=115. Desirability Of Methods.=—In the preceding paragraphs, has been
given a statement of the principal methods now in use by chemists
and others connected with fertilizer control for the scientific and
agronomic determinations of phosphoric acid, and its agricultural value.
A résumé of the important methods, in a form suited to use in a factory
for preparing phosphatic fertilizers for the market, seems desirable.
In these factories the chemists have been accustomed to use their
own, or private methods, and there has not been a general disposition
among them to publish their methods and experience for the common
benefit. For factory processes, a method should be not only reasonably
accurate, but also simple and rapid. It is evident, therefore, that
the general principles already indicated must underlie any method
which would prove useful to factory work. Albert has made a résumé of
such methods applicable for factory control, and these are given here
for convenience, although they are, in many respects, but condensed
statements of methods already described.[98]
=116. Reagents.=—_Molybdate Solution._—One hundred and ten grams of
pure molybdic acid are dissolved in ammonia of nine-tenths specific
gravity and diluted with water to one liter. The solution is poured
into one liter of nitric acid, of one and two-tenths specific gravity,
and, after standing a few days, filtered.
_Concentrated Ammonium Nitrate Solution._—Seven hundred and fifty grams
of pure ammonium nitrate are dissolved in water and made up to one
liter.
_Magnesia Mixture._—Fifty-five grams of magnesium chlorid; seventy
grams of ammonium chlorid; 130 cubic centimeters of ammonia of
nine-tenths specific gravity are dissolved and diluted with water to
one liter.
_Two and One-Half Per Cent Ammonia._—One hundred cubic centimeters of
ammonia of nine-tenths specific gravity are diluted with water to one
liter.
_Joulie’s Citrate Solution._—Four hundred grams of citric acid are
dissolved in ammonia of nine-tenths specific gravity and diluted to one
liter with ammonia of the same strength.
_Wagner’s Citrate Solution._—One hundred and fifty grams of citric acid
are exactly neutralized with ammonia, then ten grams of citric acid
added and diluted to one liter with water.
_Sodium Acetate Solution._—One hundred grams of sodium acetate,
crystallized, are dissolved in water, treated with 100 cubic
centimeters of acetic acid, and diluted to one liter with water.
_Calcium Phosphate Solution._—About ten grams of dry, pure tribasic
calcium phosphate are dissolved in nitric acid and diluted with water
to one liter. In this solution the phosphoric acid is determined
gravimetrically by the molybdate or citrate method, and the value of
the solution marked on the flask containing it.
_Titrated Uranium Solution._—Two hundred and fifty grams of uranium
nitrate are dissolved in water, twenty-five grams of sodium acetate
added, and the whole diluted to seven liters. One cubic centimeter of
this solution corresponds to about 0.005 gram of phosphorus pentoxid.
In order to determine its exact value proceed as follows: Twenty-five
cubic centimeters of the calcium phosphate solution which, for example,
has been found to contain 0.10317 gram of phosphorus pentoxid, are
neutralized in a porcelain dish with ammonia, acidified with acetic,
treated with ten cubic centimeters of sodium acetate solution, and
warmed. Through a burette as much uranium solution is allowed to flow
as is necessary to show in a drop of the solution taken out of the
dish, when treated with a drop of pure potassium ferrocyanid, a slight
brown color. In order to be certain, this operation is repeated two or
three times with new quantities of twenty-five cubic centimeters of
calcium phosphate solution. Example:
Twenty-five cubic centimeters of the calcium phosphate solution
containing 0.10317 gram of phosphorus pentoxid, gave as a mean of three
determinations 23.2 cubic centimeters of the uranium solution necessary
to produce the brown color with potassium ferrocyanid. Consequently
0.10317 ÷ 23.2 = 0.00445 gram of phosphorus pentoxid equivalent
to one cubic centimeter of uranium solution. If, for instance, a
quantity of fertilizer weighing exactly five grams, require ten cubic
centimeters of the uranium solution for the complete precipitation of
its phosphoric acid, then the quantity of phosphoric acid contained in
the fertilizer would be equivalent to 10 × 0.0045, equivalent to 0.0445
gram of phosphorus pentoxid. The fertilizer, therefore, contains eight
and nine-tenths per cent of phosphorus pentoxid.
