5.2 Gravimetric analysis1

PRECIPITATION METHOD In precipitation methods, the analyte is converted to a sparingly soluble precipitate that is filtered, wash free of impurities, and convert to a product of known composition by suitable heat treatment.

This product is then weighed.

Examples : 1. Determination of the amount of calcium in water or milk.

2. Determination of Fe content in Fe ore.

The process of determination of Ca in water: 1.

An excess amount of oxalic acid, H2C2O4 is added to a carefully measured volume of water sample.

2.

The addition of ammonia then causes essentially all the calcium in the sample to precipitate as calcium oxalate.

The reaction is: Ca2+ (aq) + C2O42- (aq)

CaC2O4 (s)

3.

The precipitate is collected in a weighed filtering crucible, dried, and then ignited at red heat. This process converts the precipitate entirely to calcium oxide.

CaC2O4 (s) +

4.

5.

Ca O (s)

CO (g) + CO2 (g)

The crucible and precipitate are cooled, weighed, and the weight of calcium oxide is determined by subtraction of the known weight of the crucible.

The calcium content is then calculated as in form of CaO.

VOLATILIZATION METHOD In volatilization methods, the analyte or its decomposition products are volatilized at a suitable temperature. The volatile product is then collected and weighed or alternatively the weight of the product is determined indirectly from the weight loss of the sample.

Examples : Determination of sodium hydrogen carbonate content of antacid tablets.

Process : 1. Weighed quantity of the finely ground sample is treated with dilute sulphuric acid to convert the sodium hydrogen carbonate to carbon dioxide.

NaHCO3 (aq) + H2SO4 (aq)

CO2 (g) +H2O (l) + NaHSO4 (aq)

2.This reaction is carried out in a flask that is connected to a weighed absorption tube that contain an absorbent that retains the carbon dioxide selectively as it is evolved from the solution by heating.

3.The difference in weight of the tube before and after absorption is used to calculate the amount of sodium hydrogen carbonate.