The filtration of the ammonium magnesium phosphate is made through
perforated crucibles. The asbestos felt is prepared in the following
way: The coarse fibers of asbestos are chopped up with a sharp knife on
a glass plate and boiled for two hours with strong hydrochloric acid;
afterwards, by repeated washing with distilled water they are freed
from acid and the too fine particles of asbestos which would tend to
make the filter too impervious. After the last wash-water is poured
off, the asbestos is suspended in water and used for making the felt on
the filter. The preparation of the crucible and the filtration under
pressure are accomplished in the usual way.
The ignition of the precipitate is accomplished in a Rössler ignition
oven, Fig. 5. When the muffle of the furnace shows a white heat or a
white-red heat it is at the proper temperature for the estimation.
At higher temperatures, the asbestos felt is easily injured.
Generally, an ignition of five minutes is sufficient, but with double
superphosphates, ten minutes are required.
=65. The Swedish Citrate Method.=[56]—This method of determining
phosphoric acid is founded on the fact that phosphoric acid in the
presence of calcium salts, without it being necessary, previously, to
convert it into phosphomolybdate, is precipitated directly by magnesia
mixture from a solution, to which ammonium citrate has been added,
provided first, that the solution contain a sufficient quantity of
sulfuric acid, and second, that only as much citrate be added as is
required to keep the calcium salts in alkaline solution.[57]
_Reagents._ (1) _Citric Acid Solution._—Prepared by dissolving 500
grams of citric acid in water, and completing to a volume of one liter.
(2) _Ten Per Cent Ammonia_ of 0.959 specific gravity.
(3) _Magnesia Mixture_, of the usual composition.
The various processes are conducted as follows:
(_a_) _Water-Soluble Phosphoric Acid._—Add twenty cubic centimeters of
citric acid solution to fifty cubic centimeters of the water-soluble
solution obtained according to the Swedish molybdenum method, and
then add thirty-three cubic centimeters of ammonia. When the mixture
has cooled, add slowly twenty-five cubic centimeters of the magnesia
mixture, and then forty-two cubic centimeters of the ammonia. Keep the
solution stirred by means of a closely clipped feather which is pressed
tightly against the sides of the beaker; by this process the phosphate
is precipitated after half an hour in pure condition and completely,
without, in the least, sticking to the wall of the beaker; filter,
wash, and ignite, as usually directed.
(_b_) _Insoluble Phosphoric Add._—Moisten, in a porcelain dish, ten
grams of the powdered sample with water; add fifty cubic centimeters
of concentrated sulfuric acid, and heat for fifteen minutes so high
that fumes of sulfuric acid will escape. When the mass has cooled,
wash it into a half liter graduated flask, fill to the mark, and shake
well. After filtration, the clear filtrate may, after some time,
turn turbid by separation of calcium sulfate, but as the ammonium
citrate, which is afterwards added, again brings the precipitate into
solution, it is of no importance. Take fifty cubic centimeters of
the solution, corresponding to one gram of the powdered sample, add
twenty cubic centimeters of the citric acid solution, neutralize the
mixture approximately, but not exactly, by ammonia; after cooling,
add twenty-five cubic centimeters of magnesia mixture; stir the fluid
by means of a feather, as described above, till no more precipitate
is formed, and finally add thirty-three cubic centimeters of ammonia
while stirring for a couple of minutes more; after half an hour the
precipitate may be separated by filtration, washed, and ignited, as
usually directed.
The above process is essentially the one used with basic slags. When
much organic matter is present, by continuing the heating with sulfuric
acid for some time, it may be destroyed.
=66. Methods Adopted by the Brussels Congress, 1894.=—The report of
the committee on methods of analysis of phosphoric acid requires the
molybdate method to be used in all cases where the quantity to be
determined is very small. In other cases the citrate method may be
employed.[58]
(1) _Soluble Phosphoric Acid._—The soluble phosphoric acid is
determined by the method adopted at Brussels in the following manner:
Five grams of the sample are rubbed to a powder in a mortar, and then
from fifty to sixty cubic centimeters of water added. After allowing
to settle for a few minutes the liquid portion is decanted upon a
filter. This operation is repeated three or four times. Finally the
solid portions are washed upon the filter, and the washing with water
is continued until the filtrate amounts to about three-quarters of a
liter. A few drops of hydrochloric acid are added until the filtrate
is perfectly clear, and the volume is then made up to one liter. Fifty
cubic centimeters of the solution are then treated with thirty cubic
centimeters of ammonium citrate solution and one-third as much ammonia.
Afterwards thirty cubic centimeters of magnesia mixture are added, drop
by drop, with constant stirring.
For superphosphates containing more than eighteen per cent of
phosphoric acid only one gram is taken, for ordinary superphosphates
two grams, and for compound fertilizers four grams. The sample is first
treated as above for soluble acid until the filtrate amounts to 200
cubic centimeters, then clarified with a drop of nitric acid, and made
up to a quarter of a liter.