_Conduct of the Molybdenum Method._—This method rests upon the
precipitation of the phosphorus pentoxid by a solution of ammonium
molybdate in nitric acid, solution of the precipitate in ammonia, and
subsequent precipitation with magnesia.
_Manipulation._—Twenty-five or fifty cubic centimeters of a solution
of the phosphate which has been made up to a standard volume and
containing about one-tenth gram of phosphorus pentoxid, are placed
in a beaker together with 100 cubic centimeters of the molybdate
solution and treated with as much ammonium nitrate solution as will be
sufficient to give the liquid a content of fifteen per cent of ammonium
nitrate. The contents of the beaker are well mixed and warmed for about
twenty minutes at from 60° to 80°. After cooling, they are filtered and
the precipitate washed on the filter with cold water until a drop of
the filtrate saturated with ammonia does not become opaque on treatment
with ammonium oxalate. The filtrate is then washed from the filter with
two and one-half per cent ammonia solution and precipitated slowly and
with constant stirring by the magnesia mixture. After standing for two
hours the ammonium magnesium phosphate is separated by filtration,
washed with two and one-half per cent ammonia until the filtrate
contains no more chlorin, and ignited.
_Conduct of the Citrate Method._—The principle of this method depends
upon the fact that when a sufficient quantity of ammonium citrate is
added to phosphate solutions, iron, alumina, and lime are retained in
solution when, on the addition of the magnesia mixture in the presence
of free ammonia, the phosphoric acid is completely precipitated as
ammonium magnesium phosphate.
_Manipulation._—From ten to fifty cubic centimeters of the solution
of the phosphate to be determined are treated with fifteen cubic
centimeters of the Joulie citrate solution avoiding warming. A few
pieces of filter-paper, the ash content of which is known, are thrown
in and, with stirring, fifteen cubic centimeters of magnesia mixture
slowly added and if necessary also some free ammonia. By the small
pieces of filter-paper the collection of the precipitate against
the sides of the vessel and on the stirring rod is prevented and in
this way the production of the precipitate hastened. After standing
from one-half an hour to two hours the mixture is filtered, ignited,
and weighed. If it be preferred to estimate the phosphoric acid by
titration, the precipitate is dissolved in a little nitric acid, made
slightly alkaline with ammonia, and then acid with acetic and then
afterwards titrated with the standard uranium solution.
_Conduct of the Uranium Method._—The principle upon which this
method rests depends upon the fact that uranium nitrate or acetate
precipitates uranium phosphate from solutions containing phosphoric
acid and which contain no other free acid except acetic. In the
presence of ammonium salts the precipitate is uranium ammonium
phosphate having the formula PO₄NH₄UrO₂. The smallest excess of soluble
uranium salt is at once detected by the ordinary treatment with
potassium ferrocyanid.
_Manipulation._—In all cases the solution is first made slightly
alkaline with ammonia and then acid by a few drops of acetic, so that
no free mineral acid may be present.
(1) _With liquids free from iron_:
If, on the addition of ammonium or sodium acetate, no turbidity be
produced, the liquid is free from iron and alumina. In this case
from ten to fifty cubic centimeters of the solution containing about
one-tenth gram of phosphorus pentoxid are treated with ten cubic
centimeters of sodium acetate, and afterwards with a quantity of
uranium solution corresponding, as nearly as possible, to its supposed
content of phosphorus pentoxid, and heated to boiling. From the heated
liquid by means of a glass rod, one or two drops are taken and placed
upon a porcelain plate and one drop of a freshly prepared solution of
potassium ferrocyanid allowed to flow on it. If no brown color be seen
at the point of contact of the two drops, additional quantities of
the uranium solution are added and, after boiling, again tested with
potassium ferrocyanid until a brown color is distinctly visible. The
quantity of the uranium solution thus having been determined, duplicate
analyses can be made and the whole quantity of the uranium solution
added at once with the exception of the last drops, which are added as
before.
