Experiment 8-018: Determination of Hypochlorite in Bleach
Report Sheet for experiment 8-018: Extra report sheet (pdf format).
Dr J's notes: Sample calculations (pdf format).
One
of the active ingredients in household bleaches is sodium
hypochlorite, NaOCl , a strong oxidizing agent also used as a
disinfectant in swimming pools. In this experiment we will use
several oxidation-reduction reactions to analyze for this
powerful household chemical. While this experiment will test for
the active ingredient in these bleaches, one cannot necessarily
equate the amount of oxidant with the effectiveness of the
bleach, as there are several other factors, such as abrasive
polishing agents (pumice), as well as detergents to trap
(emulsify) the dirt. These other components can influence the
effectiveness of a household cleaner, but will not be analyzed
here.
The Chemistry:
The method used to analyze for hypochlorite is indirect. Initially the oxidizing ability of the OCl- is used to oxidize an aqueous solution of I- to I2:
OCl-(aq) + 2 H+(aq) + 2 I-(aq) ---------> I2(aq) + Cl-(aq) + H2O(l) [1]
Since reaction 1 proceeds to completion (100% yield), and since excess H+ and I are added ( OCl- is the limiting reagent) we can assume that the amount of I2 generated is directly related to the amount of OCl- initially present. Since there was no I2 present in the initial bleach solution, the amount of I2 now in solution must have been generated from reaction 1. If the amount of I2 in solution after reaction 1 has occurred can be determined, the amount of OCl- in the initial bleach solution can be determined.
The amount of I2 generated is then determined by a titration with sodium thiosulfate (Na2S2O3):
2 S2O32-(aq) + I2(aq) -------> 2 I-(aq) + S4O62-(aq) [2]
Like reaction 1, reaction 2 is rapid and proceeds with a yield of 100%. The problem is to know when the reaction is done, as the equivalence point (the point at which all of the I2 has reacted with the S2O32-) is not easily detected without additional chemical help. To clearly see the end point, two additional interactions will be used:
a) in aqueous solution, I2 reacts with I- to form the triiodide ion (I3), which is brown:
I2(aq) + I-(aq) -------> I3- (aq) (occurs rapidly and completely) [3]
This means that in the presence of excess I- ion, reaction 2 should be rewritten as:
2 S2O32-(aq) + I3- (aq) -------> 3 I-(aq) + S4O62-(aq) [4]
b) the triiodide ion (I3(aq)) fits nicely into the helically-coiled starch molecule, and forms an intensely blue (starch-triiodide) complex.
I3- (aq) + starch -------> deep-blue complex (also occurs rapidly) [5]
This complex is so intensely colored that iodine concentrations as low as 10-6 M can be detected by the naked eye.
To determine the amount of iodine in a solution, one must make sure the solution contains excess I- (driving reactions 1 and 3 to completion), and then titrate the resulting I2(aq) (or I-(aq)) with S2O32- (reaction 2) until the color of the triiodide complex begins to fade. Once near the end point, a few drops of starch solution are added, and the titration is continued until the intense blue color disappears. Loss of the blue color indicates that all of the I2 has been consumed by the S2O32- and that the equivalence point has been reached.
One potential glitch: if allowed to sit around, the deep blue starch-triiodide compex can get very stable; if it sits around for any length of time, it can get so stable that it will not break up its formation becomes irreversible. Remember, the success of our titration depends on the disappearance of that blue complex, so it is important that the starch not be added until the titration is almost complete, and that as soon the complex is formed, the titration be carried out as quickly as possible.
The Experiment:
This is your last set of titrations for this year (there are more to come in Chem 102 - Spring 2008).
Standardization of sodium thiosulfate
The analysis of OCl- by titration with S2O32- requires that one knows the concentration of the S2O32- solution. So, you must first standardize the sodium thiosulfate solution. A similar series of reactions is used for the standardization, only potassium iodate (KIO3) is used to oxidize iodide to iodine:
IO3- (aq) + 5 I-(aq) + 6 H+(aq) -------> 3 I2 (aq) + 3 H2O(l) [6]
Potassium iodate is a primary standard, so by dissolving a carefully weighed sample of KIO3 in a controlled volume, the concentration of IO3- ion in solution can be accurately known. With IO3- as the limiting reagent (with excess I-), the amount of I2 generated in reaction 6 can be determined. Titration of the I2 with S2O32- (reaction 2) is then used to standardize the S2O32- solution.
Analysis of a bleach sample
You will obtain a bleach sample (of a common brand name bleach), make a standard solution of the bleach (i.e. use volumetric glassware to prepare a bleach solution of known volume). You will then titrate this solution with the sodium thiosulfate you standardized in part 1. You will then use your data to calculate the % NaOCl (w/v) of the bleach brand that you chose.