How do you explain why the addition of silver nitrate to the Fe/SCN equilibrium had an effect on the equilibrium even though neither silver ion nor nitrate ion is written as part of the equilibrium reaction?

Question

anonymous · Answer

ABOUT VOLHARDT 
This german chemist introduced the employment of ferric chloride as Indicator of silver ions dosed in the analyte. 
Ferric ions are in numerical defect respect to silver ones. 
While you dose the titrimetric reactant of potassium thiocyanate (e.g. KCNS) this interacts mainly with silver ions 

Ag+(aq) + KCNS(aq) ---> AgCNS(s) + K+(aq) 

forming a white-creamish bodies at the vessel-bottom. 
At the equivalence point, silver ions disappear and thiocyanate begins its reaction with ferric ones 

Fe+++(aq) + KNCS(aq) <---> FeCNS++(aq)+ K+(aq) 

so it appears a bloody coloration. 

YOUR CASE 
Since I wrote about a chemical equilibrium interesting ferric and thiocyanate ions, you may understand that a bloody solutions of FeCNS++(aq) may decolors for addictions of silver nitrate 

FeCNS++(aq) + Ag+(aq) ---> AgCNS(s) + Fe+++(aq) 

because the unsolubility of silver thiocyanate displaces the chemical equilibrium of ferric thiocyanate toward the 
white-creamish compound : this is a well-known application of Le-Chatelier fundament, "....chemical equilibrium evolves in a manner to opposite itself to external forcing...".