(2) _Reverted Phosphoric Acid._—The filter containing the residue
is then introduced into a quarter liter flask and treated with 100
cubic centimeters of Petermann’s alkaline ammonium citrate solution,
vigorously shaken, and left at room temperature for fifteen hours.
It is then digested for an hour at 40° and filtered. Fifty cubic
centimeters of the filtrate are placed in a flask and, with constant
shaking, thirty-five cubic centimeters of magnesia mixture added.
The aqueous solution is treated in the same way. The precipitate is
collected, ignited and weighed, and multiplied by 0.64 for phosphoric
acid. The total acid is determined in the usual way.
=67. Dutch Method for Citrate-Soluble Phosphoric Acid.=[59]—The
reagents necessary are:
(1) Citrate solution, prepared according to Petermann. Dissolve 165
grams of citric acid in 700 cubic centimeters of water, mix with 250
cubic centimeters of ammonia of 0.92 specific gravity, and, after
cooling, bring to the volume of one liter.
(2) Magnesia mixture prepared according to Petermann. Dissolve 400
grams of crystallized magnesium chlorid, 800 grams of ammonium chlorid,
and 1,600 cubic centimeters of ammonia of 0.96 specific gravity in
water, and dilute to five liters.
The quantity to be taken for the analysis is five grams where the
fertilizer contains less than six per cent of phosphoric acid (mixed
fertilizers); two grams where it contains more than six and less than
fifteen per cent (common superphosphates); and one gram where it
contains more than fifteen per cent (double superphosphates). Place
the weighed substance in a mortar and cover with 100 cubic centimeters
of citrate solution. Gently rub up, wash into a half liter flask, and
heat in a water-bath for an hour to a temperature between 35° and 38°.
Allow to cool, fill up to 500 cubic centimeters, and filter through a
dry double filter. If it is not clear at the first filtration, pour
through the filter again, repeating this till clearness is attained.
Measure 100 cubic centimeters and add seventy-five cubic centimeters
of magnesia mixture, allowing the latter to flow into the former very
slowly, and constantly stirring during the influx. Allow to stand
fifteen hours, filter, wash with ammonia of 0.96 specific gravity, dry,
ignite, and weigh.
The per cent of phosphoric acid, except where otherwise indicated, is
always to be given as per cent of anhydrous acid (P₂O₅).
=68. Comparative Accuracy of the Citrate and Molybdate Methods.=—The
general use of the citrate method of determining phosphoric acid
by the German chemists has led Johnson[60] to review some trials
of that method in the Yale laboratory made as early as 1880. These
determinations have lately been repeated in comparison with the
ordinary molybdate methods with the result that in sixty-seven
determinations on bone-dust, superphosphate, cotton-hull ashes,
cottonseed-meal, tankage, bone-char, phosphatic guano, and phosphate
rock, only three citrate results differed from those obtained by
the molybdate method by more than three-tenths of one per cent. The
greatest discrepancy between the two methods was 0.41 per cent, and the
average difference was 0.09 per cent.
The citrate method was found to give poor results when iron and alumina
were present in considerable quantity. Ignited precipitates by the
citrate method were found to contain as high as four per cent of
lime, and iron and alumina in small quantities when these bodies were
abundant in the original substance.
In the molybdate method the rapid precipitation from solutions at 65°
was found to give unsatisfactory results and it was found necessary
to conduct the process at temperatures between 40° and 50°. With a
relative excess of nitric or a relative deficiency of molybdic acid
some phosphoric acid may easily escape precipitation. The chief
objection to precipitating at 65° is found in the fact that in presence
of considerable iron and alumina some of these bodies may be found
in the yellow precipitate, whence they pass to the final ammonium
magnesium phosphate.
The citrate method, therefore, only gives safe results by compensating
errors which in every class of phosphates must be empirically
determined.
The molybdate method gives results too high when iron and alumina are
present in considerable quantity and the yellow precipitate is obtained
at temperatures above 50°. On the other hand, if there, be a great
relative excess of nitric acid the results may be too low unless the
filtrates from the yellow precipitate be mixed with additional molybdic
solution and digested until no further precipitate is formed.
Comparative determinations made, by both methods, by the Association of
German Experiment Stations have led to the conclusion that both give
practically the same results when each one is conducted with the proper
precautions peculiar to it.[61] In the latter part of 1892, at the
general meeting of the Association, it was declared that the citrate
method, after having been subjected to repeated tests, was found to
be satisfactory, changing the composition of the solution so that it
might have 1,100 instead of 1,000 grams of citric acid and four liters
of twenty-four per cent ammonia to each ten liters. The data afforded
by the citrate method, when applied to an artificial mixture of known
composition, were more satisfactory than those obtained by the molybdic
process.
In this laboratory the citrate method has been found to give nearly
agreeing results with the old process. It is much shorter and less
expensive; and is recommended most favorably for practical use,
suggesting, however, that with every new kind of phosphate or
phosphatic fertilizer varying notably in composition from the standard,
the work should be checked at first by comparison with the molybdenum
method.