Question

Sodium thiosulphate is used in photography because of its

• A. Reducing behaviour
• B. oxidizing behaviour
• C. complex forming behaviour
• D. reaction with light

Answer 1

The correct option is C

complex forming behaviour

Explanation for the correct option:

Option (C): Complex forming behavior

• Sodium thiosulfate, often known as sodium sulfate, is a chemical compound with the formula ${\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3$ .
• It is most commonly encountered in its pentahydrate form, which is either white or completely colourless.
• Due to its ability to form complexes, sodium thiosulphate is commonly employed as a fixer in photography.
• It forms a silver thiosulphate complex that is soluble in nature when it combines with undecomposed silver bromide ($\mathrm{AgBr}$).

$\underset{\mathrm{Silver}\mathrm{bromide}}{\mathrm{AgBr}}+\underset{\mathrm{Sodium}\mathrm{thiosulfate}}{2{\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3}\to \underset{\mathrm{Sodium}\mathrm{silver}\mathrm{thiosulfate}\left(\mathrm{soluble}\right)}{{\mathrm{Na}}_3[\mathrm{Ag}{\left({\mathrm{S}}_2{\mathrm{O}}_3\right)}_2}+\underset{\mathrm{Sodium}\mathrm{bromide}}{\mathrm{NaBr}}$

• The complex formed is light sensitive.

Explanation for the incorrect option:

Option (A): Reducing behavior

1. Sodium thiosulfate, often known as a reducing agent, is used in flotation to manage the sulfide mineral's reductive-oxidative characteristics.
2. It's a reasonably powerful reducing agent that's used in an indirect procedure in which iodine serves as an intermediary for determining the oxidizing agents.

Option (B): Oxidizing behavior

1. Sodium thiosulfate is known as a reducing agent.

Option (D): Reaction with light

1. Thiosulfate is a light-sensitive chemical to avoid its dissociation it is kept in dark.

Therefore, option (C) is correct.