p-Block elementsHard

Question

Which of the following statement (s) is/are true for sodium thiosulphate ?

Options

A.it acts as an antichlor
B.it is used as an reducing agent in iodometric titration.
C.it reacts with hydrochloric acid to form SO2 and sulphur.
D.it is used in photography as hypo to dissolves excess of AgBr as soluble complex.

Solution

(A) It removes the chlorine from the surface of fibres (while dyeing) according to following reaction.
        Na2S2O3 + 4Cl2 + 5H2O → 2NaHSO4 + 8HCl
Therefore , it is known as antichlor.
(B) I3- + S2O32- → 3I- + S4O62-
This reaction finds application in the iodometric and iodimetric methods of titrimetric analysis.
(C) S2O32- + H+ → S↑ (white) + SO2 + SO2 + H2O (disproportionation reaction).
(D) 2S2O32- + Ag+ → [Ag(S2O3)2]3- (soluble complex) or [Ag(S2O3)3]5-
This reaction is utilized in photography where hypo is used as fixer.

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