soil where it is met with, principally in the state of anhydrous
sesquioxid or the hydrated sesquioxid of silicates. Some soils, however,
only contain iron in small proportions and it can happen that the
introduction of iron as a fertilizing element may be useful. Plants
assimilate iron only in small quantities, but it appears to be
indispensable to their development and to the proper functional activity
of their assimilating faculties. The method of estimation which is
recommended is based upon the decoloration of potassium permanganate by
iron in the ferrous state. The following description, based on the
method proposed by the French Commission, will illustrate the process to
be followed.
Ten grams of the soil are ignited in a porcelain capsule until all
organic matter is destroyed. The ignited mass is then introduced into a
flask of 100 cubic centimeters capacity with thirty cubic centimeters of
hydrochloric acid and fifteen cubic centimeters of water. It is boiled
for about half an hour. The iron oxid is dissolved and is found in
solution in the form of ferric chlorid. After filtering and washing, the
volume of the filtrate is reduced by evaporation to about twenty-five
cubic centimeters. The liquor is afterwards placed in a flask of from
100 to 150 cubic centimeters capacity, which is closed by a stopper
carrying a tube furnished with a valve destined to prevent the
re-entrance of the air. Ten cubic centimeters of dilute sulfuric acid
are added from a mixture containing twenty cubic centimeters of strong
acid and eighty cubic centimeters of water. Afterwards the iron is
reduced to the ferrous state by introducing into the flask, in
quantities of about five decigrams, metallic zinc and waiting after each
addition until the portion last added is dissolved before adding
another. This addition of zinc is continued until the iron is all
reduced. When this point is reached and the last portion of zinc added
is dissolved, the contents of the flask are transferred rapidly to a
precipitating glass of about one liter capacity, in which there has been
placed a little lately boiled but cold water. The flask is washed
several times with cold water, previously boiled, to remove from it all
traces of oxygen. The volume is made up to 500 cubic centimeters, and
afterwards, without any loss of time, by means of a graduated burette
and with constant stirring, a solution of potassium permanganate is
added which is stopped exactly at the moment when the liquor begins
taking on a light rose tint. The quantity of permanganate employed is
read from the burette and is proportional to the amount of iron
contained in the soil. A blank operation is made for the purpose of
detecting traces of iron which the zinc may contain. If, as often
happens, the soil contains a large amount of iron it is advisable to use
only one gram of it for this operation. The aspect of the earth will
indicate in general if it be very ferruginous.
_Preparation and Standardization of the Permanganate Liquor._—In one
liter of water are dissolved ten grams of crystallized potassium
permanganate and the quantity of iron which corresponds to one cubic
centimeter of this liquor is determined. It may be well enough to remark
that this liquor does not remain constant and it is necessary to titrate
it from time to time. For this purpose pure iron is taken. Piano wire
may be used, being almost pure iron. One-tenth of a gram of this wire is
dissolved in a flask in the manner recommended for treating the soil and
with the same quantities of acid and water. When the solution is
complete it is transferred to the flask to be estimated. It is made up
to one liter and permanganate added, just as in the case before
mentioned, until the rose color persists. There is thus determined the
quantity of iron which corresponds to each cubic centimeter of the
permanganate, and by a simple proportion, the quantity of iron contained
in the soil analyzed is determined.
The Italian agricultural chemists proceed essentially in the same manner
in determining the iron in soils, first igniting the sample and
afterwards extracting the iron in the ferric state with boiling
hydrochloric acid, reducing with hydrogen, and titrating with potassium
permanganate.
The following are the reactions which take place:
Fe₂O₃ + 6HCl = Fe₂Cl₆ + 3H₂O.
Fe₂Cl₆ + 2H = 2FeCl₂ + 2HCl.
10FeCl₂ + K₂Mn₂O₈ + 16HCl = 5Fe₂Cl₆ + 2KCl + 2MnCl₂ + 8H₂O.
Sulfuric may take the place of hydrochloric acid in the above reactions.
[Illustration:
FIGURE 68.
APPARATUS BY SACHSSE AND BECKER.
]
=371. Method of Sachsse and Becker.=[236]—Ferric oxid (not as silicate)
in soils can be estimated by reducing with hydrogen, and measuring the
hydrogen which is evolved by the action of the reduced iron on an acid.
