different states. That part of it which is combined with the humus
material, or with the hydrated silicates, is easily set free from its
combinations and is to be regarded as the more assimilable portion.
The potash in the soil is found chiefly in combination with silicates,
and particularly with the hydrated aluminum silicates, forming clay. As
the particles with which it is combined are found in a state of greater
or less fineness, the potash itself is set free under the influences of
the agents which are active in the soil, with greater or less rapidity,
passing into a form in which it can be utilized by plants. In silicates
which are very finely divided, such as clay, the potash becomes active
in a relatively short time, while in the débris of rocks in a less
advanced state of decomposition it may rest for an indefinite period in
an inert state. The estimation of the potash which is assimilable in the
clay is quite as important for agricultural purposes as to determine
that which may be present in the soil in firmer combination. Treating
the sample of soil with water does not furnish any useful information in
regard to the potash which it contains. Indeed, the absorbing properties
of the soil tend to prevent the elimination of the potash in this way,
even when it is found in the soluble state. It is therefore, necessary
to employ an acid to set the potash free, but variable results are
obtained, according to the employment of acids of greater or less
concentration and for longer or shorter periods of contact.
=350. Estimation of the Potash Soluble in Concentrated Acids.=—In the
method of the French agricultural chemists[224] twenty grams of the
earth are placed in a dish with a flat bottom, eleven centimeters in
diameter, and rubbed up with twenty to thirty cubic centimeters of
water. There is added carefully, and in small quantities, some nitric
acid of 36° Baumé until all effervescence has ceased, the mass meanwhile
being thoroughly stirred. When the carbonates have been decomposed,
which can be told by the cessation of the effervescence, twenty cubic
centimeters more of the same acid are added. The dish is heated on the
sand-bath for five hours, regulating the heating in such a way that
there still remains some acid at the end of the operation and the mass
is not thoroughly dry. The acid mass is then taken up with hot water,
filtered, and washed with hot water until the amount of filtrate is
about 300 cubic centimeters. The filtrate should be received in a flask
of about one liter capacity. The filtrate will contain the dissolved
potash, soda, magnesia, lime, iron and aluminum oxids, and traces of
sulfuric and phosphoric acids. For the elimination of the other
substances, with the exception of potash, soda and magnesia, a few drops
of barium nitrate are added, afterwards sufficient ammonia to render the
solution alkaline, and finally an excess of ammonium carbonate in powder
added in small portions. These materials are added successively and the
whole is left to stand for twenty-four hours. By this operation the
sulfuric acid is separated in the form of barium sulfate; the iron and
aluminum oxids are precipitated, carrying down with them the phosphoric
acid, and the lime is thrown down in the form of carbonate. The mass is
now filtered and washed several times with hot water. The filtrate
contains in addition to potash, soda, magnesia, and the ammoniacal salts
which have been introduced. The ammonium salts are destroyed by adding
aqua regia and evaporating the liquid to a very small volume, as
described in the method for the estimation of magnesia. The mass is now
evaporated in a porcelain dish with a flat bottom, of about seven
centimeters diameter, and an excess of perchloric acid added. The
evaporation is carried to dryness on a sand-bath, and the heating
prolonged until the last white fumes of perchloric acid are disengaged.
The mass is now left to cool. There are then added five cubic
centimeters of alcohol, of 90° strength. The mass is triturated by a
stirring rod, the extremity of which is flattened, in such a manner as
to reduce it all to an impalpable powder. It is then left to settle and
the supernatant liquid is decanted upon a small filter. The treatment
with alcohol of the kind, quantity, and strength described, is continued
four or five times. Afterwards, as there may still remain a trace of the
sodium and magnesium perchlorates in the interior of the crystals of
potassium perchlorate, there are added to the capsule in which all of
the alkaline residue has been collected, two or three cubic centimeters
of water, and it is evaporated again to dryness and taken up twice with
small quantities of alcohol. There are thus removed the traces of sodium
and magnesium perchlorates. By means of a jet of boiling water the
stirring rod and the filter, which contains the small quantities of
potassium perchlorate, are washed, and the liquid passing through is
received in the capsule which contains the larger part of the salt. It
is then evaporated to dryness and weighed.
