about iron (III) complex

protons from acid can protonate the oxalate ligands, yielding free (or aqua-) iron(III) ions, where the later reacts with thiocyanate to give blood red complex.


alternatively, consider the reversible reaction:
1... [Fe(C2O4)3]^(3-) <======> Fe^(3+) + 3C2O4^(2-) [aqua-ligands omitted]
2... C2O4^(2-) + 2H^(+) <======> H2C2O4

oxalic acid is a weak-to-medium organic acid.
strong acid can protonate the free oxalate ions and shift the equilibrium position of rxn.2 to the RHS.
as concentration of oxalate decreases, eqm. position of rxn.1 shifts to RHS, yielding more free iron(III) ions which reacts with thiocyanate.

without addition of acid, the complex is much more stable than its constituent ions (Kc for rxn.1 is very small); amount of free iron(III) is small too.