The hydrochloric acid solution of the phosphate must contain no free chlorin and is treated with a few drops of a methyl orange solution. Ammonia is added until nearly neutral, but the acid reaction is retained as shown by the indicator. A few cubic centimeters of ammonium acetate are added, which produce a yellow coloration of the liquid and also a complete precipitation of the iron and aluminum phosphates when warmed to 70°. At this temperature the precipitation of any calcium phosphate is avoided. A small quantity of the lime may be carried down mechanically and therefore the precipitate should be dissolved in hydrochloric acid and the precipitation again made as above after the addition of some sodium phosphate. If the original solution contain any free chlorin, as may be the case when aqua regia is employed as solvent, before beginning the separation, ammonia should be added in slight excess and the acidity restored by hydrochloric after adding the indicator. In washing the precipitates, water of not over 70° must be used. As has been shown by Hess in the work cited in the next paragraph, the statement of C. Glaser to the effect that the precipitates obtained as above are free of lime has not been proved to be strictly correct. The process, however, is a distinct improvement over the older methods and forms the basis of the amended process given below, which appears to be sufficiently accurate to entitle the acetate method to favorable consideration. =34. Method of Hess.=—Hess has lately made a thorough investigation of the standard methods of determining the iron and aluminum oxids in the presence of phosphoric acid and has shown that the assumption that the composition of the precipitate is represented by the formula Al₂(PO₄)₂ + Fe₂(PO₄)₂ is erroneous[24]. In the washing of the precipitated iron and aluminum phosphates there is a progressive decomposition of the compound with the production of a basic salt. The composition of the precipitate at the end is dependent chiefly upon the way in which the washing takes place. It is quite difficult to always secure a washing in exactly the same way and the final composition of the precipitate varies with almost every determination. It is not, therefore, an accurate proceeding to take half the weight of the precipitate as phosphoric acid or as iron oxid and alumina. In every case it is necessary to dissolve the precipitate and determine the phosphoric acid in the regular way. Hess proposes the following method for carrying out the acetate process of separation: