(9) Specific Gravity.—The estimation of the volume specific gravity is also a good indication for judging of the purity of the slag. This is best done by weighing directly a given volume. Basic slag will have a volume-weight of about one and nine-tenths, while natural phosphates will have about one and six-tenths.
(10) Conclusions.—From the above résumé of the standard methods which are in use for determining the adulteration of basic slag, it is seen that there are many cases in which grave doubt might exist even after the careful application of all the methods mentioned. If we had only to consider the adulteration of basic slag with certain of the mineral phosphates, that is, tricalcium phosphate, the problem would be an easy one, but when we add to this the fact that iron and aluminum phosphates are employed in the adulteration, and that artificial slags may be so used, the question becomes more involved.
In doubtful cases one after another of the methods should be applied until there is no doubt whatever of the judgment which should be rendered.
VOLUMETRIC DETERMINATION
OF PHOSPHORIC ACID.
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.
The direct volumetric precipitation of the phosphoric acid by a uranium salt or otherwise is practiced only when the acid is combined with the alkalies and when iron and alumina are absent and only small quantities of lime present. This method has therefore but little practical value for agricultural purposes. In all volumetric analyses the accuracy of the burettes, pipettes, and other graduated vessels should be proved by careful calibration. Many of the disagreements in laboratories where the analytical work is conducted equally well can be due to no other cause than the inaccuracy of the graduated vessels which are found in commerce. Burettes should not only be calibrated for the whole volume but for at least every five cubic centimeters of the graduation.
URANIUM METHOD AS PRACTICED
BY THE FRENCH CHEMISTS.
87. The Uranium Method.—Since the phosphoric acid of practical use for agricultural purposes is nearly always combined with lime, alumina, and iron, its volumetric estimation by means of a standard solution of a uranium salt is to be preceded by a preliminary separation by means of an ammoniacal magnesium citrate solution. The principle of the method was almost simultaneously published by Sutton,[77] Neubauer,[78] and Pincus.[79] The phosphoric acid may also be separated by means of molybdic solution or by tin or bismuth.[80] In practice, however, it has been found that when the uranium method is to be used the magnesium citrate separation is the most convenient. Since this is the method practiced almost universally in France, the method there used will be given in detail. It is based essentially on the process described by Joulie.[81]
88. Preparation of Sample.—(1) Incineration.—Since the organic matters present in a phosphatic fertilizer often interfere with the employment of uranium as a reagent, it is necessary to incinerate the sample taken for analysis.[82]
(2) Solution of the Material.—All phosphates, with the exception of certain aluminum phosphates, amblygonite for example, are easily dissolved in nitric and hydrochloric acids more or less dilute, especially on ebullition. The best solvent, however, for calcium phosphates for the uranium method is incontestably hydrochloric acid which also very easily dissolves the iron and aluminum phosphates, which are often found present with calcium phosphates.