Soluble in Alkaline Ammonium Citrate.=[241]—From twenty-five to fifty
grams of the sample of soil are triturated with 100 cubic centimeters of
alkaline ammonium citrate and placed in a flask of 250 cubic centimeters
capacity, and allowed to digest for one hour at a temperature of from
35°–40°. After cooling, make up to the mark, filter, take 200 cubic
centimeters of the filtrate, evaporate to dryness on a sand-bath in a
platinum dish, heat lightly at first, and afterwards to a higher
temperature. Take up the residue of the incineration with water and
about two cubic centimeters of nitric acid, heat a few minutes gently,
filter into a bohemian flask and precipitate with fifty cubic
centimeters of ammonium molybdate solution, and estimate the phosphoric
acid in the usual way.
=377. Method of Dyer for Total and Assimilable Phosphoric Acid.=—For the
determination of phosphoric acid, soluble in citric acid, secured as
described in paragraph =328=, 500 cubic centimeters of the filtrate
obtained, corresponding to fifty grams of the soil, are evaporated to
dryness in a platinum dish, gently ignited, extracted with hydrochloric
acid, again evaporated, ignited and extracted, and the phosphoric acid
determined as below in the method applied to the hydrochloric acid
extract of the soil itself.
The total phosphoric acid is determined in each case in ten grains of
the dried soil and also in twenty-five grams, the mean of the two
results being taken. The numbers obtained in each case are, however, all
but identical, the difference in the duplicate percentages being in most
cases only a small one in the third place of decimals.
The soil is incinerated and digested with hydrochloric acid, and
evaporated to dryness, redigested with acid, filtered, and washed. The
filtrate and washings are concentrated to a small bulk, and treated, in
the cold, with excess of a solution of ammonium molybdate in nitric
acid. After standing forty-eight hours, the liquor is decanted through a
filter, the precipitate washed several times by decantation, first with
dilute acid, then with pure water in very small doses, and finally
transferred to the filter and washed free from excess of acid. The
ammonium phosphomolybdate is then dissolved in ammonia, evaporated to
dryness in a platinum capsule, and dried to constant weight at 100°. The
residue contains three and one-half per cent of its weight of phosphoric
acid. This is the method of Hehner; and for determining small quantities
of phosphoric acid, such as occur in soils or in solutions of iron and
steel, is in the opinion of Dyer very much to be preferred to the
old-fashioned method of conversion into magnesium ammonium phosphate.
The solubility of the yellow precipitate in the small quantity of
wash-water used is in most cases negligible. As a matter of fact, the
quantity of wash-water used in these analyses was found capable of
dissolving only 0.005 gram of precipitate, of which only 0.00017 is
phosphoric acid, making an error of 0.0017 per cent on the soil if ten
grams be used, or of only 0.0006 if twenty-five grams be used. In the
citric acid experiments the solution from fifty grams of soil is used,
when the error due to solubility of precipitate shrinks to 0.0003 per
cent. The correction for this solubility is, however, made in each case.
It may be observed that the method of Hehner is not applicable if the
molybdic solution be added to a hot liquid, since, in that case, some
molybdic acid is sure to crystallize with the yellow precipitate.
Moderate and careful warming to about 35° hastens precipitation, but it
is preferable, when speed is not a special object, to precipitate cold,
and leave the beaker standing at the laboratory temperature over night,
or longer if the quantity to be determined is very minute.
=378. Methods of Berthelot and André.=—The phosphorus in the soil may be
found under three forms; _viz._,
1. Phosphoric acid in phosphates.
2. Phosphoric acid in ethers which alkalies decompose slowly and
oxidizing agents destroy with regeneration of phosphoric acid.
3. Organic compounds or mineral compounds of phosphorus which are
resolved by alkaline solutions with formation of phosphoric acid and
which are not reduced to this state by the reagents employed in the wet
way of decomposition except after a contact of indefinite length and
uncertainty.
