Why should AgCl be washed with dilute HNO3 in a gravimetric analysis?

Using nitrate acid,an alloy of silver and copper became silver ion and copper(II)ions. By adding NaCl to the the mixture, AgCl formed an insoluble precipitant. My question is why should this precipitant be washed with diluted Nitrate acid? And also would this affect the total mass of AgCl?
frog@early.com

Silver chloride precipitates aren't pure AgCl. They're a clotted mass of little AgCl particles glued together by electrolyte ions. For example, a nitrate ion stuck to one particle can attract Ag+ ions or Cu2+ ions stuck to other particles. If you wash with water, you'll dilute the layers of ions around the particles in the precipitate. The precipitate "peptizes" (falls apart into colloidal particles) and some of it slips right through the filter paper.

If you wash with nitric acid, a high electrolyte concentration is maintained and the AgCl particles stay clotted together. When you dry the precipitate, the acid decomposes and volatilizes, so it shouldn't have a strong effect on the precipitate mass. But the clotted precipitate will always be contaminated with silver nitrate. Silver nitrate has a molecular weight of 169.87, and silver chloride only 143.32, so the mass of the precipitate will be a little high.

Author: Fred Senese senese@antoine.frostburg.edu



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