ALKALIMETER (Fr. Alcalimetre ; Ger., Alkalimeter) An instrument for testing the amount of alkali present in a commercial sample which may have an admixture of impurities. It was invented by F. A. H. Descroizilles, of Dieppe, though some have claimed the discovery for Dr. Andrew Ure, of Glasgow. It consists of a graduated glass tube divided into ioo degrees and furnished at one end with a dropping nozzle. The form recommended by Dr. Fara day is shown at A, but many now prefer to use the more convenient burette pattern B, hav ing a glass tap or pinchcock at its lower end. The tube of the alkalimeter is filled with dilute sulphuric acid, containing as much of the strong acid as would suffice to neutralise a given weight, say ioo grs., of potassium carbonate or sodium carbonate. One hundred grains of the alkali to be tested is then dissolved in water, the solution being placed in a glass beaker or flask, and the acid solution is allowed to drop gradu ally into it until the mixture is neutralised. The purer the substance the more of the acid will be required. If the tube is emptied to, say, 8o deg., the alkali is known to contain 20 per cent. of impurities. The point at which neutral
isation occurs used to be determined by means of litmus or turmeric, but more sensitive and easily recognisable indicators are now employed, the principal of these being methyl-orange and phenolphthalein. A mixture of these two re agents in alcoholic solution gives a pale yellow colour to a perfectly neutral liquid, which is instantly changed to pink by the least trace of acid, or to a deep red by a trace of alkali. Com mercial potassium carbonates and sodium car bonates frequently contain a certain proportion of the sulphate or chloride, silicates, etc., and since the value of the sample depends on the proportion of carbonate present it is obviously requisite to ascertain this. Impure com mercial alkalis are, of course, scarcely suitable for photographic purposes, but the above method of testing is often useful. By using an alkaline solution of known strength instead of the acid solution, the strength of acids may be tested ; or the strength of a solution of silver may be ascertained by charging the instrument with a standard solution of sodium chloride.