It is, therefore, seen that the employment of oxidizing agents for the
valuation of phosphoric acid in soils and vegetables is not a very
reliable procedure. The same is true after incineration by which more or
less phosphorus may be lost or rendered insoluble in acids. The methods
used by Berthelot and André for the estimation of these forms of
phosphorus are as follows:[242]
_Total Phosphorus._—The sample is at first oxidized by a current of air
near a red heat and the vapors are conducted over a column of sodium or
potassium carbonate at the same temperature. The combustion is finished
in a current of pure oxygen. All phosphorus compounds, even those which
are volatile, are by this treatment converted into phosphoric acid. The
part of the acid held by the carbonate is to be determined with the
non-volatile portions.
A less certain method of oxidation consists in mixing the material with
potassium nitrate and carefully throwing it little by little into a red
hot platinum crucible.
_Estimation of the Phosphoric Acid Pre-existing as Phosphates._—The
sample is treated with a cold dilute acid incapable of exercising an
oxidizing or decomposing effect on the ethers. The dissolved acid is
precipitated and weighed in the usual way. The precipitate first
obtained should be ignited and the phosphoric acid taken up and
reprecipitated. This is necessary to remove any organic matter or silica
which the first precipitate may contain.
_Estimation of Ethereal Phosphoric Acid._—The sample is boiled for some
time with a non-oxidizing acid or with a concentrated solution of
potash. The phosphoric acid dissolved represents that which was present
as phosphates and as ethers. From this, deduct that portion pre-existing
as phosphates and the remainder represents the part derived from the
ethereal compounds.
_Estimation of Phosphorus in Organic Compounds and Special
Minerals._—From the total phosphoric acid deduct that found as
phosphates and ethers. The difference represents the quantity combined
as noted in the caption. Illustration.
A sample of soil contained:
Total phosphoric acid 0.292 per cent.
—————
Of this, pre-existing as phosphoric acid 0.109 „
As ethereal phosphoric acid 0.074 „
As organic phosphoric acid 0.109 „
—————
Sum 0.292 „
=379. Method Used at the Riga Station.=—In the method pursued at the
experimental station at Riga[243] the organic matter in the fine earth
is first destroyed by igniting twenty-five grams in a muffle. The
ignited residue is placed in a 250 cubic centimeter erlenmeyer and
digested with 150 cubic centimeters of ten per cent hydrochloric acid.
The digestion is continued for forty-eight hours with frequent shaking.
The filtrate is evaporated to dryness in a porcelain dish to separate
any dissolved silica. The residue is taken up with dilute hot nitric
acid, filtered, and the phosphoric acid precipitated with ammonium
molybdate. The final weighing is made as magnesium pyrophosphate,
following the usual procedure in respect of precipitation and washing.
Experiments show that approximately ninety-five per cent of the
phosphoric acid are obtained by one extraction and five per cent by a
second, conducted exactly as the first. Thoms draws the following
conclusions from a long series of determinations:
(1) For the simple purpose of determining the need of a soil for
phosphorus fertilizer a single extraction with ten per cent hydrochloric
acid is sufficient. The difference between the first and second
extraction; _viz._, five to six per cent is too small to be of any value
from a practical point of view.
(2) A soil which has been ignited until organic matter is destroyed
gives up to the hydrochloric acid solvent about fourteen per cent more
phosphoric acid than a non-ignited sample would.
(3) The mean temperature in the flask during extraction on a steam-bath
is 74°.
=380. Method of Hilgard.=[244]—The weighed quantity of soil (usually
from three to five grams) is ignited in a platinum crucible, care being
taken to avoid all loss. The loss of weight after full ignition gives
the amount of chemically combined water and volatile and combustible
matter.
The ignited soil is now removed to a porcelain or glass beaker, treated
with four or five times its bulk of strong nitric acid, digested for two
days, evaporated to dryness, first over the water-bath and then over the
sand-bath, moistened with nitric acid, heated and treated with water.
After standing a few hours on the water-bath it is filtered and the
filtrate is evaporated to a very small bulk (ten cubic centimeters) and
treated with about twice its bulk of the usual ammonium molybdate
solution, thus precipitating the phosphoric acid. After standing at
least twelve hours, first at a temperature of about 50°, it is filtered
and washed with a solution of ammonium nitrate acidified with nitric
acid. The washed precipitate is dissolved on the filter with dilute
ammonia water. After washing the filter carefully, the ammoniacal
solution is treated with magnesia mixture, by which the phosphoric acid
is precipitated. After allowing it to stand twenty-four hours it is
filtered, washed in the usual way, dried, ignited, and weighed as
magnesium pyrophosphate, from which the phosphoric acid is calculated.
