(a) _Water-Soluble Phosphoric Acid._—1. _Preparation of the Aqueous
Solution._—Of superphosphates and in general fertilizers containing
water-soluble phosphoric acid, a sample of twenty grams is taken,
and water poured over it in a mortar; lumps are crushed lightly, but
completely with the pestle without pulverizing it finer; the whole mass
is then washed into a graduated flask holding one liter, which at once
is filled up to the mark. The volume taken up by the residue insoluble
in water, is left out of consideration in the calculation. The sample
is left standing in the flask (which is occasionally shaken) at the
ordinary temperature of the room for two hours, and the solution is
then filtered.
2. _The Determination._—Take twenty-five cubic centimeters of the
superphosphate solution thus prepared (when a twenty per cent sample
is taken equal to one-tenth gram phosphorus pentoxid); add a quantity
of molybdic solution sufficient for complete precipitation, leave
standing for four hours in a beaker covered with a watch-glass; decant
the solution through a small filter, wash the precipitate first by
decantation, then on filter, with a mixture containing 100 parts
molybdic solution, twenty parts nitric acid of 1.2 specific gravity,
and eighty parts water, until a few drops put into alcohol, to which
some dilute sulfuric acid has been added, does not, any longer, cause
turbidity. The molybdic precipitate is now washed with but little water
from the filter into a beaker, and particles adhering to the filter are
dissolved by a hot mixture of one part ammonia and three parts water,
which is allowed to flow into the beaker till the precipitate is,
finally, completely dissolved in it. To the clear solution, add dilute
hydrochloric acid while stirring, till the yellow precipitate formed
by the acid is no longer immediately dissolved; then add from six to
eight cubic centimeters of ammonia through the filter. The volume of
the solution is not to exceed seventy-five cubic centimeters. It is now
cooled completely and one cubic centimeter of magnesia mixture is added
from a burette for every centigram of phosphorus pentoxid which it is
expected to contain, and finally one-quarter of its volume of ammonia
is added. The precipitate may be filtered after four hours. This is
washed on the filter, preferably by means of suction, with a mixture of
one part ammonia and three parts water till the filtrate is entirely
free from chlorin. After drying, heat the precipitate, first gently,
then stronger, and finally with a blast for a few minutes and then
weigh it.
Treated with hydrochloric acid it must leave no insoluble residue
(SiO₂), nor should hydrogen sulfid cause any precipitation in the
solution thus formed (MoO₃).
(b) _Total Phosphoric Acid._—1. _In Superphosphates._—For the
determination of total phosphoric acid, treat a weighed quantity of the
superphosphate with nitric acid, if necessary to bring a difficultly
soluble residue into solution, with addition of hydrochloric acid, or
of potassium chlorate, to destroy organic matter present. Dilute the
solution to a definite volume, and determine the phosphoric acid in a
measured quantity of the same, as directed under (a) 2; if hydrochloric
acid or potassium chlorate, be applied in the preparation of the
solution, however, not till the measured quantity has been repeatedly
evaporated to dryness with concentrated nitric acid.
2. _In Bone-meal._—Destroy organic matter in five grams of the sample
by ignition, dissolve the residue in nitric acid, filter from the
insoluble residue, dilute the filtrate to half a liter, take an aliquot
part containing about one-tenth gram phosphorus pentoxid and determine
the phosphoric acid as directed under (a) 2.
3. _In Fish-guano_ (and other fertilizing materials of organic
origin).—The organic matter cannot here be removed by simple ignition,
as in this way a loss of phosphorus may take place; It is therefore
destroyed either in the wet way through nitric acid and potassium
chlorate or in the dry way by fusion with a mixture of potassium
nitrate and sodium carbonate, otherwise the procedure is as in (b) 1.
4. _In Mineral Phosphates._—Determine the phosphoric acid in a solution
obtained by nitric acid; organic matter is destroyed preferably in the
wet way.
5. _In Basic Slag._—Dissolve ten grams of powdered slag by treating it
with 100 cubic centimeters of fuming hydrochloric acid with heat; wash
the solution into a graduated half liter flask, fill to the mark, shake
well, and filter. Determine the phosphoric acid in twenty-five cubic
centimeters of the clear filtrate, according to (a) 2, after having
first, however, evaporated the solution to dryness and then at least
three times evaporated the residue to dryness with concentrated nitric
acid.
=59. Method Employed by the Royal Experiment Station of Holland.=—_A.
Soluble Phosphoric Acid._[44]—The necessary reagents are:
(1) Molybdate solution, made by dissolving 150 grams of ammonium
molybdate in a liter of water and pouring the solution into a liter of
nitric acid of 1.20 specific gravity.
(2) A ten per cent solution of ammonium nitrate.
(3) Strong and dilute ammonia, the latter being between two and
five-tenths and three per cent of 0.988 specific gravity.
(4) Magnesia mixture made by dissolving 110 grams of crystallized
magnesium chlorid, 140 grams of ammonium chlorid, and 700 cubic
centimeters of ammonia of 0.96 specific gravity in water and bringing
the solution to two liters.
(5) Ammoniacal citrate solution, made by dissolving 500 grams of citric
acid in a liter of water, and mixing with four liters of ten per cent
ammonia of 0.96 specific gravity.
_Manipulation._—Place twenty grams of substance in a mortar together
with some cold distilled water or pure rain water, stir, and decant
the water and suspended matters into a liter flask. After this has
been repeated several times, rub up the residual mass and wash it all
into the flask. Fill up to about 900 cubic centimeters and allow to
stand two hours (twenty-four hours in the case of double phosphates
with more than twenty-two per cent of soluble phosphoric acid),
shaking repeatedly; or shaking continuously, for half an hour. Fill
up to the liter mark and filter through a dry filter. Take portions
of twenty-five or fifty cubic centimeters for each determination, add
100 cubic centimeters of molybdate solution for each 100 milligrams of
phosphorus pentoxid present, warm to about 80° for an hour, filter, and
wash the precipitate with the ammonium nitrate solution. Add a little
molybdate solution to the filtrate, warm, and, if a fresh precipitate
be observed, it is to be added to the first. The precipitate is to
be dissolved in ammonia, and hydrochloric acid carefully added until
the precipitate caused by it only slowly redissolves on stirring. The
phosphoric acid is precipitated from the clear liquid which is still
ammoniacal with magnesia mixture, using ten cubic centimeters for each
100 milligrams of phosphorus pentoxid present. This is added, drop
by drop, and the liquid kept stirred during the addition. Allow it
to stand at least two hours, filter, wash with dilute ammonia, dry,
and ignite. This last is done at first with a very small flame but is
finished with the blast-lamp or in a Rössler furnace. To insure burning
to whiteness, nitric acid may be used, but not more than one or two
drops.
_B. Total Phosphoric Acid._—(1) For bone and flesh-meal, fish-guano,
and similar fertilizers the reagents necessary are the same as before.
Carefully burn five grams to ash, boil the ash for half an hour with
nitric acid of 1.32 specific gravity, dilute with water, and, after
cooling, dilute to 500 cubic centimeters. Filter through a dry filter
and take fifty cubic centimeters of the filtrate. Add 100 cubic
centimeters of the molybdate solution for each 100 milligrams of
phosphorus pentoxid present. Treat further as before described.
(2) Phosphates, guanos, bone-black, etc.
One gram of substance, after powdering, and, if necessary, igniting,
is covered with four cubic centimeters of hydrochloric acid of 1.13
specific gravity and a little water and heated for an hour and a half.
Evaporate to dryness without filtration, making repeated additions of
nitric acid until no more vapors of hydrochloric acid are evolved. Boil
the residue with nitric acid, cool, make up to 100 cubic centimeters
with water, and shake. Filter and treat fifty cubic centimeters of
the resulting solution by the molybdate method and proceed further as
before described.
=60. Sources of Error in the Molybdate Method.=—When conducted with
proper care, the gravimetric molybdate method is one of the most exact
processes known to analytical chemistry.
