MISCELLANEOUS NOTES ON PHOSPHATES
AND PHOSPHATIC FERTILIZERS.
119. Time Required for Precipitation of Phosphoric Acid.—The length of time required for the complete precipitation of the phosphoric acid by molybdate mixture is perhaps much less than generally supposed. At 65° the precipitation, as shown by de Roode, is complete in five minutes.[100] In a given case the weight of pyrophosphate obtained after five minutes was 0.0676 gram, and exactly the same weight was found after twenty-four hours. In view of these facts analysts would often be able to save time by omitting the delay usually demanded by the setting aside of the yellow precipitate for a few hours in order to secure a complete separation of the phosphoric acid. In the method of the official chemists it is directed that the digestion at 65° be continued for one hour, and this time may possibly be shortened with advantage. In all cases, however, where there is any doubt in regard to the complete separation, some of the molybdate solution should be added to the filtrate and, with renewed digestion, it should be noted whether any additional precipitate be formed.
120. Examination of the Pyrophosphate.—In fertilizer control it is not usually thought necessary to examine the magnesium pyrophosphate for impurities. Among those most likely to be found is silica. It is proper, in all cases where accuracy is required, to dissolve the precipitate in nitric acid, boil for some time to convert the pyro- into orthophosphate, and reprecipitate with molybdate and magnesia mixture. This treatment will separate the silica which remains practically insoluble after the first ignition. It has been observed by some analysts that the results obtained by the official method are a trifle too high and also that on re-solution the second precipitate of pyrophosphate weighs less than the first.[101] The difference in most cases is very little but it may become a quantity of considerable magnitude in samples where soluble silica is found in notable quantities. The danger of contamination with iron, alumina, and arsenic has already been mentioned but it is not of sufficient importance to warrant further attention.
121. Iodin in Phosphates.—The presence of iodin has been detected in many natural phosphates and is of interest in the discussion of the problem of their origin.[102] A qualitative test for the detection of iodin may be applied in the following manner: Some finely ground phosphate is mixed with strong sulfuric acid and the gases arising from the reaction are aspired into some carbon disulfid or chloroform. The violet coloration arising indicates the presence of iodin. The gases carrying the iodin may also be brought into contact with starch-paste producing the well-known blue color.
The quantity of iodin present in a phosphate is rarely more than one or two-tenths of one per cent. It can be determined as a silver salt, in the absence of chlorin or by any of the standard methods found in works on qualitative analysis.
Iodin is quite a constant constituent of Florida phosphates.
For a quantitative determination, the sample is treated with an excess of strong sulfuric acid in a closed flask and during the decomposition a stream of air is aspired through the flask and caused to bubble through absorption bulbs containing sodium hydroxid in solution.
The temperature of the decomposition may be raised to about 200°. After the solution of the sample the sodium iodid formed is oxidized by heating with potassium permanganate, acidulated and mixed with a solution of potassium iodid to hold the free iodin in solution. The free iodin is determined in the usual way by titration with standard sodium thiosulfate solution. The reactions preparatory to the titration are represented by the following formulas:
2KI + H₂SO₄ = K₂SO₄ + 2HI.
2HI + H₂SO₄ = 2H₂O + SO₂ + 2I.
6I + 6NaOH = NaIO₃ + 5NaI + 3H₂O.
NaI + 2KMnO₄ + H₂O = NaIO₃ + 2KOH + 2MnO₂.
HIO₃ + 5HI = 6I + 3H₂O.
The titration is represented by the following reaction: