(1) _Soluble Acid._—The soluble phosphates are first brought into
solution in such a way that one liter of water contains the soluble
phosphoric acid from twenty grams of the substance. Twenty grams are
rubbed in a porcelain mortar with water and through a wide-necked
funnel washed into a bottle-shaped flask in which a little water has
been previously placed. The flasks employed are made of thick glass
in order to withstand shaking. After the substance is washed, the
flasks are filled to the mark and closed with rubber stoppers. They are
then placed upon a shaking rack as indicated in Fig. 3, which is also
furnished with an apparatus for separating the fine meal from basic
slag.
On a table, as shown in the figure, is fastened a movable horizontal
board by means of hinges. At the left hand of this movable board is
placed an open wooden box in which is a perforated shelf for the
purpose of holding the flasks, so as to prevent their striking together
during the shaking.
For the best results the substance to be examined should be placed
in the flask in a dry state and then 800 cubic centimeters of water
added and shaken by means of the machine indicated for half an
hour. Afterwards the flasks are filled up to the mark, well shaken,
and filtered through double folded filters into ordinary flasks of
about 400 cubic centimeters capacity. Before any of the filtrate is
collected, the first that runs through should be well shaken in the
receiving flasks and rejected. Fifty cubic centimeters of the filtrate
thus collected, corresponding to one gram of the substance, should be
used for the determination.
(2) _Total Acid._—For total phosphoric acid, including the insoluble
portions, the material is treated as follows: Five grams of the
substance are placed in a 500 cubic centimeter flask with twenty cubic
centimeters of nitric acid of 1.42 specific gravity, and fifty cubic
centimeters of pure concentrated sulfuric acid, and boiled briskly for
half an hour. With substances which contain much organic material,
a little paraffin is added to avoid frothing. Such substances also
require a larger quantity of nitric acid than that above specified. The
flasks are allowed to cool, water added, again allowed to cool, and
filled up to the mark at 17°.5. If hydrochloric instead of sulfuric
acid be used in making the above solution, when the citrate method
is employed, the results are always too high because the precipitate
contains lime and alumina in such quantities as to render any
compensation for them inaccurate. In addition to this the sulfuric
has this great advantage over the hydrochloric acid; _viz._, in not
separating silicic acid, inasmuch as the silicic acid is insoluble in
boiling sulfuric acid.
(3) _Citrate-Soluble Acid._—Two grams of the sample are digested with
100 cubic centimeters of citrate solution, 1.09 specific gravity, for
half an hour at 50° in a beaker. Afterwards the soluble matter is
separated by filtration with the aid of a filter-pump and the residue
washed with a solution of one part water and one part citrate solution
until all the dissolved phosphoric acid is removed from the filter.
Generally three or four washings are sufficient. The residue on the
filter is dried, ignited, and dissolved in a mixture of two cubic
centimeters of nitric and twenty cubic centimeters of sulfuric acid,
the solution made up to a volume of 200 cubic centimeters, filtered,
and 100 cubic centimeters of the filtrate taken for the determination.
The acid in the filtrate is nearly neutralized and fifty cubic
centimeters of citrate solution are added, and afterwards twenty-five
cubic centimeters of magnesia mixture and twenty cubic centimeters of
twenty-four per cent ammonia. After standing for forty-eight hours, the
precipitate is separated by filtration, ignited, and weighed in the
usual way. The difference between the total phosphoric acid and that in
the insoluble residue, after treatment with ammonium citrate, as above,
gives the quantity of phosphoric acid soluble in the citrate solution.
The difference between the total citrate-soluble and the water-soluble
gives the quantity of the reverted phosphoric acid.
The ammonium citrate solution used for the digestion is made as
follows: Two hundred and fifty grams of crystallized citric acid
are dissolved in half a liter of hot water, diluted with 550 cubic
centimeters of water, 276 cubic centimeters of twenty-four per cent
ammonia added, and finally, exactly neutralized by adding, little by
little, fifty per cent citric acid solution.