When there is very little magnesia present, as is generally the case,
the estimation of the potash is made without any difficulty by the
process just mentioned, but when the proportion of magnesia is high it
is found useful to separate it before the transformation into
perchlorates. The magnesia is separated by carbonating the residue as
indicated in the method for the estimation of magnesia, by treatment
with oxalic acid and ignition. By extracting the carbonates formed with
very small quantities of water, and filtering, the alkalies are obtained
free from magnesia.
It is advisable to test the purity of the potassium perchlorate formed
which sometimes contains a little silica. For this purpose it is
dissolved in boiling water, and any residue which remains is weighed,
and that weight deducted from the total weight of perchlorate. By
multiplying the weight of potassium perchlorate found by the coefficient
0.339, the quantity of potash contained in the twenty grams of earth
submitted to analysis is obtained.
_Estimation of the Potash Soluble in Cold, Dilute Acids._—(Method of
Schloesing.) Introduce 100 grams of the soil into a one or one and a
half liter flask with 600 to 800 cubic centimeters of water. A little
nitric acid, of 30° Baumé, is added until the carbonate is decomposed
and a slight acid reaction is obtained. Afterwards five cubic
centimeters of the same acid are added and it is left to digest for six
hours, shaking every fifteen minutes. Instead of taking the whole of the
wash-water for the examination, it is better to extract only a portion
of it and so dispense with washing. This process is conducted in the
following manner:
The weight P of the full flask having been determined, as much as
possible of the solution, is decanted by means of a very small siphon,
of which the flow is moderated by fixing a rubber tube with a pinch-cock
to its lower extremity. After the decantation is complete, the flask is
again weighed, giving the weight of P′; the weight of liquid taken,
therefore, is equal to P − P′. To determine the total weight of the
liquid, throw upon a filter the earthy residue insoluble in the acid,
and after washing and drying it determine its weight _r_. The weight of
the empty dry flask _p_ is also determined. The total weight of the soil
will be, therefore, P − _r_ − _p_. The part of the liquid which was
extracted from the flask, and upon which the analytical operation is to
be conducted is represented by the fraction (P − P′)/(P − _r_ − _p_).
This method avoids washing and evaporation which would be of very long
duration. It rests upon the supposition that the solid matter from which
the liquor is separated has no affinity for the dissolved substances,
and that the total of these substances has passed into the liquor, and
that the solution is homogeneous.
In the liquor first decanted as described before, the potash is
estimated. This liquor contains in addition to potash, soda, lime,
magnesia, iron and aluminum oxids, as well as phosphoric, sulfuric, and
hydrochloric acids. There is first added to it a little barium chlorid
to precipitate the sulfuric acid. It is then heated to about 40° in a
glass flask and some ammonium carbonate added in a solution containing
an excess of ammonium hydroxid. By this process the lime and baryta are
precipitated in the form of carbonates; the alumina and iron as oxids,
and the phosphoric acid in combination with the last two bases. The
magnesium carbonate is not precipitated because it is soluble in the
ammonium carbonate with which it forms a double salt.
The employment of a gentle heat favors the formation of the precipitate
of calcium carbonate in a granular form which lends itself easily to
filtration. The contents of the flask are now thrown upon a filter and
the insoluble residue washed. The filtrate contains the potash, soda,
magnesia, ammonia, and nitric and hydrochloric acids. It is concentrated
as rapidly as possible by heating in a flask, and afterwards the
ammoniacal salts are destroyed by weak aqua regia and the whole is then
transferred to a porcelain dish and evaporated to dryness. There is thus
obtained a mixture of potassium, sodium, and magnesium nitrates, from
which the potash is separated by means of perchloric acid in the manner
already described.
_Estimation of the Total Potash._—Beside the potash which can be
dissolved by the boiling concentrated acids the soil contains potash
combined with silicates, which becomes useful for plant life with
extreme slowness. It is often of great interest to estimate the total
potash contained in a soil, that is to say, the reserve for the future.
In this case it is necessary to free entirely this base from its
combinations by means of hydrofluoric acid. The operation is conducted
upon two grams of earth previously ignited and reduced to an impalpable
powder. The decomposition is conducted in a platinum capsule by
sprinkling the sample with a few cubic centimeters of hydrofluoric acid,
or solution of ammonium fluorid, and adding a few drops of sulfuric
acid. It is then evaporated to dryness and dissolved in boiling
hydrochloric acid. The part which remains insoluble is treated a second
time by hydrofluoric and sulfuric and afterwards by hydrochloric acid.