The sample of soil is weighed in a platinum boat, the boat put into a
wide glass tube and heated in a stream of dry hydrogen. While this is
going on, water is boiled in the flask _A_ (see Fig. 68) from which the
stopper has been removed, to drive out the air. When the reduction of
the ferric oxid is complete, the boat is slipped out of the tube into
the flask without interrupting the hydrogen evolution. In order to
accomplish this without allowing the reduced iron to come in contact
with the air the flask is inclined, the end of the glass tube inserted
until it is covered with water and the boat is then dropped beneath the
water. The flask is closed with a cork provided with a funnel tube, _B_,
and a delivery tube _C_; the tap _a_ is opened, and tube _b_ connected
with a carbon dioxid apparatus from which the gas is passed into _A_
until all the air is displaced. This point is determined by filling the
burette _D_ with potash-lye by aspiration at _C_ and allowing the
escaping gas from _C_ to enter the burette as indicated in the figure.
Any residual gas in _D_ is removed by aspiration at _C_ and allowing the
potash-lye in _e_ to enter in its place. The end of the tube _C_ is now
placed under the measuring tube _D_, and the clamp _f_ opened and the
tap _a_ closed. The funnel is filled with dilute, boiled sulfuric acid,
the cork of _b_ replaced and connected with the carbon dioxid apparatus.
The burner under _A_ is lighted and acid let in. By continued boiling,
all the hydrogen is driven into _D_, the carbon dioxid being absorbed.
The measuring tube is then placed in a tall cylinder of water, the
volume of gas read and reduced to 0° and 760 millimeters barometric
pressure. To be certain that all carbon dioxid is absorbed, some fresh
potash-lye may be introduced into _D_ by carefully opening _d_. The iron
is then computed from the volume and weight of the hydrogen by the
formula (1) Fe + H₂SO₄ = 2H + FeSO₄.
If the substance analyzed contains iron silicates, these may be partly
decomposed with formation of ferrous sulfate, according to the reaction
(2) 2Fe + Fe₂(SO₄)₃ = 3FeSO₄. This will redissolve a part of the
metallic iron and yield ferrous oxid. In this case the contents of the
flask are cooled in an atmosphere of carbon dioxid, made up to 500 cubic
centimeters, of which 250 cubic centimeters are quickly filtered and
titrated with permanganate. In order to properly distribute the iron in
harmony with its previously existing states the following computations
may be made:
Represent the ferrous oxid corresponding to formula (1) by x
and that „ „ „ (2) by z
and that found by titration with permanganate by a.
We have then the equation x + z = a. Since seventy-two parts by weight
of ferrous oxid formed by formula (1) are equivalent to two parts by
weight of hydrogen, x parts of ferrous oxid would set free x/32 parts of
hydrogen; and this corresponds to the hydrogen found in _D_; _viz._, b.
If a = ¹⁄₃₆ then by solving the equations: Z = a − 36b and X = 36b. The
ferrous oxid arising according to formula (2), however, is derived in
such a way that only one-third of it corresponds to metallic iron. Then:
X + (⅓)z = (⅓)a + 24b. For computing the total ferric oxid reduced by
hydrogen there must, therefore, be added twenty-four parts by weight of
hydrogen for one-third of the ferrous oxid found by titration with
permanganate, and this quantity of ferrous calculated to ferric oxid.
Some silicates, such as the micas, give ferrous oxid with hot dilute
sulfuric acid. A correction for this is made by making one or more
determinations without previously reducing with hydrogen.
The method of procedure above described appears to be capable of giving
in an easily attainable manner some valuable indications of the state in
which iron exists in a soil. While plants do not use any notable
quantity of iron during their growth nevertheless its physiological
importance is unquestioned. The chief points of difficulty to be
considered are found in the changes which the iron may undergo even
while heating in a stream of hydrogen, and the practical difficulties of
obtaining carbon dioxid entirely free of air. The latter difficulty may
be overcome by making blank experiments with carbon dioxid alone and
estimating the volume of residual gas. The total volume of hydrogen
obtained is then to be diminished by the ascertained amount.
