=63. General Principles.=—It has been seen that in the molybdic method
there is introduced a process at considerable cost, both of reagents
and time, having for its object the separation of the phosphoric acid
from all the other acids and bases which may have been present in the
original sample. The phosphorus is thus obtained in composition with
molybdenum and ammonium in a form easily soluble in ammonia, from which
it can be accurately separated by means of a soluble salt of magnesia.
The citrate method has for its object the suppression of this
intermediate step and the determination of the phosphoric acid by
direct precipitation in presence of iron, lime, and alumina. The
principle on which it is based rests on the well-known power of an
alkaline ammonium citrate to hold in solution the salts of iron,
alumina, and lime, while at the same time it permits of the separation
of phosphoric acid, as ammonium magnesium phosphate. In no case can the
citrate method be regarded as an exact analytical process, but large
experience has shown that the errors of the method are compensatory and
that it affords a good and ready method for fertilizer control.
When phosphoric acid solutions which contain no iron, lime, alumina, or
manganese, are precipitated in presence of ammonium citrate the results
obtained vary markedly with the quantity of magnesia mixture employed.
Grupe and Tollens[48] were the first to point out that a portion of
the phosphoric acid might remain in solution, but that the precipitate
might contain a sufficient excess of magnesia to compensate for the
loss. It has been further shown by Glaser[49] that a portion of the
phosphoric acid may be lost by volatilization in the citrate method.
When the ignition is carried on in a crucible where the cover is
coated with magnesia to intercept the volatilized acid, a considerable
quantity of it can be recovered by the molybdic method.
Where too little magnesia mixture is employed, therefore, two sources
of loss are to be guarded against; _viz._, a part of the phosphoric
acid may remain in solution and another part be volatilized on
ignition. The explanation of the volatilization is as follows: In the
presence of ammonium citrate, magnesium chlorid may be partly converted
into magnesium citrate and ammonium chlorid. There may be a time,
therefore, in the precipitation with not too great excess of magnesia
mixture, when proportionally there is little magnesium chlorid and much
ammonium chlorid present. The formation of a salt represented by the
formula Mg(NH₄)₄(PO₄)₂ may take place which, upon ignition, breaks up
into Mg(PO₃)₂ and finally passes into Mg₂P₂O₇ with loss of P₂O₅. This
theoretical condition has but little weight, however, practically in
the analysis of fertilizers, since in these cases a large quantity of
lime is always present. But even in these cases traces of volatile P₂O₅
may be discovered.
Wells[50] has shown that the citrate method gives good results in
certain conditions but that this accuracy is reached by a fortunate
compensation of errors. The ammonium magnesium salt does not
precipitate all the phosphoric acid in this process, but contains
enough impurities to make up for this loss.
Johnson[51] in conjunction with Osborne has shown that the results
by the citrate method practiced in accordance with the details
laid down by Vögel, are too low, but that this difficulty could be
overcome by using more and stronger magnesia mixture and a larger
quantity of strong ammonia solution. The citrate method was found to
give unsatisfactory results when iron and alumina were present in
any considerable quantity. In the examination of the final ignited
precipitate, which should be pure magnesium pyrophosphate, it was found
to consist of only 94.98 to 97.83 per cent of that salt. The chief
impurity found was calcium oxid, the percentage of which varied from
2.05 to 3.95 in six cases. There was also a considerable percentage of
loss due, probably, to magnesia and pyrophosphoric acid.
The presence of large quantities of iron and alumina also impairs the
accuracy of the molybdate method when the precipitation of the yellow
salt takes place at too high a temperature. When the temperature of
precipitation in the method is above 50° the results are likely to be
too high while a great excess of nitric acid in the reagent may produce
a contrary effect. In the latter case the filtrate from the yellow salt
should be mixed with additional quantities of molybdate solution until
no further precipitate takes place.
Many methods of conducting the citrate method have been proposed but
the best of them are based on the one elaborated at the experiment
station of Halle by Bühring, and which will be given in the next
paragraph, followed by some other methods in use in other localities.
=64. Method of the Halle Agricultural Experiment Station.=[52]—The
citrate method, as described by Morgen, is the one employed.[53] The
principle depends upon the direct precipitation of the phosphoric acid
by magnesia mixture. By the addition of a solution of ammonium-citrate
the precipitation of lime, iron, alumina, and other bases, is
prevented. The precipitate of ammonium magnesium phosphate is converted
by ignition into magnesium pyrophosphate and weighed as such. By the
use of this method a part of the phosphoric acid sometimes escapes
precipitation and a portion of the other bases is sometimes thrown down
with the precipitate. Experience has shown that by adhering to certain
precautions the weight of impurities in the precipitate may be made to
correspond exactly to the weight of the phosphoric acid which escapes
precipitation.
[Illustration: FIGURE 3.