Gravimetric analysis

Gravimetric analysis is a technique to determine the amount of an analyte based on mass. A common example of a gravimetric analysis is to determine the amount of chloride in a solution - for example, to determine the amount of salt in seawater, or to determine the mass of the counter-ion of an unknown chloride compound. But the analyte could also be another ion.

The analyte is reacted with a counter-ion with which it forms an insoluble compound, which can then be isolated and weighed. From the mass of the precipitate the amount of the analyte can be calculated. If the analyte is the chloride ion, then the counter-ion could be silver, Ag+, because AgCl is insoluble in water.

In order for the gravimetric analysis to be valid, we want all of the chloride ions to precipitate. This we obtain by letting chloride be the limiting reagent and the silver ions being in excess. In other words: There should be more silver ions than chloride ions. It is equally important that silver ion is introduced via a compound that is highly soluble in water so that the only precipitate is the silver chloride. The typical choice would be silver nitrate, AgNO3:

NaCl (aq) + AgNO3 (aq) --> Na+ (aq) + NO3- (aq) + AgCl (s)