All of the potash is thus brought into solution. The estimation of the
potash, after having obtained it in a soluble state, is conducted in the
manner previously described.
_Estimation of the Potash as Platinochlorid._—Instead of estimating the
potash as perchlorate it can also be transformed into platinochlorid.
This process gives as good results as the preceding one but it is
necessary in all cases, to separate the magnesia. After having treated
the soil as indicated in the case of the estimation of the potash as
perchlorate, the separation of the sulfuric and phosphoric acids, of
alumina and iron, of magnesia, and the destruction of the ammoniacal
salts in the manner already described, there are finally left the
alkalies potash and soda in the form of carbonates. These are
transformed into chlorids by adding hydrochloric acid; afterwards they
are evaporated to dryness and the mixture of the two chlorids weighed in
order to determine what quantity of platinum chlorid it is necessary to
add, in order that it be in excess. The quantity of chlorid to be added
is calculated so as to be in sufficient quantity to saturate the whole
of the chlorids weighed, whether they may be composed wholly of sodium
or potassium. In this way there is a certainty of having an excess of
platinum. The solution of platinum chlorid used should contain in 100
cubic centimeters seventeen grams of platinum. Each cubic centimeter of
this solution will be sufficient for a decigram of the sodium and
potassium double chlorids. After the addition of the platinum chlorid
the mixture is evaporated in a capsule with a flat bottom, on a
water-bath. It is important that the temperature should not exceed 100°.
If the temperature should go above this there would be a tendency to
form some platinum subchlorids insoluble in alcohol.
The evaporation is continued until the contents of the dish are in a
pasty condition and form a rather solid mass on cooling. It is necessary
to avoid a complete desiccation. After cooling, the residue is taken up
by alcohol of 95° strength. It is allowed to digest with alcohol of this
strength for some time, after having been thoroughly mixed and shaken
therewith in order to obtain a complete precipitation of the
platinochlorid. This digestion should take place under a small bell-jar
resting upon a piece of ground glass. The evaporation of the alcohol is
thus prevented. The mass is then washed by means of alcohol of the same
strength and the liquors decanted upon a small filter placed within
another filter of identical weight, which serves as a tare for it on the
balance. The washing is prolonged until the filtrate becomes colorless.
All of the particles in the dish should be brought upon the filter by
means of a hair-brush. The filters are now dried at a temperature not
exceeding 95° and the platinochlorid received upon the interior filter
is weighed. The precipitate may also be washed from the small filter
into the capsule in which it was formed by means of a jet of alcohol.
The alcohol is evaporated and the precipitate weighed in the capsule.
The weighing should be made rapidly on account of the hygroscopicity of
the material. The weight obtained multiplied by 0.193 gives the
corresponding quantity of potash in the soil.
_Purification of the Oxalic Acid._—The commercial oxalic acid used in
separating the magnesia, often contains lime, magnesia, and potash. When
this reagent is used in a sufficiently large quantity in the estimation
of the above substances, it is indispensable to free it entirely from
them. This is secured by submitting the oxalic acid to successive
recrystallizations which are obtained by dissolving it in warm water,
filtering and leaving to cool. The mother waters are thrown away. After
two or three successive crystallizations the traces of potash and
magnesia have disappeared and the oxalic acid obtained after ignition
leaves no trace of residue.
The purification may also be secured in the following manner: At a
temperature of 60° a saturated solution of oxalic acid is made; the
liquid is decanted, carried to the boiling point and filtered. Five per
cent of nitric acid are added and it is allowed to cool. The crystals
which are deposited are collected upon a funnel in which a plug of
cotton has been placed, and are washed with a little cold water.
_Purity of the Ammonium Carbonate._—The ammonium carbonate employed
should not leave any residue whatever after volatilization. In general,
it may be said of all the reagents employed in analyses and especially
of those employed in large quantities, that it is indispensable to be
sure that they contain no traces of the substances which are to be
estimated. The acids, ammonia, etc., should always be examined with this
point in view.
