quantities of earth are taken for analysis, from as little as half a
gram for soils which are rich in carbonate, up to five or even ten grams
for soils which are poor in carbonate. The apparatus is connected with a
mercury pump for the purpose of exhausting the air as completely as
possible therefrom. For this purpose the flask in which the carbonate is
disengaged is made in the shape of a tubulated retort. Through the
opening into the retort, a narrow tube is introduced and connected with
a small funnel by means of a rubber tube supplied with a pinch-cock.
When the retort has been connected with the mercury pump a slight vacuum
is produced and the pinch-cock is opened and forty cubic centimeters of
distilled water allowed to enter. The pinch-cock is closed soon enough
to retain a portion of the water in the funnel. The retort is then
heated and a vacuum partially produced by means of the pump. When the
flask is boiling, the steam drives out the air. A refrigerating jacket
is connected with the tube leading from the retort to the pump by means
of which the steam is condensed and falls back into the flask. After
some minutes of boiling, a vacuum is produced; the lamp is then taken
away and a cylinder, graduated at 100 cubic centimeters and filled with
mercury, is placed over the lower orifice of the pump, and there is
introduced into the apparatus, by the funnel above described, some
hydrochloric acid in small quantities, but sufficient only to saturate
the whole of the carbonate in the sample of soil taken. Usually three or
four cubic centimeters will be sufficient. The acid should be added in
such quantities as to prevent the production of any large amount of
foam. If frothing should be excessive a little oil can be added to the
flask. The whole of the carbon dioxid produced in the reaction is
withdrawn by means of the mercury pump and collected in the graduated
jar. Towards the end of the operation the flask is heated anew in order
to produce an ebullition which is continued for some time. The volume of
gas collected is measured after making the proper corrections for
pressure and temperature. Afterwards the carbon dioxid which has been
produced is absorbed by two or three cubic centimeters of a solution of
potash of 42° baumé. This potash is introduced into the graduated jar by
means of a pipette bent into the form of a =ᥩ= in the lower portion. If
the whole of the gas is not absorbed the volume which remains is read,
and this is subtracted from the original volume after having made the
proper corrections for pressure and temperature. The difference gives
the quantity of carbon dioxid contained in the amount of earth employed.
From this the actual weight of the calcium carbonate is computed. This
official French method does not appear to possess any advantage in
accuracy to the usual absorption method and is far more complicated.
=361. Estimation of the Active Calcareous Matter in Soils.=—Like other
soil elements, the calcium carbonate exists in different degrees of
fineness and availability in the soil. It must be admitted that the fine
particles play the most important rôle. The calcium carbonate, which
exists in large fragments, presents only a circumscribed surface and
remains almost inactive, although it is easily corroded by the rootlets
of plants. It is possible to estimate in a rapid way, the quantity of
fine carbonate in the soil, considering that in a time relatively short,
feeble acids act upon calcareous matter proportionally to the surface
which it presents, and that it attacks, therefore, especially the finest
particles. By measuring the amount of carbon dioxid set free under the
action of dilute acids it is possible to estimate the content of
available calcareous matter in the soil.
The apparatus of Mondesir is used for this purpose by the French
chemists. It is composed of a tubulated flask of about 600 cubic
centimeters capacity. The interior tubulature carries a manometer fixed
by means of a stopper. This is formed of a rubber tube, terminated by a
glass tube, whose extremity is united to a little rubber bag, very
flexible, placed in the interior of the flask.
_Graduation of the Apparatus._—If the apparatus is new it is necessary
to begin by graduating it. The rubber bag is filled with water, the air
being carefully excluded, in such a way that the level of the water
comes just a little above the bend in the tube. There are placed in the
flask 125 cubic centimeters of water and it is shaken for a few seconds.
The flask and the manometer are then unstoppered and the level of the
water in the manometer is made to equal the level of the water in the
flask. With a rubber ring the level of the water in the manometer tube
is marked. The manometer is then stoppered. There are then added to the
flask two-tenths gram of pure calcium carbonate. The flask is closed and
shaken for a minute. There are then added, enclosed in a little piece of
filter paper, six-tenths of a gram of pulverized tartaric acid and the
flask immediately closed and shaken several times. The manometer tube is
then uncorked and moved until the level of the water reaches the point
marked before. The difference in level after the height of the water
remains constant is then read. The depression in the level observed,
corresponds to two-tenths gram of pure calcium carbonate.