When a gelatinous residue remains on the filter after dissolving the
phosphomolybdate with ammonia it may consist either of silica not
rendered fully insoluble in the first evaporation, or, more rarely, of
alumina containing phosphate. It should be treated with strong nitric
acid, and the filtrate with ammonium molybdate; any precipitate formed
is, of course, added to the main quantity before precipitating with
magnesia solution.
=381. Separation of Phosphoric Acid From Iron and Alumina.=—The
following methods are suggested by Wolff[245] for the complete
separation of the phosphoric acid from the iron and alumina in soil
analysis, where large quantities of these bases are found in solution:
1. After the separation of the greater part of the iron and alumina the
phosphoric acid is precipitated from the solution in nitric acid by
molybdic acid. The process is carried on as follows:
The acid extract is heated in a flask to boiling and the iron oxid
completely reduced by the gradual addition of small particles of sodium
sulfite. While still warm the free acid is neutralized with soda-lye,
and ammonia added until the ferrous hydroxid and the aluminum hydroxid
are completely separated. Acetic acid is now added in excess and until
about four-fifths of the whole precipitate have passed again into
solution. Then, after boiling for a moment, the whole is quickly
filtered through a large filter with a cover, and the contents of the
filter finally washed slightly. All the phosphoric acid is thus obtained
in combination with some alumina and a very little iron. Nearly the
whole of the iron and the larger part of the alumina, by this
precipitation, are found in the filtrate and therefore cannot disturb
the estimation of phosphoric acid in succeeding portions. The filter is
now filled with boiling water and a little nitric acid added. The
precipitate is dissolved and received in a beaker. The precipitation of
the phosphoric acid is then accomplished by ammonium molybdate in the
presence of nitric acid. After twenty-four hours all the phosphoric acid
is thus precipitated and the precipitate is free from iron.
2. By the method of Schulze[246] the iron is completely, and the
alumina, with the exception of a small quantity, separated, and the
precipitation of the phosphoric acid is accomplished either by the
addition of a small quantity of tartaric acid and afterwards magnesium
sulfate, or directly by means of ammonium molybdate. The principle of
the separation depends on the fact that when the hydrochloric acid is
nearly neutralized with soda or ammonia, and the solution boiled after
treatment with ammonium formate, the greater part of the alumina remains
in solution. The precipitate is quickly filtered, washed with hot water,
dried, taken from the filter and fused in a silver crucible with pure
caustic alkali, either soda or potash. On solution and boiling with
water, the iron is completely separated from the phosphoric acid, and
from the small quantity of the alumina present the precipitation of the
phosphoric acid can now be accomplished, either by saturation of the
alkaline solution with hydrochloric acid and the direct addition of the
magnesia solution after the addition of a little tartaric acid and
ammonia, or after the addition of nitric acid by ammonium molybdate.
=382. Estimation of Phosphoric Acid in Muck and Peat Soils.=—The amount
of phosphoric acid obtained by extraction with hydrochloric or sulfuric
acid is markedly less in these soils than that obtained after the
incineration of the sample, as pointed out by Schmoeger.[247] This is
due to the fact that the phosphoric acid is ordinarily combined in the
form of nuclein. Extraction of the soils with ether shows that it is not
present in the form of lecithin. The nuclein products, as is well known,
are decomposed by heating in presence of a liquid at a high temperature
for some time. The heating can either take place in an autoclave or in
sealed glass tubes. The method is as follows:
The sample of soil is thoroughly rubbed up in a mortar with water, and
then hydrochloric acid added until one gram of the water-free peat is
suspended in about ten cubic centimeters of twelve per cent hydrochloric
acid. The sample is placed in a glass or porcelain vessel in an
autoclave and heated to 140°–160° for ten hours. The phosphoric acid is
then determined in the extract in the usual way. The percentage of
phosphoric acid determined in this way is found to correspond to the
amount determined by the incineration of the substance.
The total phosphoric acid is determined in peats by the incineration of
the sample and the estimation of the phosphoric acid in the ash. The
phosphoric acid soluble in hydrochloric acid solution is determined by
extracting a sample of the soil with twelve per cent hydrochloric acid
in the usual way. The difference between this and the total is
calculated as phosphoric acid in organic compounds.
