=87. The Uranium Method.=—Since the phosphoric acid of practical use
for agricultural purposes is nearly always combined with lime, alumina,
and iron, its volumetric estimation by means of a standard solution of
a uranium salt is to be preceded by a preliminary separation by means
of an ammoniacal magnesium citrate solution. The principle of the
method was almost simultaneously published by Sutton,[77] Neubauer,[78]
and Pincus.[79] The phosphoric acid may also be separated by means of
molybdic solution or by tin or bismuth.[80] In practice, however, it
has been found that when the uranium method is to be used the magnesium
citrate separation is the most convenient. Since this is the method
practiced almost universally in France, the method there used will be
given in detail. It is based essentially on the process described by
Joulie.[81]
=88. Preparation of Sample.=—(1) _Incineration._—Since the organic
matters present in a phosphatic fertilizer often interfere with the
employment of uranium as a reagent, it is necessary to incinerate the
sample taken for analysis.[82]
(2) _Solution of the Material._—All phosphates, with the exception
of certain aluminum phosphates, amblygonite for example, are easily
dissolved in nitric and hydrochloric acids more or less dilute,
especially on ebullition. The best solvent, however, for calcium
phosphates for the uranium method is incontestably hydrochloric acid
which also very easily dissolves the iron and aluminum phosphates,
which are often found present with calcium phosphates.
(3) _Nitric Acid._—In many laboratories nitric acid is preferred in
order to avoid, in part, the solution of ferric oxid which interferes
with the determination of phosphoric acid in certain processes. Since
it does not act in this way for the citro-magnesium uranium method,
it is preferable to employ hydrochloric acid, especially because it
dissolves the iron completely and permits thus the operator to judge of
the success of the solvent action by the completely white color of the
residue.
(4) _Pyritic Phosphates._—Certain phosphates contain pyrites which
hydrochloric acid does not dissolve, and there is left consequently,
a residue more or less colored. In this case it is necessary to add
some nitric acid and to prolong the boiling until the pyrite has
disappeared, since it might retain a small quantity of phosphoric acid
in the state of iron phosphate.
(5) _Sulfuric Acid._—Some chemists decompose the phosphates by means
of dilute sulfuric acid. This method, which is certainly able to give
good results for certain products and for certain processes, presents
numerous inconveniences which tend to render its use objectionable for
volumetric purposes. The calcium sulfate which is formed, requires
prolonged washings which lead to chances of fatal error.
If an aluminum phosphate be under examination, containing only very
little or no lime, sulfuric acid is to be preferred to hydrochloric and
nitric acids, since it attacks amblygonite, which, as has been before
stated, resists the action of the other two acids. But these are cases
which are met with very rarely, and which can always be treated by the
general method by previously fusing the material with a mixture of
sodium and potassium carbonate.
In the great majority of cases the decomposition by hydrochloric acid
is very easily accomplished by simply boiling in a glass vessel, and
without effecting the separation of the silica. This operation is only
necessary after the substance has been fused with alkaline carbonates,
or, in case of substances which contain decomposable silicates giving
gelatinous silica with hydrochloric acid.
There are two methods [see (6) and (7)] of securing a solution of
the sample taken which varies from one to five, and even ten grams,
according to the apparent homogeneity of the material to be analyzed.
(6) _Solution by Filtration and Washing._—The ordinary method can
be employed consisting in decomposing the substance by an acid,
filtering, and washing the residue upon the filter, and combining all
the wash-waters to make a determinate volume. Afterwards an aliquot
fraction of the whole is taken for the precipitation. This method is
long, and presents some chances of error, when the insoluble residue is
voluminous and contains silica which obstructs the pores of the paper
and renders the filtration difficult.
(7) _Volumetric Solution._—It is advisable to substitute volumetric
solution for solution by filtration and washing, which is accomplished
by decomposing the substances in a graduated flask, the volume being
afterwards made up to the mark with distilled water after cooling. The
solution is then filtered without washing, and by means of a pipette an
aliquot part of the original volume is taken for precipitation. Thus
all retardations in the process are avoided, and likewise the chances
of error from washing on the filter. It is true that this method may
lead to a certain error due to the volume of the insoluble matter
which is left undecomposed, but since this insoluble matter is usually
small in quantity, and since it is always possible to diminish the
error therefrom by increasing the volume of the solution, this cause
of error is much less to be feared than those due to the difficulties
which may occur in the other method. Let us suppose, in order to
illustrate the above, that we are dealing with a phosphate containing
fifty per cent of insoluble sand which may be considered as an extreme
limit. In working on four grams of the material in a flask of 100
cubic centimeters capacity, there will be an insoluble residue of two
grams occupying a volume of about one cubic centimeter, the density of
the sand being generally nearly two. The one hundred cubic centimeter
flask will then contain only ninety-nine cubic centimeters of the real
solution, and the error at the most would be 0.01. This error could be
reduced to one-half by dissolving only two grams of the material in
place of four, or by making the volume up to 200 instead of 100 cubic
centimeters.
In general it may be said that the errors which do not exceed 0.01 of
the total matter under treatment, are negligible for all industrial
products. The method of volumetric solution does not present any
further inconvenience. It deserves to be and has been generally adopted
by reason of its rapidity in all the laboratories where many analyses
are to be made. In the volumetric method great care should be taken not
to make up to the volume until after the cooling to room temperature,
which may be speedily secured by immersing the flask in cold water.
Care should also be exercised in taking the sample for analysis by
means of the pipette immediately after filtration, and filtration
should take place as soon as the volume is made up to the standard. By
operating in this way the possible variations from changes of volume
due to changes of temperature are avoided.
(8) _Examination for Arsenic Acid._—When the sample examined contains
pyrites, arsenic is often present. When the decomposition has been
effected by means of nitric acid, arsenic acid may be produced. This
deports itself in all circumstances like phosphoric acid, and if it is
present in the matter under examination it will be found united with
the phosphoric acid and determined therewith afterwards. It is easy
to avoid this cause of error by passing first a current of sulfurous
acid through the solution, carrying it to the boiling-point in order to
drive out the excess of sulfurous acid, and afterwards precipitating
the arsenic by a current of hydrogen sulfid. After filtration, the rest
of the operation can be carried on as already described.