In regard to the second point it is known that both ferrous and ferric
oxids when ignited with hydrated silicates partly decompose and form new
silicates. Care should therefore be taken not to carry the temperature
too high during the process of ignition.
=372. Carnot’s Method for Estimating Phosphoric Acid in
Soils.=—Carnot[237] proposes the following procedure for the estimation
of phosphoric acid in soils. The principle of this method depends upon
the isolation of silica by the double precipitation of phosphomolybdate.
Ten grams of the sifted soil, dried at 100°, are charred if organic
matter be present. The charred mass is next moistened with water and
afterwards with nitric acid, until the carbonates are decomposed.
Afterwards the mass is digested with ten cubic centimeters of nitric
acid for two hours at about 100°, with frequent stirring and the
addition of fresh acid, from time to time, to replace that which has
been evaporated. After filtering and washing with hot water the filtrate
is evaporated to a volume of fifty cubic centimeters and treated with
five cubic centimeters of concentrated nitric acid and half a gram of
crystals of chromic acid. After covering the dish with a funnel to
return condensed vapors its contents are heated to the boiling point for
half an hour. At the end of this time five grams of ammonium nitrate are
added and afterwards fifty cubic centimeters of molybdate solution and
the mixture kept at a temperature of about 100° for an hour. The
precipitate obtained is washed twice by decantation with water
containing one-fifth of its volume of ammonium molybdate solution. It is
then dissolved in thirty cubic centimeters of ammonia diluted with an
equal bulk of warm water. The solution and the washings should measure
eighty cubic centimeters and the ammonia therein is neutralized with
nitric acid, keeping the temperature below 40°. When the yellow
precipitate formed ceases to redissolve on stirring, a mixture of three
cubic centimeters of pure nitric acid and five cubic centimeters of
water is added, together with the same quantity of molybdate solution.
The precipitate is brought upon a filter, washed first with water
containing one per cent of nitric acid and finally with a little pure
water, and dried at 100° and weighed. The weight of the precipitate
multiplied by the factor 0.0373 gives the quantity of phosphoric acid.
The object of the second precipitation is to relieve the process of the
necessity of rendering the silica insoluble, as the presence of silica
in the solution as above treated does not interfere with the complete
precipitation of the phosphate. This was proved by the author, by the
introduction of considerable quantities of sodium silicate and these
were found not to interfere with the accuracy of the operation.
The results are as accurate as those obtained by the methods of the
consulting committee of the agricultural stations. The coefficient
employed; _viz._, 0.0373, is not the same as that recommended by the
committee; _viz._, 0.043. The committee, however, itself has recognized
the inaccuracy of the latter number. The composition of the compound
obtained by double precipitation according to Carnot is
P₂O₅24MoO₃3(NH₄)₂O + 3H₂O.
=373. Method of the Halle Experiment Station.=—The available or easily
soluble phosphoric acid in soils is estimated by Maercker and Gerlach,
as follows:[238]
Sixty grams of the air-dried soil as prepared for analysis, are placed
in an erlenmeyer with 300 cubic centimeters of two per cent citric acid
solution and digested for twenty-four hours in the cold. It is necessary
in this time to shake the flask four or five times and to put the
stoppers in loosely in order to allow the escape of any evolved carbon
dioxid. Of this mixture 200 cubic centimeters are filtered and
evaporated in a 300 cubic centimeter dish to dryness. There remains, in
most cases, a sirupy-like mass from which even by strong heating the
silica is not completely separated. In order to reach this result the
residue is treated with twenty cubic centimeters of concentrated
sulfuric and five cubic centimeters of fuming nitric acid and heated
over a bunsen. As soon as the appearance of foam denotes the beginning
of the reaction the lamp must be removed. With strong foaming and the
evolution of red-brown vapors the citric acid is completely oxidized.