=362. Estimation of the Available Calcareous Matter in the Soil.=—There
is introduced into the flask of the apparatus a quantity of soil varying
in amount in accordance with the content of carbonate which it is
supposed to contain. There are added 125 cubic centimeters of water and
the flask is shaken for a minute. As in the test given before, the level
of the water in the manometer is then made to correspond to that of the
water in the flask. The level in the manometer is marked as before with
a rubber band, and the manometer is then closed. There are then added,
contained in a piece of filter paper, two grams of pulverized tartaric
acid and the operation is finished as described before. The amount of
tartaric acid added, in general, should be three times as much as the
amount of calcium carbonate supposed to be contained in the earth. The
pressure in the manometer being proportional to the quantity of carbon
dioxid disengaged, it is easy to calculate the quantity of calcium
carbonate in a state of fine division contained in the soil taken for
the test.
In order to fill the rubber bag it is necessary to put it in its proper
place in the apparatus. The flask is filled with water in order to
flatten the rubber bag and expel the air from it. It is then closed with
a cork. Afterwards, with the aid of a small funnel and with a copper
wire placed in the tube, the lower extremity of which descends just to
the elbow, the air in the tube is replaced by water. The operation is
finished by uncorking the flask and inclining it or shaking it after a
partial vacuum has been established. It is useless to attempt to drive
off the last particles of the air. The rubber bag should have a content
of about double the volume of the whole of the interior of the
manometric tube. In the washing which is necessary between two
successive operations, it is well to fill the flask entirely with water
in order to expel all the carbon dioxid which it may contain.
The same remark may be made of this method of determination as was made
of the last one. In the present case, however, the operation is not
quite so complicated. When the apparatus is once arranged, it will admit
of rapid determinations.
=363. Lime Method at the Riga Station.=—Ten grams of the non-ignited
sample of the fine earth are digested with 100 cubic centimeters of ten
per cent hydrochloric acid, in a 250 cubic centimeter erlenmeyer for
twenty-four hours on the steam-bath, with frequent shaking. The
filtrate, with washings after the addition of five cubic centimeters
strong hydrochloric acid, is evaporated to dryness in a porcelain dish
and the residue taken up with dilute hydrochloric acid. After filtering,
ammonia is added in excess, the excess removed by evaporation, and the
mass is again filtered. In the filtrate, the lime is thrown down with
ammonium oxalate, filtered, ignited, and weighed as calcium oxid.
The above method cannot give exact results chiefly because more or less
lime may be carried down with the phosphoric acid. Also if manganese be
present it will be thrown down with the lime. These errors are
compensatory, but only by chance could the compensation lead to
exactness. It would be better in all cases to remove the iron and
alumina in such a way as would avoid loss of time.
=364. Estimation of Assimilable Lime.=—In the determination of the total
lime in soils or even of that part present as carbonate, it is not to be
assumed that the quantity assimilable by plants is known; particles of
lime minerals in soils are corroded only superficially by the rootlets
of plants and any process which would attack only the superficies of the
lime particles would thus more nearly resemble the activity of the
solvent forces of plant growth. Oxalic acid is a reagent of this kind,
attacking only the surfaces of lime particles. Reverdin and de la Harpe
guided by this fact have based a method for determining the amount of
lime present in the soil in an available state on the solvent action of
oxalic acid.[233] After the total lime content has been determined,
twenty grams of the soil sample are covered with 200 cubic centimeters
of a solution containing in molecular proportion a known quantity of
sodium oxalate and carbonate. The mixture is digested on the water-bath
for one hour. By this treatment all lime minerals are converted
superficially into oxalate while particles containing magnesia are not
affected. After filtering and washing well, the filtrate and wash-waters
are acidulated with hydrochloric acid. If any precipitate of organic
matter be produced separate it by filtration. Treat the filtrate with a
slight excess of sodium acetate by which process the excess of
hydrochloric acid is replaced with acetic after which the oxalic acid
may be separated by treatment with calcium chlorid and subsequently
titrated with potassium permanganate in presence of excess of sulfuric
acid. The oxalic acid obtained, deducted from the quantity originally
present will give the amount consumed on the surfaces of the lime
particles and consequently the amount of lime corresponding thereto
which may be considered as available for plant growth.