=89. Precipitation of the Phosphate by Magnesium Citrate.=—By means of
an accurate pipette a quantity of the solution representing from 0.125
to 0.250 gram or more is taken, according to the presumed richness of
the product to be examined. In order that the following operations
may go on well, it is necessary that the quantity of phosphoric acid
contained in the sample should be about fifty milligrams. The sample
being measured is run into a beaker, and there are added, first, ten
cubic centimeters of magnesium citrate solution, and second, a large
excess of ammonia. If the quantity of the magnesium citrate solution
be sufficient, the mixture should at first remain perfectly limpid and
only become turbid at the end of some moments and especially after the
mixture is stirred.
If there should be an immediate turbidity produced it is proof that the
quantity of magnesium citrate solution employed has been insufficient,
and it is necessary to begin again by doubling its amount. Good results
cannot be obtained by adding a second portion of the magnesium citrate
solution to the original, since the iron and aluminum phosphates which
are once formed are redissolved with difficulty. Many chemists at the
present time abstain from using the magnesium citrate solution and
replace it by a solution of citric acid and one of magnesium sulfate,
which they pour successively into the sample under examination. This
is a cause of grave errors which it is necessary to point out. Joulie
has indeed recognized the fact that the precipitation of the phosphoric
acid is not completed in presence of ammonium citrate except it is
employed in conjunction with a sufficient excess of magnesia. But the
foreign matters which accompany the phosphoric acid require different
quantities of ammonium citrate in order to keep them in solution, and
it is important to increase the magnesium solution at the time of
increasing the citric acid in order to maintain them always in the same
proportion. This is easily accomplished by measuring the two solutions,
but it is much more easily done by uniting them and adding them
together.
=90. The Magnesium Citrate Solution.=—The formula originally proposed
by Joulie, and modified by Millot, and adopted by the French
Association of Chemists, is as follows: Citric acid, 400 grams; pure
magnesium carbonate, forty grams; caustic magnesia, twenty grams;
distilled water, half a liter. After solution, add enough of ammonia
to render strongly alkaline, requiring about 600 cubic centimeters.
Make the volume up with distilled water to one and a half liters. If
the solution be turbid, it is proof that the magnesia or the carbonate
employed contains some phosphoric acid which is to be separated by
filtration, and the solution can then be preserved indefinitely.
=91. Time of Subsidence.=—When the phosphoric acid is precipitated by
the mixture above mentioned, it is necessary to allow it to subside for
a certain time under a bell jar in order to avoid the evaporation of
the ammonia. In order to give plenty of time for this subsidence, it is
well to make the precipitations in the afternoon and the filtrations
the following morning. There are thus secured twelve to fifteen hours
of repose, which is time amply sufficient for all cases.
=92. Filtration and Washing.=—Filtration is performed easily and
rapidly upon a small filter without folds placed in a funnel with a
long stem of about two millimeters internal diameter. Placed in a
series of six or eight, they allow the filtration to take place in
regular order without loss of time, the first filter being always
empty by the time the last one is filled. The supernatant liquid from
the precipitate should first be decanted on the filter, avoiding the
throwing of the filtrate on the filter which would greatly retard the
process, especially if it should contain a little silica, as often
happens.
When the clear liquid is thus decanted as completely as possible, the
rest of the precipitate is treated with water to which one-tenth of
its volume of ammonia has been added, and the washing is continued by
decantation as at first, and afterwards by washing upon the filter
until the filtered solution gives no precipitate with sodium phosphate.
Four washings are generally sufficient to attain this result.
If the operations which precede have been well-conducted, the total
phosphoric acid contained in the material under examination is
found upon the filter-paper, except the small portion which remains
adhering to the beaker in which the precipitation has been made.
The determination of the phosphoric acid comprises the following
operations: First, solution of the ammonium magnesium phosphate and
second, titration by means of a standard solution of uranium.
=93. Solution of the Ammonium Magnesium Phosphate.=—The phosphate which
has been collected upon the filter is dissolved by a ten per cent
solution of pure nitric acid. This solution is caused to pass into
the beaker in which the precipitation was made in order to dissolve
the particles of phosphate which remain adherent to its sides; and
this solution is then thrown upon the filter. The filtrate is then
received in a flask of about 150 cubic centimeters capacity, marked at
seventy-five cubic centimeters. After two or three washings with the
acidulated water, the filter itself is detached from the funnel and
introduced into the vessel which contains the solution.
The whole of the filtrate being collected in the flask it is saturated
by one-tenth ammoniacal water until a slight turbidity is produced.
One or two drops of dilute nitric acid are now added until the liquor
becomes limpid, and the flask is placed upon a sand-bath in order to
carry the liquid to the boiling-point. After ebullition there are added
five cubic centimeters of acid sodium acetate in order to cause the
free nitric acid to disappear and immediately the titration, by means
of a standard solution of uranium, is undertaken.
=94. Acid Sodium Acetate.=—The acid sodium acetate is prepared as
follows: Crystallized sodium acetate, 100 grams; glacial acetic acid,
fifty cubic centimeters; distilled water, enough to make one liter.
=95. Standard Solution of Uranium.=—A solution of uranium is to be
prepared as follows: Pure uranium nitrate, forty grams; distilled
water, about 800 cubic centimeters. Dissolve the uranium nitrate in the
distilled water and add a few drops of ammonia until a slight turbidity
is produced, and then a sufficient amount of acetic acid to cause this
turbidity to disappear. The volume is then completed to one liter with
distilled water.
The uranium nitrate often contains some uranium phosphate and some
ferric nitrate. It is important that it be freed from these foreign
substances. This is secured by dissolving it in distilled water and
precipitating it by sodium carbonate, which redissolves the uranium
oxid and precipitates the iron phosphate and oxid.
The filtered liquor is saturated with nitric acid, and the uranium oxid
reprecipitated by ammonia. It is then washed with distilled water by
decantation and redissolved in nitric acid, as exactly as possible,
evaporated, and crystallized.
The crystals are taken up with ether, which often leaves still a little
insoluble matter. The solution is filtered, and the ether evaporated.
The salt which remains is perfectly pure. It frequently happens when
the uranium nitrate has not been properly purified that the solution
prepared as has been indicated above, deposits a light precipitate of
phosphate which alters its strength and affords a cause of error. Only
those solutions should be employed which have been prepared some days
in advance, and which have remained perfectly limpid.
The solution of uranium thus obtained contains uranium nitrate, a
little ammonium nitrate, a very small quantity of uranium acetate, some
ammonium acetate, and a little free acetic acid. Its sensibility is the
more pronounced as the acetates present in it are less in quantity. It
is important, therefore, never to prepare the solution with uranium
acetate.
