(3) Nitric Acid.—In many laboratories nitric acid is preferred in order to avoid, in part, the solution of ferric oxid which interferes with the determination of phosphoric acid in certain processes. Since it does not act in this way for the citro-magnesium uranium method, it is preferable to employ hydrochloric acid, especially because it dissolves the iron completely and permits thus the operator to judge of the success of the solvent action by the completely white color of the residue.
(4) Pyritic Phosphates.—Certain phosphates contain pyrites which hydrochloric acid does not dissolve, and there is left consequently, a residue more or less colored. In this case it is necessary to add some nitric acid and to prolong the boiling until the pyrite has disappeared, since it might retain a small quantity of phosphoric acid in the state of iron phosphate.
(5) Sulfuric Acid.—Some chemists decompose the phosphates by means of dilute sulfuric acid. This method, which is certainly able to give good results for certain products and for certain processes, presents numerous inconveniences which tend to render its use objectionable for volumetric purposes. The calcium sulfate which is formed, requires prolonged washings which lead to chances of fatal error.
If an aluminum phosphate be under examination, containing only very little or no lime, sulfuric acid is to be preferred to hydrochloric and nitric acids, since it attacks amblygonite, which, as has been before stated, resists the action of the other two acids. But these are cases which are met with very rarely, and which can always be treated by the general method by previously fusing the material with a mixture of sodium and potassium carbonate.
In the great majority of cases the decomposition by hydrochloric acid is very easily accomplished by simply boiling in a glass vessel, and without effecting the separation of the silica. This operation is only necessary after the substance has been fused with alkaline carbonates, or, in case of substances which contain decomposable silicates giving gelatinous silica with hydrochloric acid.
There are two methods [see (6) and (7)] of securing a solution of the sample taken which varies from one to five, and even ten grams, according to the apparent homogeneity of the material to be analyzed.
(6) Solution by Filtration and Washing.—The ordinary method can be employed consisting in decomposing the substance by an acid, filtering, and washing the residue upon the filter, and combining all the wash-waters to make a determinate volume. Afterwards an aliquot fraction of the whole is taken for the precipitation. This method is long, and presents some chances of error, when the insoluble residue is voluminous and contains silica which obstructs the pores of the paper and renders the filtration difficult.
(7) Volumetric Solution.—It is advisable to substitute volumetric solution for solution by filtration and washing, which is accomplished by decomposing the substances in a graduated flask, the volume being afterwards made up to the mark with distilled water after cooling. The solution is then filtered without washing, and by means of a pipette an aliquot part of the original volume is taken for precipitation. Thus all retardations in the process are avoided, and likewise the chances of error from washing on the filter. It is true that this method may lead to a certain error due to the volume of the insoluble matter which is left undecomposed, but since this insoluble matter is usually small in quantity, and since it is always possible to diminish the error therefrom by increasing the volume of the solution, this cause of error is much less to be feared than those due to the difficulties which may occur in the other method. Let us suppose, in order to illustrate the above, that we are dealing with a phosphate containing fifty per cent of insoluble sand which may be considered as an extreme limit. In working on four grams of the material in a flask of 100 cubic centimeters capacity, there will be an insoluble residue of two grams occupying a volume of about one cubic centimeter, the density of the sand being generally nearly two. The one hundred cubic centimeter flask will then contain only ninety-nine cubic centimeters of the real solution, and the error at the most would be 0.01. This error could be reduced to one-half by dissolving only two grams of the material in place of four, or by making the volume up to 200 instead of 100 cubic centimeters.
In general it may be said that the errors which do not exceed 0.01 of the total matter under treatment, are negligible for all industrial products. The method of volumetric solution does not present any further inconvenience. It deserves to be and has been generally adopted by reason of its rapidity in all the laboratories where many analyses are to be made. In the volumetric method great care should be taken not to make up to the volume until after the cooling to room temperature, which may be speedily secured by immersing the flask in cold water. Care should also be exercised in taking the sample for analysis by means of the pipette immediately after filtration, and filtration should take place as soon as the volume is made up to the standard. By operating in this way the possible variations from changes of volume due to changes of temperature are avoided.
(8) Examination for Arsenic Acid.—When the sample examined contains pyrites, arsenic is often present. When the decomposition has been effected by means of nitric acid, arsenic acid may be produced. This deports itself in all circumstances like phosphoric acid, and if it is present in the matter under examination it will be found united with the phosphoric acid and determined therewith afterwards. It is easy to avoid this cause of error by passing first a current of sulfurous acid through the solution, carrying it to the boiling-point in order to drive out the excess of sulfurous acid, and afterwards precipitating the arsenic by a current of hydrogen sulfid. After filtration, the rest of the operation can be carried on as already described.