After the reaction is ended the contents of the dish are heated for
about fifteen minutes over a small flame so that a continuous, yet not
too violent evolution of sulfuric acid fumes takes place. After the
silicic acid and the greater part of the lime have been separated in
this way the contents of the dish are diluted with water, stirred with a
glass rod, washed into a 200 cubic centimeter flask, cooled, filled up
to the mark, and filtered. From the filtrate 100 cubic centimeters,
corresponding to twenty grams of the earth, are taken, made slightly
alkaline with ammonia, acidified by a few drops of hydrochloric acid,
and after cooling treated with fifty cubic centimeters of the citrate
solution and twenty-five cubic centimeters of the magnesia mixture. The
complete separation of the precipitate requires about forty-eight hours
and shaking of the precipitate is not necessary.
=374. Estimation of total Phosphoric Acid in Soils.=—In the method used
at the Halle Station[239] twenty-five grams of the soil sample are
boiled with twenty cubic centimeters of nitric acid and fifty cubic
centimeters of concentrated sulfuric acid for half an hour. With very
clayey soils only half the quantity of the sample mentioned above is
used in order to avoid the too great accumulation of soluble alumina.
The oxidation of the organic substances of the soils must be carried on
at a moderate heat to avoid foaming. During the boiling, the flask is to
be often shaken to prevent the soil constituents from accumulating too
firmly at the bottom. The total volume is finally made up to 500 cubic
centimeters.
For the estimation, 100 cubic centimeters of the solution, corresponding
to five (or two and a half) grams of the soil, are taken. In order to
nearly completely saturate the acid, the solution is treated with twenty
cubic centimeters of twenty-four per cent ammonia, care being taken that
the precipitate of iron and alumina which is formed is again completely
dissolved. The solution is cooled and treated with fifty cubic
centimeters of the citrate solution, and then with twenty cubic
centimeters of ammonia of above strength, and precipitated with the
magnesia mixture. The filtration of the precipitate should not be made
for at least forty-eight hours, during which time the flask should be
often shaken to prevent the attachment of ammonium magnesium phosphate
to its sides and bottom.
A detailed description of the citrate method for estimating phosphoric
acid will be found in the chapter devoted to this subject under
fertilizers.
=375. French Method for Phosphoric Acid.=—Phosphoric acid is found in
the soil principally in combination with alumina and iron oxid, with
organic matters, or with lime and magnesia. Whatever may be the state in
which it is found all the phosphoric acid, with the exception of that
which enters into the constitution of insoluble mineral particles, can
be brought into solution by acids and determined by some of the approved
methods. This method of solution, therefore, is capable of determining
very accurately the total proportion of phosphoric acid in the soil, but
it is incapable of rendering account to us of the state in which the
phosphorus is found and of its aptitude to be utilized by plants.
The estimation of soil phosphorus, as recommended by the French
Commission, is carried on in the following way:[240] Twenty grams of the
earth are submitted to ignition in a muffle heated to the temperature of
redness but not higher. This calcination eliminates the organic
materials, whose intervention in subsequent reactions might be able to
prevent the precipitation of a part of the phosphoric acid. The calcined
earth is placed in a capsule of about eleven centimeters diameter and
saturated with water. There is then added in small quantities, as long
as effervescence is produced, nitric acid of 36° baumé. When the
effervescence has ceased, after thorough shaking and the addition of a
new quantity of acid, it will be found that the whole of the calcium
carbonate in the soil has been decomposed. It is necessary then to
proceed to the solution of the phosphoric acid by adding twenty cubic
centimeters of nitric acid and heating on the steam-bath for five hours,
shaking from time to time, and avoiding complete desiccation. At the end
of this time the whole of the phosphoric acid has entered into solution.