(2) _Solutions containing iron and alumina._
The solution is treated with the ammonium citrate solution of Joulie,
the magnesia mixture added slowly, and the precipitate collected
on a filter and washed with two and one-half per cent ammonia. The
precipitate is then dissolved in nitric acid, made alkaline with
ammonia, and then acid with acetic. This solution is then treated with
ten cubic centimeters of sodium acetate and titrated with uranium, as
described in (1). As an alternative method, 200 cubic centimeters of
the superphosphate solution may be treated with fifty cubic centimeters
of sodium acetate, allowed to stand for some time, and filtered through
a filter of known ash content. In fifty cubic centimeters of the
filtrate, which correspond to forty cubic centimeters of the original
solution, phosphoric acid may be determined as described above. The
precipitate, consisting of iron and aluminum phosphates, is washed
three times on the filter with boiling water, dried, and ignited in a
platinum dish. The weight of ignited precipitate, diminished by the
weight of the ash contained in the filter and divided by two, gives the
quantity of phosphorus pentoxid which it is necessary to add to that
obtained by titration.
=117. Determination of the Phosphoric Acid in all Phosphates and Basic
Slags.=—
(1) _Total phosphoric Acid_:
Five grams of the fine phosphate meal, or slag meal, are moistened in
a flask of 500 cubic centimeters content with some water and boiled on
a sand-bath with forty cubic centimeters of hydrochloric acid of from
16° to 20° Beaumé. The boiling is continued until only a few cubic
centimeters of a thick jelly of silicic acid remain. After cooling,
some water is added and the phosphate shaken until the thick lumps
of silica are finely divided. The flask is then filled to 500 cubic
centimeters and its contents filtered. Fifty cubic centimeters of the
filtrate are treated with fifteen cubic centimeters of the Joulie
solution and treated in the manner described with magnesia mixture,
precipitated, ignited, and weighed. The precipitate can also be
dissolved and treated with uranium solution as described.
The method used by Oliveri may also be employed and it is carried out
as indicated in the following description:[99]
A weighed quantity of the slag is reduced to a fine powder. To five
grams of the sample is added three times its weight of potassium
chlorate and the whole is intimately mixed. The mixture is then placed
in a porcelain dish and hydrochloric acid is added, little by little,
until the potash salt is completely decomposed. It is evaporated until
the mass is dry. The material is then treated with fuming nitric
acid, and the determination of the phosphorus is made by the ordinary
gravimetric method.
By carrying on the operation as described above, a reduction of
phosphoric acid is avoided, and the presence of an abundant quantity
of potash prevents the formation of basic iron phosphate which is
insoluble in nitric acid.
(2) _Citrate-Soluble Phosphoric Acid._—One gram of the basic slag
or phosphate is placed in a 100 cubic centimeter flask and covered
with Wagner’s acid citrate solution making the total volume up to
100 cubic centimeters. With frequent shaking the flask is kept at
40° for an hour, or it may be allowed to stand for twelve hours at
room temperature with frequent shaking. In fifty cubic centimeters
of the filtrate from this flask the phosphoric acid is determined by
the magnesia mixture as described. Since, in the present case, the
precipitate of ammonium magnesium phosphate contains some silicic
acid it cannot be directly ignited but must be treated in the
following manner: The precipitate and the filter are thrown into a
porcelain dish, the filter-paper torn up into shreds with a glass
rod, the precipitate dissolved in nitric acid, neutralized with
ammonia, acidified with acetic, and treated with uranium solution.
