Alkalimetry
the process of determining the purity of alkalies. Commercial potash and soda contain foreign substances, which diminish the percentage of real alkali. The alkalimeter furnishes a simple and ready means of determining the proportion of pure carbonate of potash or soda contained in any sample. It consists of a graduated glass tube, filled with diluted sulphuric acid, and containing as much absolute sulphuric acid as would neusu sulphuric acid as would neutralize a given weight, say 100 grains, of carbonate of potash. One hundred 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 neutralize the solution, that is, take up all the alkali. The point at which neutralization is complete is determined by means of colored tests.
Formerly, the two vegetable colors, litmus and turmeric, were alone used for this purpose. It is not, however, always easy to recognize the neutral point, and other indicators (as these coloring matters are called) have come into use. The chief of these are methylorange and phenolphthalein. A mixture of the alcoholic solutions of these substances imparts a pale yellow color 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.