Titration

Redox Titration
Analysis of a Commercial Bleach
A. Purpose
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To review oxidation-reduction reactions and their stoichiometry.
To learn the concept and technique of redox titration.
To determine the percent (m/v) of an active ingredient, sodium hypochlorite
(NaOCl), in a commercial bleaching agent.

B. Theoretical Background
Whereas acid-base reactions involve the transfer of a proton, oxidation-reduction or redox reactions involve the transfer of electrons from one substance to another, resulting in changes in oxidation numbers of two or more elements in the chemical reaction. In a redox reaction the substance that donates (loses) electrons experiences an increase in oxidation number and is said to be oxidized. Such process is oxidation.
Conversely, the substance that accepts (gains) electrons experiences a decrease in oxidation number and is said to be reduced. Such process is reduction.
The oxidized substance is causing reduction and therefore called a reducing agent. The reduced substance is causing oxidation and therefore called an oxidizing agent. In redox reactions the total number of electrons donated must equal the total number of electrons gained.
In today’s experiment the mass/volume percent of the active ingredient in bleach, sodium hypochlorite (NaOCl), is determined by redox titration. The method used is iodine-thiosulfate titration. It is a very useful method, since the iodide ion, I-, is easily oxidized by almost any oxidizing agent. The analysis takes place in a series of steps as follows: 1. A diluted sample of the bleach will be allowed to react with potassium iodide in acidic solution. The iodide ion will be oxidized to iodine while the hypochlorite ion will be reduced to chloride (Equation 1).
2 H+(aq) + OCl-(aq) + 2 I-(aq) ! Cl-(aq) + I2(aq) + H2O(l)

(1)

Iodine is not very soluble in water. The reaction shown in Equation 1 is carried out with a large excess of potassium iodide so that the iodine formed is converted to the triiodide ion (Equation 2). The triiodide ion is red-brown in color.
I2(aq) + I-(aq) ! I3-(aq)
(2)
Equations 1 and 2 can be added up to get Equation 3.
2 H+(aq) + OCl-(aq) + 3 I-(aq) ! Cl-(aq) + I3-(aq) + H2O(l)

(3)

2. The triiodide is titrated with a standard solution of sodium thiosulfate
(Na2S2O3) which reduces it back to iodide ions.
I3-(aq) + 2 S2O32-(aq) ! 3 I-(aq) + S4O62-(aq)

(4)

In step 2 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 triiodide 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. However, if the starch is added to a solution which contains a great deal of iodine/iodide; the complex which forms may not be reversible. Therefore, the starch is not added until shortly before the end point is reached. The quantity of thiosulfate used in this reaction is directly related to the amount of hypochlorite initially present.
The sodium thiosulfate must be standardized prior to use. This is conveniently accomplished by titrating a known amount of triiodide ion that has been generated by the reaction shown in Equation 5. The potassium iodate used in this reaction is a primary standard. IO3- (aq) + 8 I- (aq) + 6 H+ (aq) ! 3 I3-(aq) + 3 H2O(l)
(5)
C. Procedure
I. Standardization of Thiosulfate Solution:
! Rinse the buret and fill it with the thiosulfate solution provided.
! Weigh approximately 2g of KI.
! Pipet 25.00 mL of the standard KIO3 into a 250-mL Erlenmeyer flask.
! Add ~ 20 mL of distilled water and the weighed KI sample. Swirl to dissolve.
! Add ~ 10 mL of 0.5 M H2SO4 while swirling the flask.
! Titrate immediately with the thiosulfate solution. Continue titration till the redbrown color changes to light yellow.
! Add ~ 2 mL of starch solution and continue the titration slowly until the blue color completely disappears.
! Repeat the titration two additional times.
II. Titration of Unknown Bleach Solution:
Proceed as in part I above (Standardization of Thiosulfate Solution), using 10.00 mL of a diluted bleach solution instead of the 25.00 mL standard KIO3 solution.