In case a precipitate is formed, showing the presence of alumina, nitric acid is added until the precipitate is dissolved, and then in slight excess and after evaporation the phosphoric acid separated by molybdic solution and determined as usual. From the filtrate the excess of molybdic acid is removed by hydrogen sulfid and the alumina determined in the filtrate: Or the alumina may be determined directly in the hydrochloric acid solution of the melt above obtained as aluminum phosphate by adding sodium phosphate, ammonia and acetic acid. The aluminum phosphate is separated by filtration and determined in the usual manner. The phosphoric acid is then determined in another aliquot part of the original filtrate from the first solution of the ash.
Since most ashes contain an excess of phosphoric acid above the quantity required to combine with the iron it is preferable to proceed on that basis as described in the next paragraph.
297. Method Used in this Laboratory.—The principle of the method rests on the assumption that all the phosphoric acid may be removed from the solution by the careful addition of iron chlorid. Any excess of iron is then removed by ammonium acetate and the manganese, lime, and magnesia, are separated in the filtrate. The percentage of iron is determined by reduction of the iron in another portion of the solution and titration with potassium permanganate. The process as conducted by McElroy, is as follows:[251]
Moisture.—If the ash contain much carbon the water is best determined by drying in vacuo to avoid oxidation.
Sand, Silica, and Carbon.—Place a portion of the ash in a weighed platinum dish, weigh, and cover the sample with hydrochloric acid of 1.115 specific gravity. Evaporate to dryness on the water-bath, and then heat for fifteen minutes at 105° to 110° in an air-bath. Repeat the treatment with acid and drying: Finally cover with a third portion of acid and digest on the water-bath for an hour or two. Filter into a weighed gooch and wash the residue free of chlorids. The gooch is best weighed with the dish to avoid the necessity of transferring the silica which may adhere to the sides of the former. Dry at a few degrees above the boiling temperature of water and weigh. Where the ash contains much charcoal the drying is best done in a vacuum at 60° to 70°. The increase in weight found represents sand, silica, and carbon: Burn and reweigh. The loss is carbon.
Another portion of ash is treated as before except that it is filtered through a paper filter. The filtrate is united with that of the previous sample in a graduated flask. When the washing is completed the filter is placed in the dish, a weak solution of caustic soda added, and the mixture heated on the water-bath for some time. Decant while hot through a fresh filter and retreat the residue in the dish with another portion of alkali. Finally wash with hot water till the alkaline reaction disappears, then with weak hydrochloric acid, then with water until chlorids disappear. The washed mass on the filter is transferred to a platinum dish and ignited. The weight obtained represents sand.
Separation of Phosphoric Acid.—The united filtrates from the two determinations are placed in a graduated flask and made up to the mark. An aliquot portion of this solution representing half a gram of the original ash or any other convenient quantity is transferred to a beaker and a solution of ferric chlorid added until ammonia produces a brown precipitate in the mixture. Neutralize with ammonia and hydrochloric acid alternately until the liquid is as little acid as it can be and still remain clear. Add from ten to twenty cubic centimeters of a solution of sodium acetate (1:10) and bring to a boil. The liquid should be quite dilute. Filter and wash free of chlorids with boiling water containing some sodium acetate.
Manganese.—Make the filtrate faintly alkaline with ammonia and add ammonium sulfid. Any manganese sulfid which may form is separated by filtration, treated with dilute acetic acid and the resulting solution, which should be clear, heated to boiling, nearly neutralized with caustic soda, and mixed with bromin water. The resultant manganese dioxid is to be filtered into a gooch, ignited and weighed as Mn₃O₄.
Lime.—Reacidify the filtrate from the manganese sulfid with acetic acid, heat to boiling and add ammonium oxalate: Allow to stand over night, filter through a gooch and wash with water containing acetic acid. The calcium oxalate can be weighed as such, but it is preferable to dry thoroughly and then heat in a small bunsen flame until a change can be noted passing over the precipitate. If this is carefully done the residue will be calcium carbonate. In any case the result is to be checked by igniting over the blast lamp to constant weight and weighing the lime thus obtained.
Magnesia.—In the filtrate the magnesia can be determined by sodium phosphate in the usual manner. In very accurate work the calcium oxalate obtained as directed above can be dissolved and reprecipitated, and the magnesia in the filtrate added to that in the first filtrate.