is titrated with standardized thiosulphate as in the bleach titration : I3
-
+
2-
2S2O3
-
3I3 +
S4O6
2-
And the overall reaction is : 2+
2 Cu
+ 2I- +
2S2O3
2-
2CuI(s)
+
Mole ratio of copper (II) to thiosulphate is 1:1
S4O6
2-
Procedure A. Standardisation of the Sodium Thiosulphate Solution
The burette was filled with fresh thiosulphate solution. About 2 g of solid KI was weighed into a 250ml conical flask. 50.0 ml of 0.010 M KIO3 solution was pipette into the same flask and then 10 ml of 1.0M H2SO4 was added. the flask was placed in the laboratory fume hood while adding sulphuric acid to it. The colour was changed to deep brown colour. The reaction that occurs in the flask is : -
+
IO3 (aq) + 2 I (aq) + H (aq)
-
I2 (or I3 ) + H2O(aq)
(not balanced)
The solution was immediately titrated in the flask with the thiosulphate solution until the colour of the solution is light yellow. At this point, 2 ml of starch indicator was added using a cylinder. The solution turn blue-black once the starch is added. the titration was continued after the addition of starch. The titration was stopped until its turn to colorless. The volume of titrant used was recorded. The reaction that occurs in the flask between thiosulphate and triiodide to produce iodide and tetrathionate is : 2-
-
S2O3 (aq) + I2(aq) (or I3 )
-
2-
I (aq) + S2O6 (aq)
(not balanced)
The above method was repeated to two more samples. B. Analysis of Bleach
A clean and dried 100ml volumetric flask was obtained. The weight of empty 100ml volumetric flask was recorded. 10.0ml of the first bleach sample was pipette into the preweighed 100ml volumetric above. The flask was reweighed and recorded. This will give the density of the bleach solution. The sample was diluted to the 100ml mark with distilled water. Then, it was mixed well. The pipette was rinsed with distilled water followed with a little of the diluted bleach solution and a fresh 10.0ml aliquot was pipette into a 250ml conical flask. Next, 1.0 g of potassium iodide was added and it was swirled . then, 5.0ml of 6 M HCl was added to the mixture. The sample was titrated with the standardized sodium thiosulphate solution until the amber iodine colour begins to fade to light yellow. 2 ml of the starch solution was added and the titration was continued until the dark colour of the starch-iodine complex just disappears. The titration was repeated with two more 10.0ml aliquot of the diluted bleach solution. The analysis with the second branch of bleach sample.
C. Analysis ofcopper (II) unknown
About 0.3 g of copper (II) unknown was weighed into a 250ml conical flask. The solid was dissolved with 35ml of 0.05 M sulphuric acid. 1.0 g of potassium iodide was added and it was swirled. Then, the mixture was titrated with the standardized sodium thiosulphate solution until the amber color of the triiodide begins to fade. At this point 2 ml of starch solution was added. the titration was continued until the grey starch-iodine color disappears to produce a milky white-pinkish color that marks the end point. The titration was repeated with the two more samples. The percentage by mass of copper was calculated.
Results A. Standardisation of the Sodium thiosulphate solution Titration
1
2
3
Final volume of thiosulphate solution (ml)
28.00
27.60
27.90
Initial volume of thiosulphate solution (ml)
0.00
0.00
0.00
Volume of thiosulphate solution(ml)
28.00
28.00
28.00
B. Analysis of Bleach Brand of bleach I 1. 2. 3. 4. 5.
=
DEPEX
Molarity of Na2S2O3 solution Mass of volumetric flask + 10ml bleach Mass of volumetric flask Mass 10ml bleach Density of bleach
= = = = =
0.11 M 75.5642 g 64.6280 g 10.9362 g 10.9362/ 10 = 1.09362 g/ml
Titration
1
2
3
Final volume of thiosulphate solution(ml)
7.90
7.70
7.60
Initial volumeof thiosulphate solution(ml)
0.00
0.00
0.00
Volume of thiosulphate solution(ml)
7.90
7.70
7.60
Brand of bleach II 1. 2. 3. 4. 5.
