Chemistry - Volumetric analysis, redox titrations - activity 9
Volumetric analysis, redox titrations - activity 9
The concentration of the active ingredient sodium hypochlorite NaOCl in a
bleach may be determined by means of a redox titration. Firstly, excess
potassium iodide, KI, solution is added to a diluted sample of the bleach
being analysed. This brings about the redox reaction:
The iodine liberated from this reaction is then titrated with a
standardised solution of sodium thiosulfate, Na2S2O3, according to:
The endpoint of the titration is indicated by the use of starch solution
which produces an intense blue/black colour in the presence of molecular
In a particular analysis of the amount of NaOCl present in a commercial
bleach, a 20.0 aliquot of the bleach was diluted to 250 mL with water in a
volumetric flask. Three 20.0 mL aliquots of the diluted bleach solution
were taken and analysed according the procedure described above. 0.0987 M
Na2S2O3(aq) was used in the titration.
The average titre required to reach the endpoint of the reaction was 21.8
(a) What is the oxidation state of
(i) Chlorine in ClO, (ii) sulfur in S2O3(aq).
(b) Calculate the_ n_(S2O3) required to reach the endpoint.
(c) Calculate the _n_(I2) produced during the reduction of ClO(aq) to
(d) Calculate the _n_(ClO) in the original 20.0 mL aliquot of bleach.
(e) Determine the concentration of the bleach solution with respect to
NaOCl in g L.
(f) State the effect (higher, lower or no change) that each of the two
procedures listed below would have on the calculated concentration of
sodium hypochlorite if they were performed during the volumetric analysis
Briefly explain why.
(i) Rinsing the conical flasks with distilled water, prior to the addition
of the diluted bleach.
(ii)Rinsing the pipette used to transfer the original aliquot of bleach
with distilled water, prior to the aliquot.
(iii) Rinsing the burette with water prior to filling it with
The calculations for parts (b) to (e) of this question can also be
summarised via the flowchart shown.
Answer - part 1
Answer - part 2
Answer - part 3
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