This near agreement shows the reliability of the method. The comparison of the Pemberton volumetric method with the official gravimetric method was investigated by the reporter of the Association of Official Agricultural Chemists in 1894.[87] The individual variations were found to be greater than in the regular method but the average results were nearly identical therewith. The method works far better with small percentages of phosphoric acid than with large. Where the average of the results by the official methods gave 12.25 per cent, the volumetric process gave 11.90 per cent, whereas in the determination of a smaller percentage the results were 2.72 and 2.73 per cent, respectively. Kilgore proposes a variation of the method which differs from the original in two principal points.[88] First the temperature of precipitation in the Pemberton process is 100°; but in the modified form from 55° to 60°. At the higher temperature there is danger of depositing molybdic acid.
The second difference is in the composition of the molybdate solution employed. The official molybdate solution contains about sixty grams of molybdenum trioxid in a liter while the Pemberton solution contains sixty-six grams. There is therefore not much difference in strength. The absence of nitric acid, however, from the Pemberton solution favors the deposition of the molybdic acid when heat is applied. Kilgore, therefore, conducts the analysis as follows: The solution of the sample is made according to the official nitric and hydrochloric acid method for total phosphoric acid. For the determination, twenty or forty cubic centimeters are taken, corresponding to two-tenths or four-tenths gram of the sample. Ammonia is added until a slight precipitate is produced and the volume is then made up, with water, to seventy-five cubic centimeters. Add some ammonium nitrate solution, from ten to fifteen cubic centimeters, but this addition is not necessary unless much of the nitric acid has been driven off during solution. Heat in water-bath to 60° and precipitate with some freshly filtered official molybdate solution. Allow to stand for five minutes, filter as quickly as possible, wash four times by decantation using from fifty to seventy-five cubic centimeters of water each time, and then wash on a filter until all acid is removed. The solution and titration of the yellow precipitate are accomplished as in the Pemberton method. The agreement of the results obtained by this modified method was much closer with the official gravimetric method than those obtained by the Pemberton process.
109. Estimation of Phosphoric Acid as a Lead Compound.—In the volumetric lead method, as described by Wavelet, the phosphoric acid is precipitated by the magnesium citrate solution as in the uranium method of Joulie, as practiced by the French chemists, and the washing of the precipitate and its solution in nitric acid are also conducted as in that method.[89] After solution in nitric acid ammonia is added to neutrality and the solution is then made acid with acetic. The phosphoric acid is precipitated in the acid solution by a standard solution of lead nitrate, the precipitate having the formula P₂O₅3PbO.
The end reaction is determined by placing a drop of the titrated mixture on a white greased dish in contact with a drop of a five per cent solution of potassium iodid. When all the phosphoric acid is precipitated the least excess of the lead salt is revealed by the characteristic yellow precipitate of lead iodid.
The author of the process claims that the lead phosphate is insoluble in the excess of acetic acid and that the phosphate itself does not give any yellow coloration with potassium iodid. The process is quite as exact as the uranium method and the end reaction is far sharper and the standard reagents are easily made and preserved.[90] The method described merits, at least, a comparative trial with the uranium process, but cannot be recommended as exact until further approved by experience.
The reagents employed have the following composition:
| (1) | Disodium phosphate | solution | containing | 10.085 | grams | per | liter |
| (2) | Sodium acetate | “ | “ | 50.000 | “ | “ | “ |
| (3) | Lead nitrate | “ | “ | 40.000 | “ | “ | “ |
| (4) | Potassium iodid | “ | “ | 50.000 | “ | “ | “ |
| The titrations should be conducted in the cold. | |||||||
110. Water-Soluble Phosphoric Acid.—Glaser has modified the volumetric method of Kalmann and Meissels for the volumetric estimation of water-soluble phosphoric acid so as to avoid the double titration required by the original method.[91] If methyl orange be used as an indicator in the original method, the determination does not at once lead to the tricalcium salt, but the liquid still contains, after neutralization, some monocalcium phosphate, which is determined by a further titration with phenolphthalein. In the modified method the total phosphoric acid is estimated in one operation as a tricalcium salt. This is secured, by adding, at the proper time, an excess of calcium chlorid. Two grams of the superphosphate are shaken with water several times, and, after settling, filtered, and the insoluble residue finally washed on the filter until the total volume of the filtrate is a quarter of a liter. Of this, fifty cubic centimeters are taken and titrated with tenth normal soda-lye, with addition of two drops of methyl orange, until the acid reaction has entirely disappeared. There is then added some neutral calcium chlorid solution in excess. If iron and alumina be present, a precipitate is produced of which no account need be made. The acid reaction is thus restored. Five drops of the phenolphthalein solution are added and the titration continued until the alkaline reaction is noted throughout the whole mass. Each cubic centimeter of the soda-lye corresponds, in the first titration, to 7.1, and in the second to 3.55 milligrams of phosphoric acid.
111. Estimation of Phosphoric Acid in the Presence of a Large Excess of Iron.—The method given below, due to Emmerton, depends upon the precipitation of a phosphomolybdate, of constant composition, in the presence of a large excess of iron, as in the analysis of iron and steel and iron ores.[92] The molybdenum trioxid obtained is reduced by zinc to Mo₁₂O₁₉. The action of permanganate on this compound is shown in the following equation:
5Mo₁₂O₁₉ + 17(K₂OMn₂O₇) = 60MoO₃ + 17K₂O + 34MnO.