It is taken up by warm water, filtered, and the insoluble residue washed
with small quantities of boiling water. But from the solution obtained,
which holds in addition to phosphoric acid, some oxid of iron, alumina,
lime, magnesia, etc., it is necessary to separate the silica which has
passed into solution. For this purpose the mass is evaporated to dryness
on a sand-bath, heating toward the end of the operation with precaution
and not allowing the temperature to pass beyond 110°–120°. In these
conditions there is obtained a magma which sometimes remains quite
sirupy when the earth is very highly impregnated with calcium carbonate,
but in which the silica is insoluble. It is indispensable that it be
eliminated wholly because it would introduce grave errors into the
results, as will be seen later on. If the temperature be carried too
high during the desiccation this silica would react upon the earthy
salts and alkaline earths forming silicates and it would be found
ultimately again in solution. The application of a too high temperature
would also render somewhat insoluble in nitric acid the iron and
aluminum oxids, and these would retain small quantities of phosphoric
acid. The desiccation, therefore, requires to be conducted with great
precaution. When it is accomplished there are placed in the capsule five
cubic centimeters of nitric acid and five cubic centimeters of water,
and the whole heated on the sand-bath until the entire amount of iron
oxid is dissolved, that is to say, until there is no ferruginous deposit
persisting in the liquid. The solution is then filtered and washed with
small quantities of boiling water in such a way that the total volume of
the filtrate will not exceed twenty-five to thirty-five cubic
centimeters. Afterwards there are added twenty cubic centimeters of
ammonium nitromolybdate and the whole is left at rest for twelve hours
at the ordinary temperature. At the end of this time the whole of the
phosphoric acid is precipitated in the form of ammonium
phosphomolybdate. In order to be certain that an excess of
nitromolybdate has been used in the precipitation, which is
indispensable to the total precipitation of the phosphoric acid, a few
cubic centimeters of the filtrate are removed by means of a pipette and
are mixed with their own volume of the ammonium nitromolybdate. If, at
the end of an hour or two, no precipitate is formed the operation can be
regarded as terminated.
In order to collect and weigh the ammonium phosphomolybdate some
precautions are necessary. Two smooth filters are used, one of which
serves as a counter-weight for the other on the balance. One of these
filters is placed within the other and the phosphomolybdate is collected
upon the inner filter. The part of the phosphomolybdate adhering to the
precipitating jar is detached by the aid of a stirring rod, one of the
ends of which is covered with a piece of rubber tubing. The washing is
accomplished with very small quantities of water containing five per
cent of its volume of nitric acid. When all of the precipitate is
collected upon the filter and the washing is terminated a few drops of
water are thrown upon the upper borders of the filters; this is done to
displace the acid liquor which has been used in washing. The filters are
then carried to the drying oven where they are dried at a temperature
not exceeding 90°. The application of a higher temperature would
decompose the ammonium phosphomolybdate and lead to results which would
be too low. After the drying is completed the two filters are separated
and placed upon the arms of the balance and the increase in weight
corresponds to the ammonium phosphomolybdate. This, multiplied by the
coefficient 0.043, (see page 404) gives the quantity of phosphoric acid
contained in the weight of the soil which has been employed. The
ammonium phosphomolybdate is pure if all the silica has been eliminated,
but if a part of that has remained in solution it would furnish an
ammonium silicomolybdate whose weight would be added to that of the
phosphomolybdate. The elimination of the silica, therefore, should be
made with the greatest care.
Different processes have been proposed in order to determine the forms
under which phosphoric acid should be regarded as most assimilable.
Deherain has proposed acetic acid as a solvent for this purpose. Other
scientists, oxalic or citric acid, and the ammonium oxalate or citrate.
The solubility of phosphoric acid in these different reagents gives some
information in regard to its state, but the relations which exist
between this solubility and the assimilability of the acid have not yet
been fixed.
_Preparation of the Ammonium Nitromolybdate._—One hundred grams of
molybdic acid are dissolved in 400 grams of ammonia with a density of
ninety-five. The mixture is filtered, and the filtered liquor is
received drop by drop in 1,500 grams of nitric acid of one and
two-tenths density, constantly stirring. This mixture is left standing
for some days in an unexposed locality, during which time a deposit is
formed. The clear part is decanted and used.
The above method of the French chemists unfortunately attempts to
determine the phosphorus content of the soil by weighing the yellow
precipitate and using an empirical factor for the calculation, a factor
which is probably too high.
Experience has shown that at this point it is far more accurate to
continue the process by dissolving the yellow precipitate, and
subsequently obtaining the phosphoric acid in combination with ammonia
and magnesia, or according to the process of Pemberton the content of
phosphoric acid in the yellow precipitate might be determined by
titration. In regard to the latter method which will be given in full
under fertilizers, it may be said that it has been found quite accurate
by several analysts, although it is difficult to see how a precipitate
which is so variable in its constitution as to be estimated with little
safety by weight may yet be capable of rather exact determination by
titration.