=86. Classification of Methods.=—The time required for a gravimetric determination of phosphoric acid has led analysts to try the speedier if less accurate processes, depending on the use of volumetric methods. The chief difficulty with these methods has been in securing some sharp method of distinguishing the end reaction. In most cases it has been found necessary to remove a portion of the titrated solution and prepare it for final testing by subsidence or filtration. As is well known, this method of determining the end reaction is less accurate and more time-consuming than those processes depending on a change of color in the whole mass. All the volumetric processes now in general use may be divided into two classes; _viz._, (1) the direct precipitation of phosphoric acid and the determination of the end reaction by any appropriate means, and (2) the previous separation of the phosphoric acid, usually by means of a citro-magnesium or molybdenum mixture, and in the latter case the subsequent titration of the yellow ammonium phosphomolybdate either directly or after reduction to a lower form of oxidation. In respect of extent of application by far the most important volumetric method is the one depending on titration by a uranium salt after previous separation by ammoniacal magnesium citrate. A promising method after previous separation by molybdenum is the one proposed by Pemberton, but it has not yet come into general use. For small quantities of phosphoric acid or of phosphorus, such as are found in steels and irons, the method of Emmerton, either as originally proposed or as modified by Dudley and Noyes, is in frequent use. Where volumetric methods are applied to products separated by molybdic solution, the essential feature of the analytical work is to secure a yellow precipitate of constant composition. If this could be uniformly done such methods would rival the gravimetric processes in accuracy. Hence it is highly important in these methods that the yellow precipitate should be secured as far as possible, under constant conditions of strength of solution, duration of time, and manner of precipitation. In these cases, and in such only, can the quicker volumetric methods be depended on for accurate results.