=47. Preliminary Considerations.=—The chief sources of the phosphoric
acid in commercial fertilizers are the mineral phosphates and bones. In
respect of the analyses of mineral phosphates detailed directions have
been given in the preceding pages. Bones are valuable for fertilizing
materials, both because of their content of phosphoric acid and of
their organic nitrogen. The methods of treating bones for their
phosphoric acid will be found in the general methods for fertilizing
materials, and their nitrogen content can be determined by the
processes to be described hereafter. Other fertilizing materials also
contain phosphorus, as ashes, tankage, oil cakes, and other organic
products. In general, the methods for determining the phosphoric acid
is the same in all cases, but the means of destroying the organic
matter precedent to the analysis vary in different cases. In most cases
a simple ignition is sufficient, while, if the phosphorus be found
in certain organic products, the oxidation must be accomplished by
one of the methods described in the processes adopted by the official
chemists, or by the means described in volume first, paragraph =378=
or =382=. In all cases of acid phosphates and superphosphates, the
water and ammonium citrate soluble phosphoric acid is to be determined
as well as the total. In basic slags the amount soluble in ammonium
citrate or dilute citric acid is also to be ascertained.
In all cases where soluble or so-called reverted acid is to be
considered, the analysis must be performed without previous desiccation
or ignition. If water content or loss on ignition are to be considered,
the operation to determine them must be conducted on a separate part of
the sample.
The methods of analysis which have been adopted by associations of
chemists should be given the preference in the conduct of the work,
although it must be admitted that they may contain sources of error,
and may be in no respect superior to processes employed by chemists
in their private capacity. In this country the methods adopted by
the Association of Official Agricultural Chemists should be followed
as closely as possible. The great merit of other methods, however,
must not be denied. Especially those methods which shorten the time
required or diminish the labor and expense of the analysis are worthy
of careful consideration. In factory work, for instance, it is often
far more important for the chemist to be able to rapidly determine the
phosphoric acid in a great number of samples with approximate accuracy
than to confine his work to one with absolute precision. Some of the
shorter methods, moreover, notably the citrate process, appear to be
quite, if not altogether, as reliable as the molybdate method, while in
the case of the uranium volumetric process, it must not be forgotten
that it is almost the only one practiced in France. Other volumetric
processes are given in full, as, for instance, the one perfected
by Pemberton, but data are still lacking to justify their strong
recommendation. It should be remembered that this manual is not written
for the beginner but rather for the chemist already acquainted with
the principles and practice of general chemical analysis, and it is,
therefore, expected that each analyst will make intelligent use of the
data placed at his disposal.
=48. Determination of Phosphoric Acid with Preliminary Precipitation
as Stannic Phosphate.=—This method, once much in use and highly
recommended, is now almost unknown among the processes of fertilizer
control. It was first proposed and described by Girard, and rests on
the precipitation of the phosphoric acid in a nitric acid solution by
means of metallic tin.[36] The stannic acid formed by the oxidation
of the tin unites with the phosphoric acid held in a free state by
the nitric acid. The precipitation of the phosphoric acid is said to
be complete, but a trace of it has been found in the iron and alumina
subsequently separated from the solution. The precipitate obtained is
dissolved in caustic potash, whereby soluble potassium metastannate
and phosphate are obtained. Following is the method of conducting the
analysis as described by Crookes:[37]
The phosphate should be dissolved in nitric acid, and any chlorin
present be expelled by repeated evaporations with the solvent. Finally,
to the evaporated mass the strongest nitric acid is added. Pure
tin-foil is added and heat applied. The phosphoric acid is precipitated
by the stannic acid formed. The quantity of tin used should be from six
to eight times as great as that of the phosphoric acid present.
The precipitate is collected on a filter, washed, and dissolved in
caustic potash. The solution is saturated with hydrogen sulfid, and
on adding acetic acid in slight excess the tin sulfid is separated
and removed by filtration. The whole of the phosphoric acid, supposed
to be almost free of tin, is now found in the filtrate. The filtrate
is concentrated to small bulk and any tin sulfid present separated by
filtering, and the phosphoric acid finally removed from the ammoniacal
filtrate by precipitation with magnesia mixture. The chief difficulties
of this method are to be found, on the one hand, in the retention
of some of the phosphoric acid by the iron and alumina which may be
present, and on the other, in the presence of some tin in the final
magnesium pyrophosphate. If the tin be all removed as sulfid, the
latter source of error will be avoided. It is difficult to secure pure
metallic tin, and this is another disturbing element in the process.
