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Redox Titration LAB

Abstract:

This experiment determined the percent of hypochlorite ion present in a solution of bleach. This was calculated through iodine-thiosulfate titration in which I- oxidized and allowed an observable equivalence point with thiosulfate. The percent of NaClO by mass in commercial bleach was determined to be 5.15%, with an error of 3.04% from the accepted value (5.00% by mass).

Introduction:

In this experiment the percent of sodium hypochlorite in commercial bleach by mass was determined.

In the oxidation reaction, I- ions are oxidized by the hypochlorite ions into I2. In water, iodine forms a complex triiodide ion. It then is titrated with a known concentration of sodium thiosulfate. The molar amount of the I3- is calculated from the sodium thiosulfate. From the I3-, the amount of hypochlorite ions is determined. The exact equivalence point will be visible with the help of starch as an indicator, which will turn the redish-brown triiodide blue and then clear at the equivalence point with thiosulfate. Finally, having the molar amount, and given ah a known molar mass of hypochlorite, the mass percent of ions in the bleach can be determined.

The process can be broken up into three reactions from which stoichiometric calculations allowed a determination of each molar amount. Although the procedure directly measures the amount of thiosulfate in the solution, stoichiometric ratios allow calculating up the ladder back to the amount of hypochlorite in the original bleach.

2 H+(aq) + ClO-(aq) + 2 I-(aq) Cl-(aq) + I2(aq) + H2O(l) 
 (1)

I2(aq) + I-(aq) I3-(aq) 
 (2)

I3-(aq) + 2 S2O32-(aq) 3 I-(aq) + S4O62-(aq) (3)

Based on these reactions, it is expected that more titrant during the procedure will mean more hypochlorite in bleach. Potential errors may include over titrating and missing the exact equivalence point. It will be important to increase by small increments of thiosulfate when getting close to the equivalence point.

Data:

Table 1. Thiosulfate Titration Data

	Titration1	Titration 2	Titration 3
Volume S2O32- (ml)	18.60	18.70	18.76

Results: Thiosulfate concentration: 0.1M Accepted hypochlorite mass: 5.00% Hypochlorite molar mass: 74.442 g/mol

Thiosulfate (mol)	0.00187
Hypochlorite (mol)	0.000935
Dilute [ClO]	0.0374
Actual [ClO]	0.748
NaOCl (g/L)	55.682616
% by mass	5.16%
Error %	2.60

Table 2. Titration 1 Results Table 3. Titration 2 Results Table 4. Titration 3 Results

Thiosulfate (mol)	0.00187
Hypochlorite (mol)	0.000935
Dilute [ClO]	0.0374
Actual [ClO]	0.748
NaOCl (g/L)	55.682
% by mass	5.16%
Error %	3.20

Thiosulfate (mol)	0.001876
Hypochlorite (mol)	0.000938
Dilute [ClO]	0.03752
Actual [ClO]	0.7504
NaOCl (g/L)	55.861
% by mass	5.17%
Error %	3.40

Based on the titrations and calculated concentrations, the average concentration of hypochlorite in commercial bleach was 5.15% by mass which was a percent error of 3.04% from the accepted 5.00% value.

Discussion:

As with many titrations, a valid source of error was judging the precise equivalence point. Although in this experiment the equivalence point was quite visible with a distinct color change from blue to clear, it still lacks the precision that would guarantee 100% molar equivalence. In turn, the equivalence point may have been noted too late which would result in an increased calculated hypochlorite concentration.