In a volumetric precipitation method, ferric alum TS is used as an indicator if the titrant is which?

In a volumetric precipitation method, ferric alum TS is used as an indicator if the titrant is which?

• A. AgNO3
• B. BaCl2
• C. NH4Cl
• D. NH4SCN

Answer: (D)

In this volumetric precipitation setup, the endpoint is detected by the color change produced when iron(III) from ferric alum reacts with thiocyanate to form a red ferric thiocyanate complex (FeSCN2+). That means the titrant must supply thiocyanate ions. Ammonium thiocyanate provides SCN−, so when it’s used as the titrant, the Fe3+ in the ferric alum indicator reacts to give the visible red color at the endpoint. The other options don’t supply thiocyanate (or aren’t the intended titrant for this indicator system), so they wouldn’t produce the characteristic FeSCN2+ coloration needed to signal the end point.

In this volumetric precipitation setup, the endpoint is detected by the color change produced when iron(III) from ferric alum reacts with thiocyanate to form a red ferric thiocyanate complex (FeSCN2+). That means the titrant must supply thiocyanate ions. Ammonium thiocyanate provides SCN−, so when it’s used as the titrant, the Fe3+ in the ferric alum indicator reacts to give the visible red color at the endpoint. The other options don’t supply thiocyanate (or aren’t the intended titrant for this indicator system), so they wouldn’t produce the characteristic FeSCN2+ coloration needed to signal the end point.