Alkalimetry. Commercial potash and soda always contain greater or less quantities of foreign substances, such as sulphate of potash, common salt, silicates, oxide of iron, water, &c. which diminish the percentage of real alkali in a given weight. It is important, then, for the manufacturer to have some simple and ready means of determining the proportion of pure carbonate of potash or soda contained in any sample, that he may be able to judge of its value. Ordinary chemical analysis takes too much time. The alkalimeter serves this purpose. It consists of a graduated glass tube, filled with diluted sulphuric acid, and containing as much absolute sulphuric acid as would neutralise a given weight, say 100 grains, of carbonate of potash. 100 grains of the article to be judged of is then dissolved in water, and as much acid is gradually added to it from the tube as to neutralise the solution, that is, take up all the alkali. The purer the article, the more of the acid will be required; and if the tube, which is divided into 100 degrees, has been emptied to the 80°, the impure article contains 80 per cent. of pure carbonate of potash. The point at which neutralisation is complete is determined by means of coloured tests. Formerly, the two vegetable colours, litmus and turmeric, were alone used for this purpose, addition of an alkali rendering litmus blue and turmeric reddish brown; while under the influence of acids the former changes to a red, the latter to a yellow (red in the case of boracic acid). It is not, however, always easy to recognise the neutral point, and other indicators (as these colouring matters are called) have come into use. The chief of these are methyl-orange and phenolphthalein. A mixture of the alcoholic solutions of these substances imparts a pale yellow colour to strictly neutral liquids, which is changed to deep red by the least trace of alkali, and to pink by a trace of acid.
This method of determining the strength of alkalies is called the alkalimetric process; but the alkalimeter is not confined in its use to the estimation of the strength of alkaline substances. It is likewise employed in the determination of the strength of acids, such as sulphuric acid, hydrochloric acid, nitric acid, and acetic acid (vinegar). For this end, the graduated instrument is charged with a solution of an alkali of known strength, such as a given weight of crystallised carbonate of soda (washing soda), dissolved in water, and according to the number of divisions of the liquid poured from the alkalimeter, the strength of the acid into which the alkaline liquid has been decanted is calculated. The latter application of this instrument is called acidimetry. Again, the same graduated glass tube has been recently employed in many other ways, such as the determination of the strength of a solution of silver, by charging the instrument with a known or standard solution of common salt; and for this purpose it is used largely by the assayers to the Royal Mint, and other metallurgic chemists. This mode of analysis is every day becoming of more and more importance, and, in fact, has given rise to a new department of analytical chemistry, which has been designated volumetric analysis.