=
CLOROX
Molarity of Na2S2O3 solution Mass of volumetric flask + 10ml bleach Mass of volumetric flask Mass 10ml bleach Density of bleach
= = = = =
0.11 M 72.6107 g 61.1781 g 11.4326 g 11.4326 / 10 = 1.14326 g/ml
Titration
1
2
3
Final volume of thiosulphate solution(ml)
7.00
14.10
21.20
Initial volumeof thiosulphate solution(ml)
0.00
7.00
14.10
Volume of thiosulphate solution(ml)
7.00
7.10
7.20
C. Analysis of copper(II) unknown Molarity of Na2S2O3 solution
=
0.1816 M
Titration
1
2
3
Mass of copper (II) compound taken(g)
0.3007
0.3005
0.3011
Final volume of Na2S2O3 solution (ml)
13.40
25.40
37.20
Initial volume of Na2S2O3 solution (ml)
2.00
14.00
26.00
Volume of Na2S2O3 solution (ml) used
11.40
11.40
11.20
Discussion In this experiment the amount of hypochlorite ion present in a solution of bleach is determined by an oxidation-reduction titration, the iodine-thiosulfate titration procedure. In acid solution, hypochlorite ions oxidize iodide ions to form iodine, I2. The iodine that forms is then titrated with a standard solution of sodium thiosulfate. The analysis takes place in a series of steps: (1) Acidified iodide ion is added to hypochlorite ion solution, and the iodide is oxidized to iodine. +
-
-
-
2 H (aq) + ClO (aq) + 2 I (aq)
Cl (aq) + I2(aq) + H2O(l)
(2) Iodine is only slightly soluble in water. It dissolves very well in an aqueous solution of iodide ion, in which it forms a complex ion called the triiodide ion. The triiodide ion is yellow in dilute solution, and dark red-brown when concentrated. -
-
I2(aq) + I (aq)
I3 (aq)
(3) The triiodide is titrated with a standard solution of thiosulfate ions, which reduces the iodine back to iodide ions: -
2-
-
I3 (aq) + 2 S2O3 (aq)
2-
3 I (aq) + S4O6 (aq)
During this last reaction the red-brown color of the triiodide ion fades to yellow and then to the clear color of the iodide ion. It is possible to use the disappearance of the color of the I 3 ion as the method of determining the end point, but this is not a very sensitive procedure. Addition of starch to a solution that contains iodine or triiodide ion forms a reversible blue complex. The disappearance of this blue colored complex is a much more sensitive method of determining the end point. The quantity of thiosulfate used in step (3) is directly related to the amount of hypochlorite initially present. To know the copper (II) unknown is, we calculate first the average molarity of thiosulphate which is 5.667 M. from this equation : -
H2O + ClO + 2S2O3
2-
2-
-
-
S4O6 + Cl + 2OH
-
We can get the number of moles of ClO . After this, we can calculate the mass of hypochlorite in an unknown copper(II). From the mass we will divide with the mass used for this experiment and multiply by 100 to get the percentage. The percentage get is 58.7 % copper salts. To know the unknown copper (II), we compare with the percentage of copper (II) given. From
this calculation, the nearest percentage in this experiment is suit with copper (II) chloride dehydrate. Actually, the percentage we get have biggest difference with the given because all the sources of errors happen while handle this experiment. The first error is while we pipette the bleach to 10mL, we not get the accurate volume needed because of reading of the meniscus for pipette. Beside, for the repeated sample we do not remember to add starch to know the end point but we just let the color of iodine changes to colorless. The mass of sample weight also not accurate because we do not follow the method to clean and dry the volumetric flask before weight it. The 3 replicate sample of copper (II) unknown is not precise with each other because it left in the weighing boat. All the error we done effect the value/ data we get is not accurate. For the next experiment, we must beware of the weight getting to avoid too much differences from the theoretical value. Read and understand well the needed of the experiment’s procedure first before handle this. Make sure used the correct weight/volume of sample needed to avoid wasting of chemicals. Waste all the chemicals used in the safe place provided in the lab. Conclusion The experiment is very important because it gives the idea of oxidation and reduction at the same time it views a clear about iodometric reaction. From the calculation, the percentage of the NaClO in Depex is 6.8% while in Clorox is 6.5 %. The percentage of copper in an unknown copper sample is 58.7%. this percentage shows that the unknown copper (II) is actually copper (II) chloride dehydrate. References 1. http://www.jesuitnola.org/upload/clark/aplabs.htm#Analysis_of_a_Commercial_Bleach_ 2. http://kinardf.people.cofc.edu/153L_HONS_HonorsChemistryLaboratory/Exp4_Chlorine BleachTitration.pdf 3. http://chem.lapeer.org/Chem2Docs/APChem2Manual.html#bleach