=96. Typical Solution of Phosphoric Acid.=—In order to titrate a
solution of uranium, it is necessary to have a standard solution of
phosphoric acid; that is to say, a solution containing a precise
and known quantity of that acid in a given volume. This solution is
prepared by means of acid ammonium phosphate, a salt which is easily
obtained pure and dry. Sometimes as it may contain a small quantity
of neutral phosphate which modifies the relative proportions of
phosphoric acid and ammonia, and it is indispensable to have its
strength verified. The titer of the typical solution should be such
that it requires for the precipitation of the phosphoric acid which
it contains, a volume of the solution of uranium almost exactly equal
to its own, in order that the expansions or contractions which the
two liquors undergo, by reason of changes in the temperature of the
laboratory, should be without influence upon the results.
The solution of uranium prepared as has been indicated above,
precipitates almost exactly five milligrams of phosphoric acid per
cubic centimeter; the typical solution of phosphoric acid is prepared
with eight and one-tenth grams of acid ammonium phosphate pure and dry,
which is dissolved in a sufficient quantity of distilled water to make
one liter.
The acid ammonium phosphate containing 61.74 per cent of anhydrous
phosphoric acid, the quantity above gives exactly five grams of that
acid in a liter, or five milligrams in a cubic centimeter.
=97. Verification of the Strength of the Standard Solution of
Phosphoric Acid.=—The strength of the standard solution of phosphoric
acid is verified by evaporating a known volume, fifty cubic centimeters
for example, with a solution of ferric hydroxid containing a known
quantity of ferric oxid. The mass having been evaporated to dryness,
and ignited in a platinum crucible, gives an increase in the weight of
the iron oxid exactly equal to the amount of anhydrous phosphoric acid
contained therein, both the nitric acid and ammonia being driven off by
the heat.
To prepare the solution of ferric hydroxid, dissolve twenty grams of
iron filings in hydrochloric acid. The solution is filtered to separate
the carbon, and it is converted into ferric nitrate by nitric acid,
and the solution diluted with distilled water, and the ferric oxid
precipitated by a slight excess of ammonia. The precipitate, washed by
decantation with distilled water until the wash-water no longer gives a
precipitate with silver nitrate, is redissolved in nitric acid, and the
solution is concentrated or diluted, as the case may be, to bring the
volume to one liter.
In order to determine the quantity of ferric oxid which it contains,
fifty cubic centimeters are evaporated to dryness, ignited, and weighed.
A second operation like the above is carried on by adding fifty cubic
centimeters of the standard solution of phosphoric acid, and the
strength of the solution thus obtained is marked upon the flask.
If the operation have been properly carried on, three or four
duplicates will give exactly the same figures. If there are sensible
differences, the whole operation should be done over from the first.
=98. Titration of the Solution of Uranium.=—In a 150 cubic centimeter
flask marked at seventy-five cubic centimeters, are poured ten cubic
centimeters of the standard solution of phosphoric acid measured with
an exact pipette; five cubic centimeters of the acid sodium acetate
are added, and distilled water enough to make about thirty cubic
centimeters, and the whole carried to the boiling-point. The titration
is then carried on by allowing the solution of uranium to fall into the
flask from a graduated burette, thoroughly shaking after each addition
of the uranium, and trying a drop of the liquor with an equal quantity
of a ten per cent solution of potassium ferrocyanid upon a greased
white plate. Since the quantity of the uranium solution present will
be very nearly ten cubic centimeters at first, nine cubic centimeters
can be run in without testing. Afterwards, the operation is continued
by adding two or three drops at a time until the test upon the white
plate with the potassium ferrocyanid shows the end of the reaction.
When there is observed in the final test a slight change of tint, the
flask is filled up to the mark with boiling distilled water and the
process tried anew. If in the first part of the operation the point
of saturation have not been passed, it is still usually necessary to
add a drop or two of the uranium solution in order to produce the
characteristic reddish coloration, and this increase is rendered
necessary by the increase in the volume of the liquid. Proceeding
in this manner two or three times allows the attainment of extreme
precision, inasmuch as the analyst knows just when to look for the
point of saturation.
_Correction._—The result of the preceding operation is not absolutely
exact. It is evident indeed that in addition to the quantity of uranium
necessary for the exact precipitation of the phosphoric acid, it has
been necessary to add an excess sufficient to produce the reaction upon
the potassium ferrocyanid.
This excess is rendered constant by the precaution of operating always
upon the same volume; namely, seventy-five cubic centimeters. It can be
determined then once for all by making a blank determination under the
same conditions but without using the phosphoric acid.
The result of this determination is that it renders possible the
correction which it is necessary to make by subtracting the quantity
used in the blank titration from the preceding result in order to
obtain the exact strength of the uranium solution.
The operation is carried on as follows: In a flat-bottomed flask of
about 150 cubic centimeters capacity and marked at seventy-five cubic
centimeters, by means of a pipette, are placed five cubic centimeters
of the solution of sodium acetate; some hot distilled water is added
until the flask is filled to the mark, and it is then placed upon a
sand-bath and heated to the boiling-point. It is taken from the fire,
the volume made up to seventy-five cubic centimeters with a little hot
distilled water, and one or two drops of the solution of uranium are
allowed to flow into the flask from a graduated burette previously
filled exactly to zero. After each drop of the solution of uranium
the flask is shaken and the liquid tried upon a drop of potassium
ferrocyanid, as has been previously indicated. For a skilled eye, four
to six drops are generally necessary to obtain the characteristic
coloration; that is from two-tenths to three-tenths of a cubic
centimeter. Beginners often use from five-tenths to six-tenths, and
sometimes even more.
The sole important point is to arrest the operation as soon as the
reddish tint is surely seen, for afterwards the intensity of the
coloration does not increase proportionally to the quantity of liquor
employed.
It is well to note that at the end of some time the coloration becomes
more intense than at the moment when the solutions are mixed, so that
care must be taken not to pass the saturation-point. This slowness of
the reaction is the more marked as there is more sodium or ammonium
acetate in the standard solutions. This is the reason that it is
important to introduce always the same quantity; namely, five cubic
centimeters. This is also the reason why the uranium acetate should
not be employed in preparing the standard solution of uranium which
ought to contain the least possible amount of acetate in order that
the necessary quantity which is carried into each test should be as
small as possible and remain without appreciable influence. If it
were otherwise, the sensibility of the reaction would be diminished
in proportion as a larger quantity of uranium solution was employed,
giving rise to errors which would be as much more important as the
quantities of phosphoric acid to be determined were greater. The
correction for the uranium solution having been determined it is
written upon the label of the bottle containing it.
_Causes of Errors._—In the work which has just been described, some
causes of error may occur to which the attention of analysts should be
called.