=365. Method of the Halle Station for Lime.=[234]—a. _In Phosphates,
Limestones, etc._—Four grams of the prepared substance are heated with
fifty cubic centimeters of hydrochloric acid and five cubic centimeters
of nitric acid, in a porcelain dish on the water-bath to dryness, and
left for a few hours at 105° for the purpose of separating the silicic
acid. The dry residue is moistened with hot water and a few drops of
hydrochloric acid, and allowed to stand for some time with frequent
stirring. The contents of the dish are then washed into a half-liter
flask, filled up to the mark and the separated silicic acid removed by
filtration. If the silicic acid is not taken into account, the solution
can be made directly in a half-liter flask.
After filtration, an aliquot part of the filtrate is neutralized in a
500 or 250 cubic centimeter flask with ammonia, again acidified with a
few drops of hydrochloric acid and allowed to stand six hours at least,
in the cold, with ammonium acetate. For each four grams of the substance
fifty cubic centimeters of an ammonium acetate solution are used, made
by dissolving in one liter of water 100 grams of ammonium acetate. If
phosphoric acid is present in excess, iron and aluminum oxids are
precipitated completely as phosphates. If iron and aluminum oxids are in
excess, the excess must be precipitated by ammonia. If it is feared that
in the subsequent precipitation of the lime by ammonium oxalate there
may be still some phosphoric acid in solution, before precipitation with
ammonium acetate the proper amount of ferric chlorid is added and the
iron is afterwards precipitated with ammonia. It is certain that in the
presence of oxalic acid and phosphoric acid the lime is precipitated as
oxalate, but should it be feared that traces of calcium phosphate are
precipitated with the iron and aluminum phosphates the precipitate of
iron and aluminum phosphates may be dissolved in hydrochloric acid,
neutralized with ammonia, again acidified and a second time precipitated
with ammonium acetate and the filtrate added to that first obtained.
For the further estimation the filtrates are united and a quantity
corresponding to a given part of the original sample, and being in
volume from fifty to one hundred cubic centimeters is made slightly acid
with acetic acid and while hot precipitated with dilute ammonium
oxalate. The filtrate must contain acetic acid since calcium oxalate is
best precipitated from a slightly acetic acid solution. The filtering of
the calcium oxalate should not take place until from six to twelve hours
after precipitation, and during this time it should stand in a warm
place. Filter paper of the best quality should be used for the purpose.
The dried precipitate is brought into a platinum crucible together with
the filter; the filter is first incinerated over an ordinary bunsen and
the calcium oxalate converted into calcium oxid by ignition for fifteen
minutes over the blast. It is then cooled in a well-closed desiccator
and weighed as oxid. If in the precipitation of the iron and aluminum
phosphates sodium acetate be employed instead of ammonium acetate, the
precipitation must take place hot and filtration also be accomplished on
a hot filter.
b. _Estimation of Lime in Soils._—For the estimation of lime in soils
there may be used either the acid soil-extract, prepared as under the
direction for the estimation of potash, or twenty grams of the soil may
be treated with hydrochloric acid and a few drops of nitric acid, and
evaporated to dryness in a porcelain dish and the silicic acid separated
as described for the estimation of lime in phosphates and limestones. In
the case of soils, iron and aluminum oxids can be precipitated directly
with ammonia since the small quantity of phosphoric acid usually
contained in soils is not sufficient to influence in any way the
estimation of the lime. For example suppose there is 0.10 per cent of
phosphoric acid contained in a soil. In case the whole of this
phosphoric acid is taken down with the lime it would only amount to
about 0.10 per cent of calcium oxid precipitated as phosphate. This
case, however, is very improbable since it is much more likely that the
iron and aluminum phosphates will be precipitated and the whole of the
phosphoric acid be carried down with them instead of being precipitated
with the lime.
The precipitation of the lime and its subsequent treatment are to be
conducted as just described.