Or the total phosphoric acid is determined by treating the soil with
twelve per cent hydrochloric acid, in the proportion of one gram of soil
to ten cubic centimeters of the acid, and the solution is placed in an
autoclave and heated for ten hours to 140°–160°, as above described. The
phosphoric acid is then determined by the usual method. The difference
between the total phosphoric acid as thus determined and the phosphoric
acid soluble in hydrochloric acid is calculated as phosphoric acid in
organic compounds.
=383. Method of Goss.=—On account of the length of time required to
determine the phosphoric acid in soils by the usual methods, Goss[248]
has proposed the following modification which in his hands has given
satisfactory results:
Weigh ten grams of the air-dried soil, which has been sifted through a
one millimeter mesh sieve, and transfer to a pear-shaped, straight
necked, kjeldahl digestion flask, which has been marked to hold 250
cubic centimeters. Add approximately seven-tenths gram of yellow
mercuric oxid and twenty to thirty cubic centimeters of concentrated
sulfuric acid, as for the determination of nitrogen. Twenty cubic
centimeters of acid are nearly always sufficient, but in the case of
unusually finely divided clay soils containing little or no sand it is
necessary to use thirty cubic centimeters to prevent caking of contents
of flask. In doubtful cases twenty cubic centimeters of acid should
first be added and at the end of five or ten minutes, if contents show a
tendency to cake, ten cubic centimeters more should be introduced.
Thoroughly mix the contents of the flask by shaking, place on a suitable
support over a burner, boil for one hour, cool, add about 100 cubic
centimeters of water, five cubic centimeters of concentrated
hydrochloric acid, and two cubic centimeters of concentrated nitric
acid, boil gently for two minutes to oxidize iron, cool, make up to
volume, and filter through a dry folded paper until perfectly clear. In
order to secure a clear filtrate it will usually be found necessary to
pour the first portion of the filtrate back through the paper three or
four times. Transfer 100 cubic centimeters of the filtrate to an
ordinary flask of about 450 cubic centimeters capacity, add strong
ammonia until a permanent precipitate forms, then six or eight cubic
centimeters of nitric acid to dissolve the precipitate, and boil until
clear. In the case of many soils it is not absolutely necessary to
oxidize with hydrochloric and nitric acids, as a clear solution can be
secured at this point without further oxidation. In the case of some
soils, however, and especially in subsoils, the solution cannot be
cleared up even by prolonged boiling with nitric acid, but if the
solution have been previously oxidized, a clear solution can be secured
without any difficulty whatever. Remove the flask from the lamp and
after two minutes add seventy-five cubic centimeters of molybdate
solution, place the unstoppered flask in an open water-bath kept at a
temperature of 80° for fifteen minutes, shaking vigorously four or five
times while in the bath; then remove, let stand ten minutes to allow
precipitate to settle, filter through a nine centimeter filter avoiding
too strong a pressure at first, wash the flask and precipitate
thoroughly with ammonium nitrate solution, place the flask in which the
precipitation was made under the funnel, shut off pump and close all
valves to filtering jar to form an air-cushion and prevent too rapid
filtration, fill paper two-thirds full of hot water, add a few cubic
centimeters of strong ammonia, aid solution, if necessary, by stirring
precipitate with a small glass rod.
