Question

Why is commercial hydrochloric acid often in yellow color?

Answer 1

The reason for the yellow colour of commercial hydrochloric acid is due to the presence of ${\mathbf{FeCl}}_{\mathbf{3}}$ (ferric chloride).

$$\begin{gathered} 1{\mathrm{FeCl}}_3.6{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)+6{\mathrm{SOCl}}_2\left(\mathrm{l}\right)\to {\mathrm{FeCl}}_3\left(\mathrm{s}\right)+12\mathrm{HCl}\left(\mathrm{g}\right)+6{\mathrm{SO}}_2\left(\mathrm{g}\right) \\ (\mathrm{Ferric}(\mathrm{Thionyl}(\mathrm{Ferric}(\mathrm{Hydrochloric}\left(\mathrm{sulphurdioxide}\right) \\ \mathrm{chloride})\mathrm{chloride})\mathrm{chloride})\mathrm{acid}) \end{gathered}$$

It contains impurities of iron that make it look yellowish and after dilution, it will still look pale yellow. That's why, the pure acid solution is colourless but commercial acid is yellow in colour.

Hence, commercial hydrochloric acid is yellow in colour.