=366. Estimation of the Magnesia.=—Magnesia is a much more rare element
in the soil than lime. It is usually necessary to operate upon
considerable quantities of earth in order to determine the magnesia with
any degree of precision. From ten to twenty grams of the soil are taken.
The decomposition is accomplished as in the case of lime. A few drops of
barium nitrate are added for the purpose of precipitating any sulfuric
acid present. Some ammonia and ammonium carbonate are added to
precipitate the iron and aluminum oxids, the lime and the excess of
barium introduced, as well as the phosphoric acid. The operation is best
conducted on a dilute solution having a volume of from 400 to 500 cubic
centimeters. The solution from which the lime has been precipitated,
contains with the magnesia, large quantities of ammoniacal salts which
it is necessary to destroy. For this purpose the solution is
concentrated in a flask until its volume is about ten cubic centimeters.
About ten cubic centimeters of nitric acid are added and the whole
brought to the boiling point. Afterwards a few drops of hydrochloric
acid are added. Continuing the heating, hydrochloric acid is added in
small portions and, from time to time, some nitric acid until the
bubbles indicating the setting free of gaseous nitrogen, resulting from
the action of the nascent chlorin upon the ammonia, have completely
ceased to appear. The whole is then evaporated on a sand-bath in a
porcelain dish in order to separate the silica. The residue is taken up
by water containing a few drops of nitric acid. It is filtered and
evaporated to dryness in a covered porcelain dish. Upon the residue four
or five grams of oxalic acid, in a state of powder, are placed. A little
water is added in such a way that the moist mass covers entirely the
matter in the dish. In order to avoid all losses there is placed upon
the dish a funnel which serves as a cover. The dish is heated on a
sand-bath, but when the film which is formed begins to break there are
added from time to time, a little more oxalic acid and water until there
is no longer any disengagement of the vapor of nitric acid. Afterwards
it is evaporated to dryness and the heat raised to a low redness. The
magnesia is found in a free state or mixed with alkalies. It is washed
with a small quantity of water and collected upon a very small filter
paper. The filter paper is dried, burned, the ignition carried to
redness and afterwards cooled and weighed. In order to test the purity
of the magnesia it is transformed into sulfate by the addition of a few
drops of sulfuric acid. The excess of sulfuric acid is driven off by
heating moderately by means of a gas-burner moving it in a circular
manner round the bottom of the capsule and lifting the cover from time
to time in order to allow the vapors of sulfuric acid to escape. The
weight of the magnesium sulfate should correspond to that of the
magnesia from which it was formed.
Magnesia exists most often in the soil in the state of carbonate or
silicate. In this last state it is especially abundant in some soils,
such as those which are derived from mica schists, serpentines, etc. In
treating earth of this last quality with concentrated, nitric acid there
is dissolved also a notable part of the magnesia of the silicates. If,
however, it is treated for some minutes only with dilute hydrochloric
acid the amount of magnesia present as carbonate alone can be estimated
separately.
=367. Estimation of Magnesia in Soils.=—_Method of the Halle
Station._—For the estimation of magnesia the sample of soil or
fertilizer is brought into solution in the same way as is given for the
estimation of lime. After the separation of the silicic acid, the iron
and alumina are precipitated with sodium acetate. In the case of
phosphoric fertilizers, ferric chlorid should first be added in order
that the excess of phosphoric acid shall be in all cases certainly
combined with the iron. After this the lime is separated as usual with
ammonium oxalate. After the precipitation of the lime, the magnesia is
precipitated in an ammoniacal solution with sodium phosphate and the
ammonium magnesium phosphate estimated exactly as in the case with the
estimation of phosphoric acid, as magnesium pyrophosphate.
A simpler method for the estimation of magnesia consists in
precipitating it as ammonium magnesium phosphate in the presence of a
solution of ammonium citrate, the other bases remaining in solution. In
this case the operation is carried on in an inverse way as described
under the estimation of phosphoric acid, the proper quantity of the acid
solution being neutralized with ammonia and after the addition of sodium
phosphate, the required quantity of citrate solution added and a further
excess of ammonia supplied.
