What is the reaction to increase the intensity of red colour by adding `HgCl_2` positive in equilibrium of red colour by adding `HgCl_2` positive in equilibrium of
`Fe_((aq))^(3+) + SCN_((aq))^(-) hArr [Fe(SCN)]_((aq))^(2+)`

Explanation

`HgCl_2` is added in solution So, `Hg^(2+)` ions are increases and combine with `SCN^-` and form `(Hg(SCN)_4)^(2-)`. `[SCN]^(2-)` decreases. To minimize this effect `[Fe(SCN)]^(2-)` form by forward reaction. The solution becomes more red.