The first is the error which may arise from the consumption of the
small quantity of uranium phosphate which is taken with a stirring rod
when the liquid is tested with potassium ferrocyanid. It is very easy
to be assured that the end of the reaction has really been reached. For
this purpose it is only necessary to note the quantity of the solution
already employed and to add to it afterwards four drops; shake, and
make a new test with a drop of the potassium ferrocyanid placed near
the spot which the last one occupied. If a decidedly reddish tint
does not appear at the moment of removing the glass rod, it is to be
concluded that the first appearance was an illusion, and the addition
of uranium is to be continued. If, on the contrary, the coloration
appear of a decided tint, the preceding number may be taken for exact.
It is then always beneficial to close the titration by this test of
four supplementary drops which will exaggerate the coloration and
confirm the figure found.
The second cause of error, and one moreover which is the most
frequently met with, consists in passing the end of the reaction by
adding the uranium too rapidly. In place of giving then a coloration
scarcely perceptible, the test with the drop of potassium ferrocyanid
gives a very marked coloration. In this case the analysis can still
be saved. For this purpose the analyst has, at his disposal, a tenth
normal solution prepared with 100 cubic centimeters of the standard
solution of phosphoric acid diluted to one liter with distilled water.
Ten cubic centimeters of this tenth normal solution are added, and the
titration continued. At the end, the amount of additional phosphoric
acid used is subtracted from the total.
A third cause of error is found in the foam which is often found in
the liquid, due to the shaking. This foam may retain a portion of the
last drops of the solution of uranium which fall upon its surface and
prevent its mixture with the rest of the liquid. If the glass stirring
rod in being removed from the vessel pass through this froth charged
with uranium, the characteristic coloration is obtained before real
saturation is reached. Consequently it is necessary to avoid, as much
as possible, the formation of the foam, and especially to take care
never to take the drop for test after agitation except in the middle of
the liquid where the foam does not exist.
Suppose the titration has been made upon ten cubic centimeters of the
normal solution of phosphoric acid in the conditions which we have
just indicated, and the figure for the uranium obtained is 10.2 cubic
centimeters; if now the correction, which may be supposed to amount
to two-tenths cubic centimeter, be subtracted there will remain ten
cubic centimeters of the uranium solution which would have precipitated
exactly fifty milligrams of phosphoric acid.
The quantity of phosphoric acid which precipitates one cubic centimeter
of the solution will be consequently expressed by the proportion
⁵⁰/₁₀ = five milligrams, which is exactly the strength required. In
the example which has just been given, the inscription upon the flask
holding the standard solution would be as follows: Solution of uranium,
one cubic centimeter equals five milligrams of phosphorus pentoxid;
correction, two-tenths cubic centimeter.
=99. Titration of the Sample.=—The strength of the solution of uranium
having been exactly determined, by means of this solution the strength
of the sample in which the phosphoric acid has been previously prepared
as ammonium magnesium phosphate is ascertained. In this case the
quantity of phosphoric acid being unknown, it is necessary to proceed
slowly and to duplicate the tests in order not to pass beyond the point
of saturation. From this there necessarily results a certain error in
consequence of the removal of quite a number of drops of the solution
of the sample before the saturation is complete. It is therefore
necessary to make a second determination in which there is at once
added almost the quantity of the solution of uranium determined by the
first analysis. Afterwards the analysis is finished by additions of
very small quantities of uranium until saturation is reached. Suppose,
for instance, that the sample was that of a mineral phosphate, five
grams of which were dissolved in 100 cubic centimeters, and of which
ten cubic centimeters of the solution prepared as above required 15.3
cubic centimeters of the standard solution of uranium. We then would
have the following data:
Mineral phosphate, five grams of the material dissolved in
twenty cubic centimeters of hydrochloric acid.
Water, sufficient quantity to make 100 cubic centimeters.
Quantity taken, ten cubic centimeters = 0.50 gram of the
sample taken.
Solution of uranium required 15.3 cubic centimeters.
Correction 0.2 “ “
----
Actual quantity of uranium solution 15.1 “ “
Strength of the solution of uranium, one cubic centimeter
= fivemilligrams P₂O₅.
Then P₂O₅ in 0.50 gram of the material = 5 × 15.1 = 75.50
milligrams.
(75.5 x 100)
Then the per cent of P₂O₅ = ----------- = 15.10.
50
The sample under examination ought always to be prepared in duplicate,
either by making a single precipitation and re-solution of the ammonium
magnesium phosphate which is made up to a certain volume and an
aliquot portion of which is taken for the analysis, or by making two
precipitations under the conditions previously described. When the
content of phosphoric acid in the material under examination is very
nearly known, the double operation may be avoided, especially if it be
required to have rapid and only approximate analyses, such as those
which are made for general control and for the conduct of manufacturing
operations. But when analyses are to be used to serve as the basis of
a law or for the control of a market, they should always be made in
duplicate, and the results ought not to be accepted when the numbers
obtained are widely different, since the agreement of the two numbers
will show that the work has been well executed.
This method of analysis, much longer to describe than to execute, gives
results perfectly exact and always concordant when it is well carried
out, provided that the standard solutions, upon which it rests for its
accuracy, are correctly prepared and frequently verified in the manner
indicated.
The strength of the solution of uranium ought to be verified every
three or four days. The strength of the standard solution of phosphoric
acid should be verified each time that the temperature of the
laboratory undergoes any important change. A solution prepared, for
example, in winter when the temperature of the laboratory is from 15°
to 18° would no longer be exact in summer when the temperature reaches
28° or 30°.
=100. Condition of Phosphoric Acid in Superphosphates.=—Superphosphates
are the products of the decomposition of phosphates by sulfuric or
hydrochloric acid. They contain phosphoric acid combined with water,
with lime, with magnesia, and with iron and alumina in various
proportions.
These combinations may be classed in three categories: First, those
compounds soluble in water; second, those insoluble in water, but
very soluble in ammoniacal salts of the organic acids such as the
citrate and oxalate; and third, phosphates not soluble in any of the
above-named reagents.
In the products soluble in water are met free phosphoric acid,
monocalcium phosphate, acid magnesium phosphate, and the iron and
aluminum phosphates dissolved in the excess of phosphoric acid. In
the products insoluble in water but soluble in the ammonium citrate
are found bicalcium phosphate and iron and aluminum phosphates, which
together constitute the phosphates called reverted.
These compounds reduced to a very fine state of division in the process
of manufacture are considered to contain phosphoric acid of the same
economic value.