It can not be recommended for the work which agricultural analysts are
usually called on to perform.[38]
=49. Water-Soluble Phosphoric Acid.=—The method of procedure
recommended by the Association of Official Chemists is as follows:[39]
Place two grams of the sample in a nine centimeter filter; wash with
successive small portions of cold water, allowing each portion to pass
through before adding more, until the filtrate measures about 250 cubic
centimeters. If the filtrate be turbid, add a little nitric acid. Make
up to any convenient definite volume; mix well; take any convenient
portion and proceed as under total phosphoric acid.
=50. Citrate-Insoluble Phosphoric Acid.=—Heat 100 cubic centimeters of
strictly neutral ammonium citrate solution of 1.09 specific gravity
to 65° in a flask placed in a bath of warm water, keeping the flask
loosely stoppered to prevent evaporation. When the citrate solution
in the flask has reached 65°, drop into it the filter containing the
washed residue from the water-soluble phosphoric acid determination,
close tightly with a smooth rubber stopper, and shake violently until
the filter paper is reduced to a pulp. Place the flask back into the
bath and maintain the water in the bath at such a temperature that the
contents of the flask will stand at exactly 65°. Shake the flask every
five minutes. At the expiration of exactly thirty minutes from the time
the filter and residue were introduced, remove the flask from the bath
and immediately filter as rapidly as possible. It has been shown by
Sanborn, in this laboratory, that the filtration is greatly facilitated
by adding asbestos pulp. Wash thoroughly with water at 65°. Transfer
the filter and its contents to a crucible, ignite until all organic
matter is destroyed, add from ten to fifteen cubic centimeters of
strong hydrochloric acid, and digest until all phosphate is dissolved;
or return the filter with contents to the digestion flask, add from
thirty to thirty-five cubic centimeters of strong nitric, and from
five to ten cubic centimeters of strong hydrochloric acid, and boil
until all the phosphate is dissolved. Dilute the solution to 200 cubic
centimeters. If desired, the filter and its contents can be treated
according to methods (1), (2), or (3), under total phosphoric acid.
Mix well; filter through a dry filter; take a definite portion of the
filtrate and proceed as under total phosphoric acid.
In case a determination of citrate-insoluble phosphoric acid be
required in non-acidulated goods it is to be made by treating two
grams of the phosphatic material, without previous washing with water,
precisely in the way above described, except that in case the substance
contain much animal matter (bone, fish, etc.), the residue insoluble
in ammonium citrate is to be treated by one of the processes described
below under total phosphoric acid, (1), (2), or (3).
=51. Total Phosphoric Acid.=—In case of ignition the residual material
is to be dissolved in hydrochloric acid. The following methods of
treating the raw material, using two grams in each case, may be
employed: (1) Evaporate with five cubic centimeters of magnesium
nitrate, ignite, and dissolve in hydrochloric acid. (2) Boil in a
Kjeldahl flask graduated to 250 cubic centimeters, with from twenty
to thirty cubic centimeters of strong sulfuric acid, adding from two
to four grams of sodium or potassium nitrate at the beginning of the
digestion and a small quantity after the solution has become nearly
colorless; or adding the nitrate in small portions from time to time.
After the solution is colorless, add 150 cubic centimeters of water and
boil for a few minutes, cool, and make up to volume. (3) Digest with
strong sulfuric acid and such other reagents as are used in either the
plain or modified Kjeldahl or Gunning methods for estimating nitrogen.
Do not add any potassium permanganate, but after the solution has
become colorless add about 100 cubic centimeters of water and boil
for a few minutes, cool, and make up to a convenient volume; two
and five-tenths grams of substance and a digestion flask graduated
to 250 cubic centimeters are recommended. This method will be found
convenient when both the nitrogen and the total phosphoric acid are to
be determined in a fertilizer. In this case, after diluting the volume
and mixing, a part for the estimation of nitrogen, may be removed with
a pipette and the remainder then filtered through a dry filter and a
portion taken for the determination of the total phosphoric acid. (4)
Dissolve in thirty cubic centimeters of concentrated nitric acid and a
small quantity of hydrochloric acid. (5) Add thirty cubic centimeters
of concentrated hydrochloric acid, heat, and add cautiously, in small
quantities at a time, about five-tenths gram of finely-pulverized
potassium chlorate. (6) Dissolve in from fifteen to thirty cubic
centimeters of strong hydrochloric and from three to ten cubic
centimeters of nitric acid. This method is recommended for fertilizers
containing much iron or aluminum phosphate. Boil until all phosphates
are dissolved and all organic matter is destroyed; cool and dilute
to 200 or 250 cubic centimeters; mix and pass through a dry filter;
take an aliquot part of the filtrate corresponding to a quarter, half,
or one gram, neutralize with ammonia, and clear with a few drops of
nitric acid. In case hydrochloric or sulfuric acid have been used as a
solvent, add about fifteen grams of dry ammonium nitrate.
To the hot solutions, for every decigram of phosphorus pentoxid that is
present, add fifty cubic centimeters of molybdic solution. Digest at
about 65° for an hour, filter, and wash with water or ammonium nitrate
solution. Test the filtrate by renewed digestion and the addition of
more molybdic solution. Dissolve the precipitate on the filter with
ammonia and hot water and wash into a beaker to a bulk of not more than
100 cubic centimeters. Nearly neutralize with hydrochloric acid, cool,
and add magnesia mixture from a burette; add slowly (about one drop per
second), stirring vigorously. After fifteen minutes add thirty cubic
centimeters of ammonia solution of 0.95 density. Let stand for some
time; two hours are usually enough. Filter, wash with dilute ammonia,
ignite gently at first and then at white heat for ten minutes, and
weigh. For the quantity of magnesia mixture to be added see paragraph
=21=.
=52. Citrate-Soluble Phosphoric Acid.=—The sum of the water-soluble and
citrate-insoluble subtracted from the total gives the citrate-soluble
phosphoric acid.
=53. Preparation of Reagents.=—(1) _Ammonium Citrate Solution._—(_a_)
Mix 370 grams of commercial citric acid with 1,500 cubic centimeters
of water, nearly neutralize with commercial ammonia, cool, add ammonia
until exactly neutral (testing with saturated alcoholic solution of
corallin) and bring to a volume of two liters. Test the specific
gravity, which should be 1.09 at 20°, before using.
(_b_) _Alternate Method._—To 370 grams of commercial citric acid add
commercial ammonia, of 0.96 specific gravity, until nearly neutral;
reduce the specific gravity to nearly 1.09 and proceed as follows:
Prepare a solution of fused calcium chlorid 200 grams to the liter,
and add four volumes of strong alcohol. Make the mixture exactly
neutral, using a small amount of freshly prepared corallin solution as
a preliminary indicator, and test finally by withdrawing a portion,
diluting with an equal volume of water, and testing with cochineal
solution. Fifty cubic centimeters of this solution will precipitate
the citric acid from ten cubic centimeters of the citrate solution. To
ten cubic centimeters of the nearly neutral citrate solution add fifty
cubic centimeters of the alcoholic calcium chlorid solution, stir well,
filter at once through a folded filter, dilute with an equal volume
of water, and test the reaction with neutral solution of cochineal.
If acid or alkaline, add ammonia or citric acid, as the case may be,
to the citrate solution, mix, and test again as before. Repeat this
process until a neutral reaction of the citrate solution is obtained.
At the end the specific gravity must be 1.09 at 20°.
(2) _Molybdic Solution._—See paragraph =22=.
(3) _Ammonium Nitrate Solution._—Dissolve 200 grams of commercial
ammonium nitrate in water and bring to a volume of two liters.
(4) _Magnesia Mixture._—See paragraph =22=.
(5) _Dilute Ammonia for Washing._—See paragraph =22=.
(6) _Magnesium Nitrate._—Dissolve 320 grams of calcined magnesia in
nitric acid, avoiding an excess of the latter; then add a little
calcined magnesia in excess, boil, filter from the excess of magnesia,
ferric oxid, etc., and bring to volume of two liters.
