Question

The following $\text{N}{{\text{a}}_{\text{2}}}{{\text{S}}_{\text{2}}}{{\text{O}}_{\text{3}}}$ is used in photography because of its:
(A) Reducing behaviour
(B) Oxidising behaviour
(C) Complex forming behaviour
(D) Reaction with light

Answer 1

Hint: $\text{N}{{\text{a}}_{\text{2}}}{{\text{S}}_{\text{2}}}{{\text{O}}_{\text{3}}}$ Or sodium thiosulphate which is commonly known as hypo is used in the fixing of image. It dissolves unaffected $\text{AgBr}$ but leaves metallic silver unchanged.
A photography film consists of a light sensitive emulsion of fine particles of silver salts in gelatine spread on a clear cellulose strip or a glass plate. The grain size is very important to photographers, this affects the quality of picture produced. $\text{AgBr}$ Compound is mainly used as the light sensitive material in photography.

Complete step by step solution:
-The solubility of $\text{AgBr}$ (silver bromide) in hypo solution due to the formation of a colourless complex $\text{N}{{\text{a}}_{\text{3}}}\text{Ag(}{{\text{S}}_{\text{2}}}{{\text{O}}_{\text{3}}}{{\text{)}}_{\text{2}}}$, which is represented by following reaction – $\text{AgBr}\,\text{+2N}{{\text{a}}_{\text{2}}}{{\text{S}}_{\text{2}}}{{\text{O}}_{\text{3}}}\,\to \,\text{N}{{\text{a}}_{\text{3}}}\text{Ag(}{{\text{S}}_{\text{2}}}{{\text{O}}_{\text{3}}}{{\text{)}}_{\text{2}}}\text{+}\,\text{2}\,\text{NaBr}$
-In day photography if the film was brought out in latent image stage, the unexposed part of the emulation would turn black and thus destroy the picture. To prevent the happening any unchanged silver halides are removed by placing the film in a fixer solution. A solution of sodium thiosulphate is used as a fixer. It forms a soluble halide complex with silver halide. After fixing the film, the film can be safely brought out into daylight.
-The presence of excess sulphur makes sodium thiosulphate a useful reducing agent (the substance which donates electrons in a chemical reaction).
$\text{2N}{{\text{a}}_{\text{2}}}{{\text{S}}_{\text{2}}}{{\text{O}}_{\text{3}}}\,+{{\text{I}}_{\text{2}}}\,\,\to \,\text{N}{{\text{a}}_{\text{2}}}{{\text{S}}_{4}}{{\text{O}}_{6}}\,+\,2\text{NaI}$
This reaction is applied in volumetric estimation of iodine. In this reaction sodium thiosulphate acts as a reducing agent.

So the answer to this question is option (A).

Note: Electron donors are reducing agents. Ammonium thiosulphate is mainly preferred in respect to sodium thiosulphate for fixing the photographic films.
Sodium thiosulphate is also used as an antichlor to remove the excess chlorine from bleached fabric.