The method is applicable to products corresponding to the general formula M2O . xSiO2, where the values of x lie between 1,5 and 3,5 in the case of sodium silicates and 1,9 and 3,9 in the case of potassium silicates. Because of the difficulty ancountered in dissolving fine particles of these products, the sample is first sieved to remove these fines. The procedure consists in dissolution of the test portion in carbon dioxide-free water, by treatment in an autoclave. Filtration and weighing of the insoluble matter on a tared filter crucible. Dilution of the filtrate to a given volume, from which aliquot samples are taken in order to carry out the different determinations.