The Halle methods of separating the water and citrate-soluble acids
appear to be less complete and reliable than those in use by the
Official Agricultural Chemists of this country. The precipitation of
basic phosphates, when large quantities of water are used at once in
separating soluble acid, must tend to diminish the quantity obtained,
while the lack of care in assuring the neutrality of the citrate
solution might lead to varying results.
(4) _Double Superphosphates._—In the case of double superphosphates,
which sometimes contain large quantities of pyrophosphate, the solution
is made in the usual way so that in 100 cubic centimeters there will
be contained two grams of the substance. Usually ten grains are taken
and the volume made up to half a liter. Twenty-five cubic centimeters
of the filtrate are diluted with seventy-five cubic centimeters of
water and the pyro converted to orthophosphoric acid by heating with
ten cubic centimeters of strong nitric acid on a sand-bath. The heating
should be continued until the volume be reduced to twenty-five cubic
centimeters. The strongly acid liquid is made alkaline with ammonia,
and afterwards slightly acid with nitric, and the rest of the process
is carried on in the usual way.
(5) _Phosphoric Acid in the Residue of Superphosphate Manufacture._—In
the mixture of superphosphates and gypsum, the residue of the
manufacture of double superphosphates, the phosphoric acid is estimated
in the following manner: Five grams of the substance are placed in a
dish, rubbed up with absolute alcohol, and washed into a 250 cubic
centimeter flask. The flask is filled with absolute alcohol to the
mark, closed with a stopper, and with frequent shaking, allowed to
stand for two hours; it is thereupon filtered as quickly as possible;
fifty cubic centimeters of the filtrate corresponding to one gram
of the substance, are taken for the estimation. This fifty cubic
centimeters is evaporated on a sand-bath to a sirupy consistence,
diluted with water, and treated, as in the case of the soluble
phosphates above mentioned. In all cases as described above, after
the solutions are obtained they are treated with the ammonium citrate
solution and the phosphoric acid estimated as in the first instance
given.
(6) _Solutions Employed._—
(a) The citrate solution is made as follows: 1,500 grams of
citric acid are dissolved in water, treated with five liters
of twenty-four per cent ammonia, and made up to fifteen liters.
(b) The magnesia mixture is made as follows: 500 grams of
magnesium chlorid, 1,050 grams of ammonium chlorid, three
and five-tenths liters of twenty-four per cent ammonia, and
six and five-tenths liters of distilled water are used.
In the case of the superphosphates fifty cubic centimeters of the
citrate solution are employed and with the basic slags 100 cubic
centimeters; and in both cases twenty-five cubic centimeters of the
magnesia mixture.
(7) _Details of the Manipulation._—On the addition of the citrate
solution there should be no permanent troubling of the liquid but
there should be a total clearing up thereof. In order to facilitate
this, after the addition of the citrate solution, the flasks should be
gently shaken in order to distribute the solution throughout the mass.
Solutions from bone-black superphosphates show sometimes, after the
addition of the citrate solution, a more or less strong opalescence,
but this opalescence does not influence the results. Should it
happen that with superphosphates which are made from raw material
containing large excesses of iron or clay, fifty cubic centimeters of
the citrate solution are not sufficient to prevent the other bases
from being precipitated, an additional quantity up to twenty-five
cubic centimeters may be added. The addition of the magnesia mixture
must follow as quickly as possible after the addition of the citrate
solution to avoid a separation of crystalline calcium phosphate. On the
addition of the citrate solution there is always a rise in temperature.
Inasmuch as the precipitation of the phosphoric acid with magnesia must
take place in the cold, the liquid must be cooled after the addition
of the citrate,[54] and the cooling should take place as quickly as
possible.
The above method was adopted by the chemical section of the
International Agricultural Congress held at Vienna, September, 1890.[55]
[Illustration: FIGURE 4.