The phosphoric acid may also be estimated by the gravimetric method
by dissolving the precipitate again in hydrochloric or nitric acid,
evaporating to dryness, and drying for one hour at from 110° to 120°,
dissolving again in hydrochloric acid, filtering, and washing the
precipitate well. The filtrate, which is now free from silica, can be
treated with Joulie’s solution, precipitated with magnesia mixture, the
precipitate washed, ignited, and weighed as described. The molybdenum
method is preferred in the estimation of citrate-soluble phosphoric
acid, especially in slags. For this purpose fifty cubic centimeters
of the filtrate from the solution of one gram of slag in 100 cubic
centimeters of Wagner’s citrate liquid are treated with 100 cubic
centimeters of molybdenum solution and thirty cubic centimeters of
ammonium nitrate solution, warmed for twenty minutes at 80°, filtered
after cooling, and the yellow precipitate washed with cold water. The
water will gradually dissolve all the silicic acid from the yellow
precipitate and carry it into the filtrate. The yellow precipitate
is then dissolved in two and one-half per cent liquid ammonia and
precipitated with magnesia mixture and the precipitate washed, ignited,
and weighed in the way described.
=118. Determination of Phosphoric Acid in Superphosphates.=—(1)
_Citrate-Soluble Phosphoric Acid._—Five grams of the superphosphate are
rubbed with 100 cubic centimeters of Wagner’s acid citrate solution in
a mortar and washed into a flask of 500 cubic centimeters content and
diluted to 500 cubic centimeters with water. With frequent shaking the
flask is allowed to stand for twelve hours, after which its contents
are filtered. Fifty cubic centimeters of the filtrate are treated
with ten cubic centimeters of the Joulie solution and fifteen cubic
centimeters of the magnesia mixture and, if necessary, made distinctly
alkaline with ammonia, vigorously stirred, and, after two hours,
filtered. The precipitate is washed, ignited, and weighed as described,
or titrated, after solution in nitric acid and the addition of sodium
acetate, with uranium solution.
Example:
The weighed precipitate has 0.1272 gram Mg₂P₂O₇ then the phosphate
contains 12.72 × 2 × 0.64 = 16.28 per cent of citrate-soluble P₂O₅.
(2) _Water-Soluble Phosphoric Acid._—Twenty grams of superphosphate
are rubbed in a mortar and washed into a flask of one liter content
and made up to the mark with water. After two hours’ digestion with
frequent shaking, the contents of the flask are filtered through a
folded filter. Twenty-five cubic centimeters of the filtrate equivalent
to five-tenths gram of the substance are precipitated with magnesia
mixture, the precipitate filtered, washed, ignited, and weighed, or the
moist filtrate may be dissolved upon the filter with a little nitric
acid, treated with sodium acetate, and titrated, as described, with
uranium solution.
Example: 14.5 cubic centimeters of the uranium solution are required
for the precipitate from twenty-five cubic centimeters of the original
solution = 0.5 gram superphosphate; it contains then 14.5 × 0.00445 =
0.0645 gram P₂O₅. Consequently the superphosphate contains 12.90 per
cent of water-soluble P₂O₅.
_Total Phosphoric Add._—Twenty grams of the superphosphate are boiled
with fifty cubic centimeters of hydrochloric acid of from 16° to 18°
Beaumé for about ten minutes and, after cooling, made up to one liter
with water and filtered. Twenty-five cubic centimeters of the filtrate
are treated with ten cubic centimeters of Joulie’s citrate solution,
a few pieces of filter-paper thrown in, fifteen cubic centimeters
of magnesia mixture added, and the whole thoroughly stirred. After
standing two hours the contents of the flask are filtered and the
precipitate is washed with dilute ammonia and the filter and the
precipitate are placed in a platinum crucible. The crucible is heated
slowly until the moisture is driven off and the filter burned. Then the
temperature is gradually raised to a white heat. The residue is cooled
and weighed. Example:
The precipitate weighs, after the subtraction of the filter ash, 0.1390
gram; then the superphosphate contains 13.90 × 2 × 0.64 = 17.79 per
cent phosphoric acid.