Page 47 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102

Chloroform is used to dissolve tribromophenol and to act as
indicator

Br2 + 2I– → I2 + 2Br–
I2 + 2S2O32– → S4O62– + 2I–

Properties and Applications of Volumetric Reducing Agents
Sodium thiosulfate, Na2S2O3

Properties

Moderately strong reducing agent

Standard solution and stability

Standard solution is prepared by dissolving Na2S2O3 in freshly
boiled water (WHY).

Factors affect stability

1. Acidity causes disproportionation)

S2O32− + H+ HSO3− + S(s)

HSO3− consumes twice quantity of I2 consumed by S2O32−

HSO3− + I2 + H2O HSO4− + 2H+ + 2I−

(2S2O32– + I2 S4O62– + 2I–)

So, to prevent the formation of HSO3-, st solution is prepared in
freshly boiled water to get rid of dissolved CO2 which makes the
solution acidic. Also pH is adjusted for optimum stability (9-10) by
adding NaHCO3 or borax.

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