Question

Sodium thiosulfate is prepared by:
(A) Reducing ${\text{N}}{{\text{a}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$ solution with ${{\text{H}}_{\text{2}}}{\text{S}}$.
(B) Boiling ${\text{N}}{{\text{a}}_{\text{2}}}{\text{S}}{{\text{O}}_3}$ solution with ${\text{S}}$ in alkaline medium.
(C) Neutralizing ${{\text{H}}_{\text{2}}}{{\text{S}}_2}{{\text{O}}_{\text{3}}}$ solution with
${\text{NaOH}}$.
(D) Boiling ${\text{N}}{{\text{a}}_{\text{2}}}{\text{S}}{{\text{O}}_3}$ solution with ${\text{S}}$ in acidic medium.

Answer 1

The chemical formula for sodium thiosulfate is ${\text{N}}{{\text{a}}_{\text{2}}}{{\text{S}}_{\text{2}}}{{\text{O}}_{\text{3}}}$. The preparation of sodium thiosulfate is done using sodium sulphite.

Step by step answer:
Sodium thiosulfate $\left( {{\text{N}}{{\text{a}}_{\text{2}}}{{\text{S}}_{\text{2}}}{{\text{O}}_{\text{3}}}} \right)$ is prepared by boiling ${\text{N}}{{\text{a}}_{\text{2}}}{\text{S}}{{\text{O}}_3}$solution with ${\text{S}}$ in alkaline medium.
The reaction is as follows:
${\text{N}}{{\text{a}}_{\text{2}}}{\text{S}}{{\text{O}}_3}{\text{ + S }}\xrightarrow[{{\text{Boil}}}]{{{\text{O}}{{\text{H}}^ - }}}{\text{ N}}{{\text{a}}_{\text{2}}}{{\text{S}}_2}{{\text{O}}_3}$
Thus, the correct option is (B) boiling ${\text{N}}{{\text{a}}_{\text{2}}}{\text{S}}{{\text{O}}_3}$ solution with ${\text{S}}$ in alkaline medium.

Additional Information: Sodium thiosulfate is generally found in its pentahydrate form. Thus, the formula for sodium thiosulfate is ${\text{N}}{{\text{a}}_{\text{2}}}{{\text{S}}_{\text{2}}}{{\text{O}}_{\text{3}}} \cdot {\text{5}}{{\text{H}}_2}{\text{O}}$. Sodium thiosulfate is also known as sodium hyposulfite.
Sodium thiosulfate is a crystalline substance and is soluble in water. It is white or colourless.
Sodium thiosulfate is used as a reducing agent. It is used in an indirect process where iodine is used as an intermediate to determine the oxidizing agents. Starch decomposes in solutions that have high amounts of iodine.
Sodium sulphate is used in swimming pools to neutralize the effect of chlorine present in water. It is used as an antidote to cyanide poisoning.
Sodium thiosulfate is used to reduce iodine to iodide before iodine forms a complex with starch and forms a blue-black coloured solution.
Sodium thiosulfate is used to control the reductive-oxidative property of the surface of sulphides during the flotation process.

Note: Another method of preparing sodium thiosulfate is as follows:
Sodium thiosulfate can be prepared by boiling sodium hydroxide $\left( {{\text{NaOH}}} \right)$ with sulphur. The reaction is as follows:
${\text{6NaOH + 4S }} \to {\text{ N}}{{\text{a}}_2}{{\text{S}}_2}{{\text{O}}_3}{\text{ + 2N}}{{\text{a}}_2}{\text{S + }}{{\text{H}}_2}{\text{O}}$.
Sodium thiosulfate is an inorganic salt. It contains sodium and thiosulfate ions in the ratio ${\text{2:1}}$.