As pointed out by Hilgard, aluminum is sometimes carried down with the
phosphoric acid upon precipitating with molybdate solution, in which
case some of the phosphoric acid will not be dissolved in the treatment
with ammonia. This will be indicated, first, by the appearance of a
white precipitate upon dissolving the yellow precipitate in ammonia;
and, second, by the difficulty experienced afterward in washing. If such
a precipitate be present in any appreciable quantity, proceed as
follows:
After washing out all the ammoniacal solution in the usual manner, place
a small beaker under the funnel, close all valves, fill the filter
one-third full of hot water, add the same amount of concentrated
hydrochloric acid, proceed as if dissolving phosphomolybdate in ammonia,
and receive final solution and washings in flask used. As soon as the
yellow precipitate is dissolved open the valve to filtering jar but do
not turn on the pump; after the solution has all passed through rinse
the filter once with a small amount of hot water; after the last portion
has passed through remove the flask and place a No. 1 lipped beaker
under the funnel and heat the solution in the flask to boiling. If the
solutions have not been oxidized, a blue color is sometimes present upon
dissolving the yellow precipitate in ammonia. This can be discharged by
boiling the ammoniacal solution for a minute or two and shaking at the
same time. Again pour the solution through the filter, avoiding use of
pump at first, otherwise loss from spattering is likely to ensue, wash
out flask and filter with a small amount of hot water, (the total
filtrate should not exceed fifty cubic centimeters), add hydrochloric
acid to contents of beaker while hot, until yellow color appears, then
add a few drops of ammonia until solution clears, cool, add fifty cubic
centimeters of filtered magnesia mixture from a burette, a drop at a
time with constant stirring, let stand fifteen minutes, add twenty cubic
centimeters of strong ammonia specific gravity nine-tenths, let stand
over night, filter, wash precipitate with dilute ammonia, dry, ignite
intensely over blast-lamp for ten minutes, cool in desiccator and weigh
Mg₂P₂O₇ secured.
_Time of Digestion._—Experience has shown that very little phosphoric
acid is extracted from the sample by digestion with sulfuric acid after
the first thirty minutes.
_Time Required to Precipitate Phosphomolybdate._— When the yellow
precipitate is obtained according to the method of Goss practically the
whole of it will be thrown down in five minutes.
_Agreement with Standard Methods._—Comparative tests of the Goss method
against standard methods have shown that it gives almost identical
results with them. The variations were never more than 0.02 to 0.03 per
cent.
While this method has not been sufficiently tried to receive
unconditional recommendation it possesses merits which entitle it to the
attention of analysts.
=384. Estimation of the Sulfuric Acid.=—Sulfuric acid is generally
present in small proportions in soils. Since the plants have need of
sulfur it is proper to inquire into the presence of the compound which
is its principal source. It is in combination with lime that sulfuric
acid exists almost always. In addition to this there is also some sulfur
combined with the organic matter of the soil.
By digesting a soil for six hours with hot, concentrated nitric acid the
sulfates are dissolved, and there is transformed into sulfuric acid an
important part of the sulfur which is combined with the humic
substances. The quantity of soil to be operated upon should be about
fifty grams.
After filtering and washing with hot water the filtered liquor is
collected, in the French Commission method,[249] in a flask and carried
to boiling, and five cubic centimeters of a saturated solution of barium
chlorid or sufficient to be in slight excess are added. The boiling is
continued for some minutes and the flask is allowed to stand for
twenty-four hours. The filtrate is received upon a filter and washed
with boiling water. The filter is dried and incinerated, allowed to
cool, and as there may have been a slight reduction of the sulfate a few
drops of nitric acid are added and a drop of sulfuric acid. It is now
evaporated to dryness on a water-bath, heated to redness for a few
moments, cooled and weighed. The weight of the barium sulfate obtained
multiplied by 0.3433, gives the quantity of sulfuric acid obtained from
the fifty grams of soil.
If it is desired to estimate only the sulfur which exists in the form of
sulfate it is necessary to treat the soil with hydrochloric acid in a
very dilute state, heating for a few moments only and afterwards
precipitate by barium nitrate. If, on the other hand, it is desired to
estimate the total sulfur which is sometimes of great interest, it is
necessary to employ the process of Berthelot and André.
=385. Method of Berthelot and André.=—Sulfur may exist in the soil in
three forms; _viz._,
1. Mineral compounds, consisting generally of sulfates and sometimes of
sulfids.
2. Sulfur, existing in ethereal compounds or their analogues, as in
urine.
3. Organic compounds containing sulfur.
_Estimation of Total Sulfur._—The principle on which this operation, as
described by Berthelot and André, rests is that already described for
phosphorus; _viz._, oxidation in a current of oxygen and passing the
vapors over a column of alkaline carbonate at or near a red heat.[250]
The ordinary methods of oxidation in the wet way give generally inexact
results.
_Estimation of Sulfur Pre-existing as Sulfates._—The sample is treated
with cold, dilute hydrochloric acid. The filtrate is treated with barium
chlorid, the precipitate collected, dried, ignited, washed with a
mixture of sulfuric and hydrofluoric acids to remove silica, and
afterwards weighed as barium sulfate.