There are, however, some sources of error in the process which should
be avoided as carefully as possible or taken into account.
1. _Error Due to Occluded Silica._—When silica passes into solution in
the original sample, and this may be the case especially with mineral
phosphates, it may appear both in the yellow precipitate and in the
final magnesium pyrophosphate. In all such cases the residue, after
ignition, should be dissolved in hydrochloric acid, and any insoluble
residue weighed as silica and deducted from the first weight. If the
silica be removed by evaporating the solution of the original material
to dryness, and igniting to destroy organic matter, care must be taken
to reconvert all phosphoric acid into the ortho form by long boiling
with nitric acid before precipitation.
Another method of avoiding any trouble from silica consists in using
sulfuric and a little nitric acid as the solvent for the original
substance. Silica is not soluble in hot concentrated sulfuric acid. The
volume of the sulfuric should be about ten times that of the nitric
acid used, and the boiling be continued until sulfuric vapors are
evolved.
2. _Error Due to Arsenic._—Only in rare cases will arsenic be found in
phosphatic fertilizing materials. In case of pyritic phosphates, the
iron disulfid may carry arsenic. The solution in such a case is best
accomplished in hydrochloric acid. If aqua regia be used, all nitric
acid should be removed by repeated evaporation with hydrochloric. The
arsenic can then be precipitated in the hot dilute hydrochloric acid
solution by hydrogen sulfid.
3. _Error Due to Occluded Magnesia._—The danger of contamination of
the yellow precipitate with magnesium oxid has been pointed out by
some authors. The re-solution of the precipitate followed by a second
precipitation is the usual remedy proposed. Lorenz states that this
source of error may be entirely avoided by the addition of two per cent
of citric acid to the phosphomolybdate solution.[45]
4. _Error Due to Volatility of Phosphoric Acid._—This source of error
has been made the subject of a special study by Neubauer.[46]
From the results, a table has been constructed, the use of which is
recommended for phosphoric acid determinations. The source of error
in this method lies exclusively in the loss of phosphoric acid by
volatilization. The magnesia-covered crucible lid offers a very good
control of this error, and its use is recommended to the analyst. Of
course, the presence of sulfur in the gas used for ignition is liable
to disturb this check.
The following course of procedure in the determination of phosphoric
acid can be recommended to avoid or correct this error:
Separate the phosphoric acid in the form of the yellow precipitate
and wash this latter in the usual way. Too high a heat should not be
employed, nor should the solutions be allowed to stand too long lest
excess of molybdic acid separate. Dissolve the phosphomolybdate in 100
cubic centimeters of cold two and five-tenths per cent ammonia and add
as many cubic centimeters of the usual magnesia mixture (fifty-five
grams magnesium chlorid and seventy grams ammonium chlorid dissolved
in a liter of two and five-tenths per cent ammonia) as there are
centigrams of phosphorus pentoxid present. Addition should not be
made faster than ten cubic centimeters per minute. Stir during the
addition. After the precipitation, stir briskly once more and then
allow to stand at least three hours. Wash with two and five-tenths per
cent ammonia till the chlorin reaction disappears, dry the filter,
and introduce into a well-cleaned crucible which has been thoroughly
ignited. Place the lid at an angle, carbonize the filter, and
gradually raise the heat, though not higher than a medium red heat,
till the pyrophosphate becomes completely white. When this happens
bring the blast into action and ignite to constant weight. The weight
finally accepted must not change even after half an hour’s ignition.
Upon this requirement especial stress must be laid. Pure magnesium
pyrophosphate does not suffer any loss even after several hours’
ignition nor does a good platinum crucible. To the weighed amount of
pyrophosphate, add the correction given in the table. For example, if
the weight be 250 milligrams, the correction to be added is four and
two-tenths milligrams, and the correct weight is then 254.2 milligrams.
Multiplication of the sum by sixty-four gives the amount of phosphorus
pentoxid in the weight taken for analysis.