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+.
In
TodayÕs Experiment