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Gravimetric analysis : As the name suggests, gravimetric analysis involves very accurate weighing of a chemical as a means of working out something about that chemical. A common type of gravimetric analysis involves precipitation of a single component (usually an ion) from a known amount of a compound or mixture. This is accomplished by reacting a solution of the test compound with a solution containing an ion that forms an insoluble salt with the component you wish to test for.
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In this experiment the sample containing an alkali sulfate is dried, weighed and dissolved in dilute HCl.The precipitation was occur. Barium chloride solution is added in excess to precipitate barium sulfate, and the precipitate is digested in the hot solution. After the precipitate is filtered through a paper filter which is then ignited and completely ashed. From the weight of the sample and weight of the precipitate, the percentage of sulfate in the sample is calculated. The precipitation reaction is the following in below:

Ba2+ (aq) + SO42- (aq) —> BaSO4 (s)

Variations in the acidity, temperature, manner of addition of the precipitant and time of digestion markedly affect the filterability of the barium sulfate precipitate and the extent to which various foreign ions are coprecipitated. Foreign anions such as nitrate, chlorate and chloride are coprecipitated as their barium salts, and the ignited precipitate contains the salt or oxide as an additive impurity. The coprecipitation of chloride can be decreased by slow addition of the precipitant. Since nitrate and chlorate interfere even at low concentrations, they should be removed from the solution before precipitation. Foreign cations such as ferric iron, calcium and, to a lesser extent, the alkali metals are coprecipitated as the sulfates. These are substitutional impurities, and the magnitude of the error depends upon the differences between the weight of the foreign sulfate or oxide and the weight of an equivalent amount of barium sulfate. The presence of ferric iron can produce errors as high as 2% in the determination.