46. Modification of Wyatt.—By reason of the tediousness of the method of Chatard given above, Wyatt has sought to shorten the process by the following modification:[35]
Five grams of the finely ground phosphate are fused in a platinum dish with fifteen grams of the mixed carbonates of sodium and potassium and three grams of fine sand. After fusing very thoroughly with a strong heat for a quarter of an hour, the dish is removed from the fire and cooled. Its contents, dissolved in hot water, are then put into a half liter flask, and a considerable excess of ammonium carbonate is added to the liquid. All the soluble silica falls out of solution, and the flask, after cooling, is made up to the mark with distilled water, well shaken, and then set aside for twenty-four hours to settle. At the end of this time 200 cubic centimeters are carefully decanted through a filter; the filter is well washed, and the filtrate, after being nearly neutralized with hydrochloric acid, is treated with an excess of calcium chlorid solution.
The precipitate, consisting of phosphate, fluorid, and some calcium carbonate, is allowed to settle, and is then carefully washed with boiling water, first by decantation several times, and finally on the filter. After being properly dried in the gas-oven, calcined, and cooled, the residue is treated with acetic acid, placed upon the water-bath, and evaporated to complete dryness.
The calcium acetate is now well washed out by several treatments with boiling water, and the residue is brought upon a filter, dried, calcined, and weighed. The weight represents the calcium phosphate and fluorid contained in two grams of the original sample; and if the calcium phosphate in the residue be determined, according to the usual methods, the difference will be calcium fluorid and may be thus estimated.
Example.—Assuming the calcined residue of calcium phosphate and fluorid in two grains of the original sample to have amounted to one and six-tenths gram and the calcium phosphate in this quantity to have been determined as 1.540 gram, the calcium fluorid is thus proved to be 0.060 gram, and, therefore, 2: 0.60::100: x = 3 per cent calcium fluorid which, multiplied by 0.4897, gives 1.46 per cent of fluorin.
The above method, while shorter, is not to be preferred to the Chatard process when great accuracy is desired. All the soluble silica may not fall out of the solution as Wyatt says. Finally the fluorin is calculated from small differences in the weight of very heavy precipitates and all the error of the process may be found affecting the numbers for fluorin. For commercial purposes, however, the method is to be recommended for its comparative brevity.
GENERAL METHODS FOR ESTIMATING
PHOSPHORIC ACID IN FERTILIZERS.
47. Preliminary Considerations.—The chief sources of the phosphoric acid in commercial fertilizers are the mineral phosphates and bones. In respect of the analyses of mineral phosphates detailed directions have been given in the preceding pages. Bones are valuable for fertilizing materials, both because of their content of phosphoric acid and of their organic nitrogen. The methods of treating bones for their phosphoric acid will be found in the general methods for fertilizing materials, and their nitrogen content can be determined by the processes to be described hereafter. Other fertilizing materials also contain phosphorus, as ashes, tankage, oil cakes, and other organic products. In general, the methods for determining the phosphoric acid is the same in all cases, but the means of destroying the organic matter precedent to the analysis vary in different cases. In most cases a simple ignition is sufficient, while, if the phosphorus be found in certain organic products, the oxidation must be accomplished by one of the methods described in the processes adopted by the official chemists, or by the means described in volume first, paragraph 378 or 382. In all cases of acid phosphates and superphosphates, the water and ammonium citrate soluble phosphoric acid is to be determined as well as the total. In basic slags the amount soluble in ammonium citrate or dilute citric acid is also to be ascertained.
In all cases where soluble or so-called reverted acid is to be considered, the analysis must be performed without previous desiccation or ignition. If water content or loss on ignition are to be considered, the operation to determine them must be conducted on a separate part of the sample.
The methods of analysis which have been adopted by associations of chemists should be given the preference in the conduct of the work, although it must be admitted that they may contain sources of error, and may be in no respect superior to processes employed by chemists in their private capacity. In this country the methods adopted by the Association of Official Agricultural Chemists should be followed as closely as possible. The great merit of other methods, however, must not be denied. Especially those methods which shorten the time required or diminish the labor and expense of the analysis are worthy of careful consideration. In factory work, for instance, it is often far more important for the chemist to be able to rapidly determine the phosphoric acid in a great number of samples with approximate accuracy than to confine his work to one with absolute precision. Some of the shorter methods, moreover, notably the citrate process, appear to be quite, if not altogether, as reliable as the molybdate method, while in the case of the uranium volumetric process, it must not be forgotten that it is almost the only one practiced in France. Other volumetric processes are given in full, as, for instance, the one perfected by Pemberton, but data are still lacking to justify their strong recommendation. It should be remembered that this manual is not written for the beginner but rather for the chemist already acquainted with the principles and practice of general chemical analysis, and it is, therefore, expected that each analyst will make intelligent use of the data placed at his disposal.