Question

We did a lab on equilibrium and this is one of the following questions:

When Ag ions are added to FeSCN ions (which are red in colour), the colour disappears, and a white precipitate of AgSCN forms.

The question is.. Explain why Fe can be used as an indicator in a reaction to determine the concentration of an unknown solution of Ag by reaction it with a solution of KSCN of known concentration.

Answer 1

I don't see this as a problem. The whole idea is that Ag ion reacts and ppts with SCN ion to form AgSCN (due to the extreme insolubility of AgSCN Ksp = 1+ x 10^-12). The next drop of SCN ion, after all of the Ag ion has reacted, is free to form the complex with Fe(III) of FeSCN^+2. You have no FeSCN^+2 ions at the beginning of the titration, only at the end of the titration. Until the end point is reached, Ksp is so small for AgSCN that FeSCN^+2 doesn't form until all of the Ag ion is gone.