=368. Estimation of Manganese.=—The estimation of manganese in the
presence of Fe₂O₃, Al₂O₃, CaO, etc., presents peculiar difficulties. In
ordinary alluvial clays the quantity of manganese is proportionately
small and its estimation may be neglected. In volcanic clays the
quantity of manganese, in proportion to the lime and magnesia, is much
larger. The method used for estimating manganese is that of Carnot.[235]
The hydrochloric acid extract of the soil is evaporated to dryness and
heated with potassium bisulfate in order to destroy the organic
substance, the neutralized solution of the residue precipitated with
twenty cubic centimeters of hydrogen peroxid solution and thirty cubic
centimeters of ammonia. The colorless filtrate gives, with nitric acid
and bismuth peroxid, no trace of reaction for manganese. The
precipitate, washed by decantation, is carried into a carbon dioxid
apparatus and treated with oxalic acid and dilute sulfuric acid. From
the amount of carbon dioxid obtained, the quantity of manganese is
calculated on the supposition that the precipitate corresponds to the
formula Mn₆O₁₁.
=369. Estimation of the Manganese by the French Method.=—Manganese
exists in all plants and its presence in small quantities seems
necessary to vegetation. The method of estimation adopted by the French
Commission is the one proposed by Leclerc and is applicable even when
the base exists in small quantities.
Twenty grams of the soil are taken and the organic matter destroyed by
incineration. In a flask of 200 cubic centimeters capacity, are placed
thirty cubic centimeters of water and, little by little, some
hydrochloric acid for the purpose of decomposing the calcium carbonate.
When effervescence has ceased ten cubic centimeters of the same acid are
added and boiled for half an hour, filtered, washed, and the wash-water
and filtrate evaporated to dryness in a porcelain dish. Afterwards there
are added twenty cubic centimeters of nitric acid of one and two-tenths
density, and ten cubic centimeters of water. The liquor is boiled with
constant shaking. Afterwards there are thrown in, in two or three
portions, ten grams of lead dioxid. The boiling is stopped just at the
moment when all the lead oxid is introduced into the liquor and the
mixture is then shaken vigorously. The manganese is transformed by this
treatment into a highly oxygenized compound having a deep rose
coloration. It is transferred immediately afterwards to a graduated
cylinder of 100 cubic centimeters capacity, with the wash-waters the
volume is completed to 100 cubic centimeters and it is vigorously
stirred with a rod, flattened at its extremity, in order to obtain a
complete homogeneity of the liquid. The stirring rod is withdrawn and
the liquid left to settle. At the end of some minutes the principal part
of the liquid is clear, and it is decanted by means of a pipette
graduated at fifty cubic centimeters, and this quantity of the clear
liquid is poured into a small glass precipitating jar to which is added
immediately, with constant stirring, a solution of mercurous nitrate
from a graduated burette. The addition of the nitrate is arrested at the
moment when the rose color of the liquor disappears, and the volume of
the mercurous nitrate employed is read from the burette.
It is now necessary to determine the strength of the mercurous nitrate,
that is the quantity necessary to decolorize one milligram of manganese.
For this purpose dissolve by means of five cubic centimeters of
hydrochloric acid 150 milligrams of manganese dioxid, which is prepared
perfectly pure by means of precipitation. When the solution is complete
evaporate to dryness, add one cubic centimeter of sulfuric acid and heat
on a sand-bath until white fumes of sulfuric acid appear. Redissolve in
water and make the volume up to 100 cubic centimeters. Each cubic
centimeter of this solution should contain one milligram of manganese.
Take five cubic centimeters of this solution, equivalent to five
milligrams of manganese, treat in a capsule with twenty cubic
centimeters of nitric acid and ten cubic centimeters of water,
afterwards add ten grams of lead dioxid, carrying on the operation
exactly as described above. Fifty cubic centimeters, taken as before
described, are then decolorized by the solution of mercurous nitrate,
and thus it is easy to calculate the quantity of manganese which
corresponds to one cubic centimeter of the mercurous nitrate employed.
By a simple proportion the quantity of manganese contained in the twenty
grams of earth to be analyzed is calculated.
The mercurous nitrate is prepared by dissolving five grams of
crystallized mercurous nitrate in one liter of water; it is allowed to
repose for some time and is preserved in a well-stoppered flask.