=54. Official Methods with Norwegian Fertilizers.=—The Director of
the Chemical Control Station of Norway, expresses the opinion, that
for Norwegian, Swedish, Danish, and German conditions, the American
methods for the determination of phosphoric acid, notwithstanding
their analytical exactness, are quite inapplicable.[40] In those
countries are found many, in part, poorly pulverized and badly mixed
manures, such as ammonium superphosphate, potassium superphosphate, and
potassium ammonium superphosphate, and these can not usually be so well
pulverized and mixed that one can take out a true average sample of
from two to two and five-tenths grams. Care in the analysis is useless
when the material employed does not represent the average condition of
the materials investigated. Therefore, in the countries named, often
from ten to twenty grams, and almost never less than five grams of
substance are taken in the preparation of the solutions, except for
instance, in the determination of nitrogen and reverted phosphoric acid.
=55. The Molybdic Acid Method, as Practiced by Direction of the Union
of the German Experiment Stations.=—The method adopted by the German
Experiment Stations is essentially that used at Halle.[41] The samples
are brought into solution in the following way: For the estimation of
phosphoric acid in bone-meal, fish-guano and raw phosphates, and the
total phosphoric acid in superphosphates, five grams of the sample are
dissolved in fifty cubic centimeters of aqua regia, made of three parts
of hydrochloric acid of 1.12 specific gravity and one part of nitric
acid of 1.25 specific gravity, or the solution may be made of a mixture
of twenty cubic centimeters of nitric acid of 1.42 specific gravity
and fifty cubic centimeters of sulfuric acid of 1.8 specific gravity.
The boiling should continue for half an hour. The solution is made up
to half a liter and filtered. Fifty cubic centimeters of the filtrate
containing the phosphoric acid, with double superphosphates twenty-five
cubic centimeters, are digested with 200 cubic centimeters of ammonium
molybdate solution for three hours at 50° in a water-bath and, after
cooling, filtered, so that as little as possible of the precipitate is
collected upon the filter, which is made of strong paper.
The yellow precipitate is washed by decantation in the flask nine times
with twenty cubic centimeters of molybdic solution diluted with one
volume of water and the filter washed out once with the same quantity
of liquid. The funnel, with the filter, is immediately placed upon
the flask and the portion of the precipitate collected in the filter
dissolved in five per cent ammonia, which is easily accomplished by
throwing ammonia upon it from a wash-bottle. Afterwards the filter is
washed with a sufficient quantity of hot water and finally removed. The
contents of the flask are neutralized warm, with hydrochloric acid, the
acid being added until the precipitate first formed, after continued
shaking, is again dissolved in the liquid. The solution is then cooled
and treated, drop by drop, with constant stirring, with twenty cubic
centimeters of magnesia mixture. Finally twenty-five cubic centimeters
of dilute ammonia solution are added, the precipitate is not shaken,
and, after two hours, is filtered through a gooch.
For the filtering of the ammonium magnesium phosphate by the molybdic
method, freshly prepared felts are always employed since the remarkably
fine crystalline precipitates will pass through a filter which has once
been used. It is necessary also that special precautions be taken in
the ignition. The crucible should be heated in a platinum cap, which
has the purpose of protecting the contents of the crucible from the
access of reducing gases during the ignition. After redness has been
reached the cap can be removed and the crucible transferred to a blast
where it is strongly ignited for ten minutes before weighing. The
precipitate should be pure white.
The molybdic solution is prepared as follows: 150 grams of ammonium
molybdate are dissolved in a liter of water, and after the solution is
completely cooled, poured into a liter of nitric acid of 1.2 specific
gravity.
=56. Estimation of Soluble Phosphoric Acid.=—1. The extraction of the
superphosphates is made as follows: Twenty grams of the superphosphates
are placed in a liter flask with 800 cubic centimeters of water and
shaken continuously for thirty minutes. The flask is then filled with
water to the mark and the whole again thoroughly shaken and filtered.
For shaking, a machine is recommended, driven by hand or water power.
The normal rate of the machine is fixed at 150 turns per minute.
2. The solution of the total superphosphates, obtained as above, must
be boiled with nitric acid before the precipitation of the phosphoric
acid in order to convert any phosphoric acid present as pyrophosphoric
into tribasic phosphoric acid. For each twenty-five cubic centimeters
of the superphosphate solution ten cubic centimeters of concentrated
nitric acid are added and the mixture boiled.
3. The precipitation of the phosphoric acid is conducted by the
molybdenum method as usually practiced.
4. For the estimation of iron and alumina in each of the
superphosphates the Glaser method is recommended provisionally.
=57. Methods for Phosphoric Acid used in the Norway Stations.=[42]—1.