=101. Determination of the Total Phosphoric Acid in Superphosphates
and Fertilizers.=—The process is carried on exactly as for an ordinary
phosphate, and with all the care indicated in connection with the
sampling, the incineration, the solution by means of hydrochloric acid,
and the separation of the phosphoric acid in the state of ammonium
magnesium phosphate, and finally in the titration by uranium.
=102. Determination of Soluble and Reverted Phosphoric Acid.=—To make
this determination a method unique and applicable to all cases consists
in extracting, at first, the soluble constituents in distilled water,
and following this operation by digestion in the ammonium citrate. The
products soluble in water can be determined either separately or at the
same time as the products soluble in the ammonium citrate according
to the taste of the people interested, without its being necessary to
modify very greatly the method of operation.
The determination of the soluble phosphoric acid comprises first, the
solution of the soluble constituents in distilled water; second, the
solution of the reverted phosphates in ammonium citrate; third, the
precipitation of the phosphoric acid dissolved in the two preceding
operations, and its determination.
=103. Preparation of the Sample for Analysis.=—The sample sent to the
chemical expert is prepared as has been indicated; that is to say,
it is poured on a sieve of which the meshes have a diameter of one
millimeter, and sifted upon a sheet of white paper. The parts which
do not pass the sieve are broken up either by the hand or in a mortar
and added, through the sieve, to the first portions. The product is
well mixed and, in this state, the mass presents all the homogeneity
desirable for analysis.
Some fertilizers are received in a pasty state which does not permit
of their being sifted. It is necessary in such a case to mix them
with their own weight either of precipitated calcium sulfate dried at
160° or with fine sand washed with hydrochloric acid and dried, which
divides the particles perfectly and permits of their being passed
through the meshes of the sieve.
=104. Extraction of the Products Soluble in Distilled Water.=—The
substance having been prepared as has just been indicated, one and a
half grams are placed in a glass mortar. Twenty cubic centimeters of
distilled water are added, and the substance gently suspended therein.
After standing for one minute, the supernatant part is decanted into a
small funnel provided with a filter-paper and placed in a flask marked
at 150 cubic centimeters. This operation is repeated five times and
is terminated by an intimate breaking up of the matter with distilled
water. When the volume of 100 cubic centimeters of the filtrate has
been obtained, the residue in the mortar is placed on the filter, and
the washing is continued until the total volume reaches 150 cubic
centimeters. The filtrate is shaken in order to render the liquor
homogeneous, and is transferred to a precipitating glass of about 300
cubic centimeters capacity.
=105. Solution of the Reverted Phosphates by Ammonium Citrate.=—The
filter from the above process is detached from the funnel and is
introduced into a flask marked at 150 cubic centimeters together with
sixty cubic centimeters of alkaline ammonium citrate prepared in the
following manner:
Pure citric acid, 400 grams.
Ammonia of 22°, 500 cubic centimeters.
The ammonia is poured upon the citric acid in the form of crystals in
a large dish. The mass becomes heated, and the solution takes place
rapidly. When it is complete and the solution is cold it is poured
into a flask of one liter capacity, and the flask is filled up to the
mark with strong ammonia. It is preserved for use in a well-stoppered
bottle. The solution must be strongly alkaline.
The flask in which the filter-paper is introduced, together with
the ammonium citrate, is stoppered and shaken violently in order to
disintegrate the filter-paper and put the reverted phosphates in
suspension. There are added then about sixty cubic centimeters of
distilled water, and the flask is shaken and left for twelve hours
at least, or at most for twenty-four hours. The volume is made up to
150 cubic centimeters with distilled water, and, after mixture, the
solution is filtered.
There are thus obtained two solutions which can be precipitated
together or separately according to circumstances. The most usual
process is to combine the two equal volumes of twenty-five, fifty, or
one hundred cubic centimeters, representing one-quarter, one-half,
or one gram of the material according to its presumed richness, in
a precipitating flask to which are added from ten to twenty cubic
centimeters of the solution of magnesia made up as follows:
Magnesium carbonate, 50 grams.
Ammonium chlorid, 100 “
Water, 500 cubic centimeters.
Hydrochloric acid, 120 “ “
After complete solution of the solid matters in the above, add 100
cubic centimeters of ammonia of 22° strength, and distilled water
enough to make one liter.
The solutions are thoroughly mixed in a precipitating glass, an excess
of ammonia added, and allowed to stand for twelve hours under a bell
jar. The phosphoric acid contained in the liquor is separated as
ammonium magnesium phosphate. It is collected upon a small filter,
washed with a little ammoniacal water, redissolved, and titrated with
the uranium solution in the manner already indicated.
_Example_: The following is an example of this kind of a determination:
(1) One and one-half grams of the superphosphate and distilled water
enough to make 150 cubic centimeters.
(2) Filter-paper with reverted phosphates, sixty cubic centimeters of
ammonium citrate, and a sufficient quantity of distilled water to make
150 cubic centimeters.
Aqueous solution (1) 25 cc }
} = 0.25 grams of the sample.
Citrate solution (2) 25 cc }
Add magnesium solution twenty cubic centimeters and ammonia in excess,
and allow from twelve to twenty-four hours of digestion, then filter
and wash, dissolve and titrate.
Required of solution of uranium 8.55 cubic centimeters (1 cubic
centimeter = 5 milligrams P₂O₅).
Correction 0.20.
Remainder 8.35 × 0.005 = 0.04175 gram P₂O₅ for 0.25 gram of the sample.
Then 0.04175 ÷ 0.25 = 16.7 per cent.
From the above data there would be 16.7 per cent of phosphoric acid
soluble in water and in ammonium citrate.
If it be desirable to have separately the phosphoric acid soluble in
water, a separate precipitation is made of the aqueous solution alone
by means of the magnesium citrate solution. The precipitate washed with
ammoniacal water is redissolved and titrated in the manner indicated.
In subtracting from the figures obtained with the two solutions
together the number obtained for the phosphoric acid soluble in water,
the number representing the phosphoric acid soluble in ammonium citrate
alone, is obtained.
It is to be noted that the determinations with uranium require always
two successive titrations. It would therefore be an advantage in all
operations to precipitate a weight of ammonium magnesium phosphate
sufficient for allowing this precipitate to be dissolved and made
up to 100 cubic centimeters on which amount it would be possible to
execute two, three, or four determinations, and thus to obtain a figure
absolutely incontestable.
=106. Conclusions.=—It has been seen from the above data that the
French chemists have worked out the uranium volumetric method with
great patience and attention to detail. Where many determinations are
to be made it is undoubtedly possible for an analyst to reach a high
degree of accuracy as well as to attain a desirable rapidity, by using
this method. For a few determinations, however, the labor of preparing
and setting the standard solutions required would be far greater
than the actual determinations either by the molybdate or citrate
gravimetric methods. For control work in factories and for routine
work connected with fertilizer inspection, the method has sufficient
merit to justify a comparison with the processes already in use by the
official chemists of this country.
The use of an alkaline ammoniacal citrate solution, however, for the
determination of reverted acid renders any comparison of the French
method with our own impossible. On the other hand the French method for
water-soluble acid is based on the same principle as our own; _viz._,
washing at first with successive small portions of water, and thus
avoiding the decomposition of the soluble phosphates, which is, likely
to occur when too great a volume of water is added at once.
In the matter of the temperature and time as affecting the solubility
of reverted acid, the French method is also distinctly inferior to our
own. The digestion is allowed to continue from twelve to twenty-four
hours, at the pleasure of the analyst, and meanwhile it is subjected to
room temperature. It is not difficult to see that this treatment in the
same sample would easily yield disagreeing results between twelve hours
at a winter temperature and twenty-four hours at summer heat.
THE DETERMINATION OF PHOSPHORIC ACID BY TITRATION OF THE YELLOW
PRECIPITATE.
=107. Pemberton’s Volumetric Method.=—In order to shorten the work of
determining the phosphoric acid, numerous attempts have been made to
execute the final determination directly on the yellow precipitate
obtained by treating a solution of a phosphate with ammonium molybdate
in nitric acid. The composition of this precipitate appears to be
somewhat variable, and this fact has cast doubt on the methods of
determination based on its weight. Its most probable composition is
expressed by the following formula, (NH₄)₃PO₄(MoO₃)₁₂. For convenience
in writing reactions this formula should usually be doubled. Pemberton
has described a volumetric determination of phosphoric acid in the
yellow precipitate which has the merit of being rapid.[83]
In this laboratory the method has not given very satisfactory results
when compared with the molybdate gravimetric process. It has however
attracted so much attention from analysts as to merit description, and
the details of the process are therefore given.
=108. The Process.=—One gram of phosphate rock, or from two to
three grams of phosphatic fertilizer, are dissolved in nitric acid,
and, without evaporation, diluted to 250 cubic centimeters. Without
filtering, twenty-five cubic centimeters are placed in a four-ounce
beaker and ammonia added until a slight precipitate begins to form.
Five cubic centimeters of nitric acid of one and four-tenths specific
gravity are then added, and afterwards ten cubic centimeters of
saturated solution of ammonium nitrate and enough water to make the
volume about sixty-five cubic centimeters. The contents of the beaker
are boiled, and while still hot, five cubic centimeters of the aqueous
solution of ammonium molybdate added. Additional quantities of the
molybdate are added, if necessary, until the whole of the phosphorus
pentoxid is thrown-out.
After allowing to settle for a moment the contents of the beaker are
poured upon a filter seven centimeters in diameter. The precipitate is
thoroughly washed with water, both by decantation and on the filter.
The filter with its precipitate is transferred to a beaker and titrated
with standard alkali, in the presence of phenolphthalein. Each cubic
centimeter of alkali employed should correspond to one milligram of
phosphorus pentoxid, (P₂O₅).
The reagents employed have the composition indicated below:
_Ammonium Molybdate._—Ninety grams of the crystals of ammonium
molybdate are placed in a large beaker and dissolved in a little less
than one liter of water. The beaker is allowed to stand over night
and the clear liquor decanted. Any undissolved acid is brought into
solution in a little ammonia water and added to the clear liquor. If a
trace of phosphoric acid be present a little magnesium sulfate is added
and enough ammonia to produce a slight alkaline reaction. The volume
of the solution is then made up to one liter. Each cubic centimeter
of this solution is capable of precipitating three milligrams of
phosphorus pentoxid.
_Standard Potassium Hydroxid._—This solution is made of such strength
that one cubic centimeter is equivalent to one milligram of phosphorus
pentoxid. Treated with acid of normal strength, 100 cubic centimeters
are required to neutralize 32.37 cubic centimeters thereof.
_Standard Acid._—This should have the same strength, volume for volume,
as the standard alkali solution. It is made by diluting 323.7 cubic
centimeters of normal acid to one liter.
_Indicator._—The indicator to be used is an alcoholic solution of
phenolphthalein, one gram in 100 cubic centimeters of sixty per cent
alcohol, and half a cubic centimeter of this should be used for each
titration.
Thomson has shown[84] that of the three hydrogen atoms in phosphoric
acid two must be saturated with alkali before the reaction with
phenolphthalein is neutral. Therefore, when the yellow precipitate
is broken up by an alkali, according to the reaction to follow, only
four of the six molecules of ammonium are required to form a neutral
ammonium phosphate as determined by the indicator employed. The
remaining two molecules of ammonium unite with the molybdenum forming
also a salt neutral to the indicator.
Phenolphthalein is preferred because, as has been shown by Long, its
results are reliable in the presence of ammonium salts unless they
be present in large quantity, and if the solution be cold and the
indicator be used in sufficient quantity.[85] To prepare the indicator
for this work, one gram of phenolphthalein is dissolved in 100 cubic
centimeters of sixty percent alcohol. At least one-half of a cubic
centimeter of the solution is used for each titration.
The advantages claimed for the method are its speed and accuracy. Much
time is saved by avoiding the necessity for the removal of the silica
by evaporation. The results of analyses with and without the removal of
the silica are practically identical. When the silica is not removed it
is noticed that the filtrate from the yellow precipitate has a yellow
tint.
The reaction is represented by the following formula:
(NH₄)₆(PO₄)₂(MoO₃)₂₄ + 46KOH = (NH₄)₄(HPO₄)₂ + (NH₄)₂MoO₄
+ 23K₂MoO₄ + 22H₂O.
From this reaction it is seen that the total available acidity of one
molecule of the yellow precipitate titrated against phenolphthalein is
equivalent to twenty-three molecules of potassium hydroxid.
_Calculation of Results._—The standard alkali is of such strength
that one cubic centimeter is equal to one per cent of phosphoric acid
when one gram of material is employed and one-tenth of it taken for
each determination. In a given case one gram of a sample was taken
and one-tenth of the solution used. Fifty cubic centimeters of alkali
were added to the yellow precipitate. It required thirty-two cubic
centimeters of standard alkali to neutralize the excess.
The alkali consumed by the yellow precipitate was 50 - 32 = 18. The
sample therefore contained eighteen per cent of phosphoric acid.
_Comparison with Official Method._—A comparison of the Pemberton
volumetric with the official method of the Association of Agricultural
Chemists has been made by Day and Bryant.[86] The comparisons were made
on samples containing from 1.45 to 37.28 per cent of phosphoric acid
and resulted as follows:
Per cent Per cent
P₂O₅, P₂O₅,
Substance. Official. Pemberton.
No. 1. Florida rock 1.45 1.32
“ 2. “ “ 4.40 4.53
“ 3. Sodium phosphate 19.78 19.99
“ 4. “ “ 19.72 19.73
“ 5. Florida rock 37.28 37.22
This near agreement shows the reliability of the method. The comparison
of the Pemberton volumetric method with the official gravimetric
method was investigated by the reporter of the Association of Official
Agricultural Chemists in 1894.[87] The individual variations were found
to be greater than in the regular method but the average results were
nearly identical therewith. The method works far better with small
percentages of phosphoric acid than with large. Where the average
of the results by the official methods gave 12.25 per cent, the
volumetric process gave 11.90 per cent, whereas in the determination
of a smaller percentage the results were 2.72 and 2.73 per cent,
respectively. Kilgore proposes a variation of the method which differs
from the original in two principal points.[88] First the temperature
of precipitation in the Pemberton process is 100°; but in the modified
form from 55° to 60°. At the higher temperature there is danger of
depositing molybdic acid.
The second difference is in the composition of the molybdate solution
employed. The official molybdate solution contains about sixty grams
of molybdenum trioxid in a liter while the Pemberton solution contains
sixty-six grams. There is therefore not much difference in strength.
The absence of nitric acid, however, from the Pemberton solution favors
the deposition of the molybdic acid when heat is applied. Kilgore,
therefore, conducts the analysis as follows: The solution of the sample
is made according to the official nitric and hydrochloric acid method
for total phosphoric acid. For the determination, twenty or forty cubic
centimeters are taken, corresponding to two-tenths or four-tenths
gram of the sample. Ammonia is added until a slight precipitate is
produced and the volume is then made up, with water, to seventy-five
cubic centimeters. Add some ammonium nitrate solution, from ten to
fifteen cubic centimeters, but this addition is not necessary unless
much of the nitric acid has been driven off during solution. Heat in
water-bath to 60° and precipitate with some freshly filtered official
molybdate solution. Allow to stand for five minutes, filter as quickly
as possible, wash four times by decantation using from fifty to
seventy-five cubic centimeters of water each time, and then wash on a
filter until all acid is removed. The solution and titration of the
yellow precipitate are accomplished as in the Pemberton method. The
agreement of the results obtained by this modified method was much
closer with the official gravimetric method than those obtained by the
Pemberton process.
=109. Estimation of Phosphoric Acid as a Lead Compound.=—In the
volumetric lead method, as described by Wavelet, the phosphoric acid
is precipitated by the magnesium citrate solution as in the uranium
method of Joulie, as practiced by the French chemists, and the washing
of the precipitate and its solution in nitric acid are also conducted
as in that method.[89] After solution in nitric acid ammonia is added
to neutrality and the solution is then made acid with acetic. The
phosphoric acid is precipitated in the acid solution by a standard
solution of lead nitrate, the precipitate having the formula P₂O₅3PbO.
The end reaction is determined by placing a drop of the titrated
mixture on a white greased dish in contact with a drop of a five per
cent solution of potassium iodid. When all the phosphoric acid is
precipitated the least excess of the lead salt is revealed by the
characteristic yellow precipitate of lead iodid.
The author of the process claims that the lead phosphate is insoluble
in the excess of acetic acid and that the phosphate itself does not
give any yellow coloration with potassium iodid. The process is quite
as exact as the uranium method and the end reaction is far sharper and
the standard reagents are easily made and preserved.[90] The method
described merits, at least, a comparative trial with the uranium
process, but cannot be recommended as exact until further approved by
experience.
The reagents employed have the following composition:
(1) Disodium phosphate solution containing 10.085 grams per liter
(2) Sodium acetate “ “ 50.000 “ “ “
(3) Lead nitrate “ “ 40.000 “ “ “
(4) Potassium iodid “ “ 50.000 “ “ “
The titrations should be conducted in the cold.
=110. Water-Soluble Phosphoric Acid.=—Glaser has modified the
volumetric method of Kalmann and Meissels for the volumetric estimation
of water-soluble phosphoric acid so as to avoid the double titration
required by the original method.[91] If methyl orange be used as an
indicator in the original method, the determination does not at once
lead to the tricalcium salt, but the liquid still contains, after
neutralization, some monocalcium phosphate, which is determined by a
further titration with phenolphthalein. In the modified method the
total phosphoric acid is estimated in one operation as a tricalcium
salt. This is secured, by adding, at the proper time, an excess of
calcium chlorid. Two grams of the superphosphate are shaken with water
several times, and, after settling, filtered, and the insoluble residue
finally washed on the filter until the total volume of the filtrate
is a quarter of a liter. Of this, fifty cubic centimeters are taken
and titrated with tenth normal soda-lye, with addition of two drops of
methyl orange, until the acid reaction has entirely disappeared. There
is then added some neutral calcium chlorid solution in excess. If iron
and alumina be present, a precipitate is produced of which no account
need be made. The acid reaction is thus restored. Five drops of the
phenolphthalein solution are added and the titration continued until
the alkaline reaction is noted throughout the whole mass. Each cubic
centimeter of the soda-lye corresponds, in the first titration, to 7.1,
and in the second to 3.55 milligrams of phosphoric acid.
=111. Estimation of Phosphoric Acid in the Presence of a Large Excess
of Iron.=—The method given below, due to Emmerton, depends upon the
precipitation of a phosphomolybdate, of constant composition, in the
presence of a large excess of iron, as in the analysis of iron and
steel and iron ores.[92] The molybdenum trioxid obtained is reduced by
zinc to Mo₁₂O₁₉. The action of permanganate on this compound is shown
in the following equation:
5Mo₁₂O₁₉ + 17(K₂OMn₂O₇) = 60MoO₃ + 17K₂O + 34MnO.
Seventeen molecules of permanganate are equal to sixty molecules of
molybdenum trioxid. The iron or steel is dissolved in nitric acid,
evaporated to dryness, heated, and redissolved in hydrochloric acid,
then treated again with nitric acid and evaporated until a clear and
concentrated solution is obtained free from hydrochloric acid.
The solution obtained is diluted to forty cubic centimeters with water
and washed into a 400 cubic centimeter flask, making the total volume
about seventy-five cubic centimeters. Add strong ammonia, shaking
after each addition, until the mass sets to a thick jelly from the
ferric hydroxid. Add a few more cubic centimeters of ammonia and shake
thoroughly, being sure the ammonia is present in excess. Add next
nitric acid gradually, with shaking, until the precipitate has all
dissolved; add enough more nitric acid to make the solution a clear
amber color. The volume should now be about 250 cubic centimeters.
Bring the solution to 85° and add, at once, forty cubic centimeters
of molybdate solution of the following strength: Dissolve 100 grams
of molybdic acid in 300 cubic centimeters of strong ammonia and 100
cubic centimeters of water, and pour the solution into 1,250 cubic
centimeters of nitric acid (1.20); close the flask with a rubber
stopper, wrap it in a thick cloth, and shake violently for five
minutes. Collect the precipitate on a filter, using pump, and wash with
dilute nitric acid (1 HNO₃: 50 H₂O). If a thin film of the precipitate
should adhere to the flask it can be removed by the ammonia in the next
operation. Wash the molybdate precipitate into a 500 cubic centimeter
flask with dilute ammonia (1 H₃N: 4 H₂O), using about thirty cubic
centimeters. Add hot dilute sulfuric acid (1 H₂SO₄: 4 H₂O) and cover
the flask with a small funnel. Add ten grams of granulated zinc and
heat until rapid action begins, and then heat gently for five minutes.
The reduction is then complete. During the reduction the colors, pink,
plum, pale green, and dark green, are seen in the molybdate solution,
the latter color marking the end of the reaction.
To remove the zinc, pour through a large folded filter, wash with
cold water, and fill up the filter once with cold water. But little
oxidation takes place in this way. A port-wine color is seen on the
filter, but this does not indicate a sufficient oxidation to make an
error.
In titrating, the wine color becomes fainter and finally the solution
is perfectly colorless and shows a single drop in excess of the
permanganate. The permanganate solution, for convenience, is made so
that one cubic centimeter is equal to 0.0001 gram of phosphorus. With
iron its value is one cubic centimeter equals 0.006141 gram of iron;
and one cubic centimeter equals 0.005574 gram of molybdenum trioxid.
=112. Variation of Dudley and Noyes.=—The method of Emmerton to
determine small quantities of phosphoric acid, or of phosphorus in
presence of a large excess of iron, has been modified by Dudley and
Pease,[93] and by Noyes and Royse.[94] As modified, the method is
not intended for fertilizer analysis, but the principle on which it
rests may some time, with proper modifications, find application in
fertilizer work. The reduction is accomplished in a Jones’ tube,[95]
much simplified, so as to render it suitable for common use. The
molybdic acid is reduced to a form, or series of forms, corresponding
to molybdenum sesquioxid, as in the Emmerton method, and subsequently
as in that method, titrated by a set solution of potassium permanganate.
The iron or steel filings, containing phosphorus, are brought into
solution by means of nitric acid. For this purpose two grams of them
are placed in a half liter flask together with fifty cubic centimeters
of nitric acid of 1.18 specific gravity. The mixture is boiled for one
minute, and ten cubic centimeters of permanganate solution of one and
a quarter per cent added. Boil again until the pink color disappears.
Ferrous sulfate solution is next to be carefully added, shaking
meanwhile, until the solution clears. Cool to 50° and add eight cubic
centimeters of ammonia of 0.90 specific gravity, stopper the flask,
and shake until any precipitate which may form is redissolved. Cool or
warm, as the case may be, until the solution is as many degrees above
or below 60° as the molybdic solution is above or below 27°. Add sixty
cubic centimeters of molybdic solution, stopper, and shake on a machine
or by hand for five minutes. After remaining at rest for five minutes
pour into a nine centimeter filter of fine texture and wash with the
acid ammonium sulfate solution in quantities of from five to ten cubic
centimeters each time. The filtrate and washings must be perfectly
bright. Continue the washings until the filtrate gives no color with
hydrogen sulfid.
Dissolve the yellow precipitate with twelve cubic centimeters of 0.96
ammonia diluted with an equal volume of water, and wash the filter
with 100 cubic centimeters of water. Finally add to the filtrate and
wash-water eighty cubic centimeters of water and ten of strong sulfuric
acid. Pass the mixture through the Jones’ reducing tube and follow it
with 200 cubic centimeters of water, taking care that no air enter the
tube during the operations. The solution collected in the flask should
be at once titrated with potassium permanganate.
_Solutions used_: (1) _Nitric acid._—One part of nitric acid of 1.42
specific gravity and two parts of water by volume. The specific gravity
of the mixture is about 1.18.
(2) _Permanganate solution for oxidizing._—Dissolve 12.5 grams of
potassium permanganate in one liter of water.
(3) _Ferrous sulfate._—Fresh crystals not effervesced and free from
phosphorus.
(4) _Ammonia._—The strong ammonia used should have a specific gravity
of about 0.90 and the dilute of 0.96 at 15.5°.
(5) _Molybdic Solution._—Dissolve 100 grams of molybdic anhydrid in
400 cubic centimeters of ammonia of 0.96 specific gravity and pour the
solution slowly, with constant stirring, into one liter of nitric acid
of about 1.20 specific gravity. Heat the mixture to 45° and add one
cubic centimeter of a ten per cent solution of sodium phosphate, stir
vigorously, and allow to stand in a warm place for eighteen hours.
Filter before using.
(6) _Add Ammonium Sulfate._—To half a liter of water add 27.5 cubic
centimeters of 0.96 ammonia and twenty-four cubic centimeters of strong
sulfuric acid, and make the volume one liter with water.
(7) _Potassium Permanganate for Titration._—Dissolve four grams of
potassium permanganate in two liters of water, heat nearly to boiling
for an hour, allow to stand for eighteen hours, and filter on asbestos
felt. The solution must not come in contact with rubber or other
organic matter. The solution may be standardized with thoroughly
air-dried ammonium oxalate in solution with a little dilute sulfuric
acid and with ammonium ferrous sulfate partly crystallized in small
crystals from a slightly acid solution. The crystals should be well
washed and quickly air-dried in a thin layer. The factors 1/1 1/4 2/2
and 1/7 should be used respectively to calculate the iron equivalent.
The phosphorus equivalent is obtained by multiplying the iron
equivalent by 31 ÷ (36 × 56) = 0.01538.
[Illustration: FIGURE 7.