Explanation:
In photography \(\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}\) (hypo or sodium thiosulphate) is used as complexing agent. It dissolves unreduced silver bromide by forming a complex.
\[\begin{array}{*{20}{l}}
{{\rm{AgBr}} + 2{\rm{N}}{{\rm{a}}_2}\;{{\rm{S}}_2}{{\rm{O}}_3} \to }\\
{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\rm{Hypo}}}\\
{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\rm{N}}{{\rm{a}}_3}[{\rm{Ag}}{{({{\rm{S}}_2}{{\rm{O}}_3})}_{2]}}] + {\rm{NaBr}}}\\
\begin{array}{l}
\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\rm{Sodium}}\,\\
\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\rm{argentothiosulphate}}
\end{array}\\
{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,({\rm{soluble}}\,\,{\rm{complex}})}
\end{array}\]
\({\rm{Therefore}},{\rm{N}}{{\rm{a}}_2}\;{{\rm{S}}_2}{{\rm{O}}_3}\) is used as a complexing agent.