_Description of the Method for Total Phosphoric Acid._—For determining
the phosphoric acid in bone-meal, fish-guano, and superphosphates, five
grams of the substance, with twenty cubic centimeters of nitric acid of
1.42 specific gravity, and fifty cubic centimeters of sulfuric acid of
1.8 specific gravity, are boiled half an hour in a half liter flask,
diluted with water, and after cooling, made up to the mark. Fifty cubic
centimeters of the filtrate are made alkaline with ammonia, then acid
with nitric acid, precipitated with fifty cubic centimeters of molybdic
solution for every one-tenth gram of phosphorus pentoxid present,
heated over the water-bath for one hour, and allowed to stand twelve
hours more, when the supernatant liquid is separated by decantation the
precipitate washed thoroughly with dilute molybdate solution (1: 4)
dissolved in warm dilute ammonia, and the filter washed with hot water.
The ammoniacal solution is neutralized with hydrochloric acid, cooled,
mixed, drop by drop, with constant stirring, with from ten to twenty
cubic centimeters of magnesia mixture, and after a quarter of an hour
one-third the volume of ten per cent ammonia is added. This is allowed
to stand two hours, is filtered, washed with five per cent of ammonia
until the disappearance of the chlorin reaction, dried, burned in an
open crucible over a bunsen, and finally for a quarter of an hour, in a
covered crucible heated over the blast.
2. _Water-Soluble Phosphoric Acid._—To twenty grams of the substance
in a liter flask, are added 800 cubic centimeters of water, and shaken
every fifteen minutes for two hours; the volume made up to the mark
and the phosphoric acid in fifty cubic centimeters of the filtrate,
equaling one gram substance, is determined as under total.
3. _Reverted Citrate-Soluble Phosphoric Acid._—Two and five-tenths
grams substance are rubbed up with water, then washed upon the filter
with about 100 cubic centimeters of water, the residue on the filter
washed into a flask with a part of the measured citrate solution,
and digested one hour at 35° to 40° with 200 cubic centimeters of
Petermann’s citrate solution. The water and citrate extracts are made
up to a quarter of a liter each, and the phosphoric acid determined in
from twenty-five to fifty cubic centimeters, according to the quantity
present.
_Solutions._ 1. _Molybdate Solution._—375 grams of ammonium molybdate
are dissolved in two and five-tenths liters of water, and the solution
poured into two and five-tenths liters of nitric acid of 1.20 specific
gravity.
2. _Magnesia Mixture._—275 grams of crystallized magnesium chlorid and
350 grams of ammonium chlorid are dissolved in 3250 cubic centimeters
of water and filled up to five liters with ammonia of 0.96 specific
gravity.
3. _Petermann’s Solution._—One kilogram of citric acid is dissolved
in about two liters of water and 1350 cubic centimeters of ammonia
of 0.925 specific gravity and filled up with water to 5750 cubic
centimeters. The solution then has a specific gravity of 1.09; 300
cubic centimeters of ammonia of 0.925 specific gravity are now added.
=58. Swedish Official Method for Determination of Phosphoric
Acid.=[43]—The Swedish chemists determine phosphoric acid in
fertilizers both by the molybdate and the citrate methods. These
methods carefully conducted according to the directions given below,
give very concordant results. In doubtful cases the former method is
taken as the deciding one, it having proved by long practice to give
very satisfactory results.
_Reagents for the Molybdate Method._—1. _Molybdic Solution._—Prepared
by dissolving 100 grams of finely powdered molybdic acid with heat,
in 400 grams of eight per cent ammonia of 0.967 specific gravity
and pouring the solution into 1,500 grams of nitric acid of one and
two-tenths specific gravity; or else by dissolving 150 grams ammonium
molybdate in one liter of hot water, and pouring the solution into one
liter of nitric acid of 1.2 specific gravity. Prepared in this way,
the molybdic solution will contain, in the former case five per cent,
in the latter case from five to six per cent of molybdic acid, and 100
cubic centimeters of it are required for precipitating one-tenth gram
of phosphorus pentoxid.
2. _Magnesia Mixture._—Prepared from 110 grams of crystallized
magnesium chlorid, 140 grams ammonium chlorid, 700 grams of eight per
cent ammonia of 0.967 specific gravity and 1,300 grams of distilled
water. The mixture is filtered after a few days, if necessary; ten
cubic centimeters of the same are required for precipitating one-tenth
gram of phosphorus pentoxid.