Figure. 5.
Rössler Ignition Furnace.
The filtration of the ammonium magnesium phosphate is made through perforated crucibles. The asbestos felt is prepared in the following way: The coarse fibers of asbestos are chopped up with a sharp knife on a glass plate and boiled for two hours with strong hydrochloric acid; afterwards, by repeated washing with distilled water they are freed from acid and the too fine particles of asbestos which would tend to make the filter too impervious. After the last wash-water is poured off, the asbestos is suspended in water and used for making the felt on the filter. The preparation of the crucible and the filtration under pressure are accomplished in the usual way.
The ignition of the precipitate is accomplished in a Rössler ignition oven, [Fig. 5]. When the muffle of the furnace shows a white heat or a white-red heat it is at the proper temperature for the estimation. At higher temperatures, the asbestos felt is easily injured. Generally, an ignition of five minutes is sufficient, but with double superphosphates, ten minutes are required.
65. The Swedish Citrate Method.[56]—This method of determining phosphoric acid is founded on the fact that phosphoric acid in the presence of calcium salts, without it being necessary, previously, to convert it into phosphomolybdate, is precipitated directly by magnesia mixture from a solution, to which ammonium citrate has been added, provided first, that the solution contain a sufficient quantity of sulfuric acid, and second, that only as much citrate be added as is required to keep the calcium salts in alkaline solution.[57]
Reagents. (1) Citric Acid Solution.—Prepared by dissolving 500 grams of citric acid in water, and completing to a volume of one liter.
(2) Ten Per Cent Ammonia of 0.959 specific gravity.
(3) Magnesia Mixture, of the usual composition.
The various processes are conducted as follows: