The iron (III) ion
also introduces a complication because of its
reaction with water to form iron hydroxide, which
is insoluble in water:
Fe3+ (aq) + 3 H2O (l) <==> Fe(OH)3 (s) + 3 H+ (aq)
To avoid
precipitation of iron (III) hydroxide, you
will include excess nitric acid (HNO3) in all
solutions, to shift this equilibrium far
to the left.
Because neither
hydrogen ions nor nitrate ions are components of the
iron (III) thiocyanate equilibrium, nitric
acid does NOT affect the equilibrium position
of
the reaction that produces FeSCN2+.