_Estimation of the Soda._—It is often of interest to estimate the soda
in the soil, not that it is an element of any great fertility but rather
because it is hurtful when in excess. It is determined in the residue
obtained in the estimation of potash and is estimated by difference. The
weight of the mixture of sodium and potassium chlorids being known when
the potash is determined, the weight of its chlorid is to be deducted
from the weight of the two chlorids and thus the direct weight of the
sodium chlorid is obtained.
A better way is to make a direct estimation. The soda is found entirely
dissolved in the alcoholic solution obtained by washing the potash salt
as before described, for the separation of the potassium platinochlorid.
This alcoholic liquor is evaporated to dryness on a water-bath, in a
bohemian flask of about 100 cubic centimeters capacity. The residue
obtained consists of sodium platinochlorid and a little platinum
chlorid. There is now fitted to the bohemian flask a cork stopper
carrying two tubes. The apparatus is placed upon a water-bath and kept
at about 100°. Through the tube which reaches to the bottom of the
bohemian flask, a current of pure hydrogen is passed. The hydrogen
passes off through the second tube. The hydrogen completely reduces the
salts of platinum. In order that the decomposition may go on more
rapidly a few drops of water are added. When the whole mass in the flask
has become black owing to the separation of the platinum, it is shaken,
evaporated to dryness and hydrogen passed through a second time. This
operation is repeated three or four times, being stopped when the water
no longer shows a yellow color. There is then in the flask only a
mixture of reduced platinum and sodium chlorid. No trace of sodium
chlorid has been lost because the temperature has never exceeded 100°.
The sodium chlorid is dissolved by washing with water and filtered. The
liquor, which must be absolutely colorless, is evaporated to dryness in
a platinum capsule and weighed. There is thus obtained the weight of the
sodium chlorid. For verification, the sum of the weight of potassium
chlorid calculated from the platinochlorid and the weight of the sodium
chlorid should be equal to the initial weight of the mixture of the two
chlorids.
=351. Potash Methods of the German Experiment Stations.=[225]—_a._ To
one volume of air-dried fine earth which is obtained by sifting through
a three millimeter sieve, two volumes of twenty-five per cent
hydrochloric acid are added, or more if the soil contains much
carbonate. The acid is allowed to act with frequent stirring for
forty-eight hours at room temperature.
_b._ To one volume of the soil, as above prepared, are added two volumes
of hydrochloric acid and allowed to stand for three hours with frequent
shaking, at the temperature of boiling water.
_c._ (Halle method.) One hundred grams of the fine earth are treated
with 500 cubic centimeters of forty per cent hydrochloric acid, made up
to one liter with water and allowed to stand for forty-eight hours with
frequent shaking. After filtering, a large aliquot part of the filtrate
is evaporated for the estimation of the potash. The evaporated residue
is washed into a half-liter flask in which the sulfuric acid is
precipitated with barium hydroxid the flask filled to the mark and an
aliquot part of the filtrate in a half-liter flask, treated with
ammonium carbonate, filtered and the potash estimated as platinochlorid
by the usual method.
=352. Method of Raulin for the Estimation of Potash in Soils.=[226]—The
process rests upon the very feeble solubility in aqueous solution of
potassium phosphomolybdate, while sodium, magnesium, calcium, iron, and
aluminum phosphomolybdates are more or less soluble. The process does
not require complicated separation and permits of the treatment of a
small quantity of soil, since the weight of the phosphomolybdate
obtained is equivalent to nineteen times that of the potash.
The reagent is prepared by dissolving 100 grams of crystallized ammonium
molybdate in as little water as possible and adding six and a half grams
of neutral crystallized ammonium phosphate dissolved in a little water.
Aqua regia is now added cold and some ammonium phosphomolybdate is
precipitated. The mixture is heated, adding a little aqua regia from
time to time, until the solution of the precipitate is accomplished. The
whole is then evaporated to dryness, the final temperature of
evaporation not being carried above 70°. Four hundred cubic centimeters
of water are now added and five cubic centimeters of nitric acid, and
the contents of the dish heated and filtered. The reagent is then ready
for use.
The liquid to be used for washing the potassium phosphomolybdate is
prepared by dissolving twenty grams of sodium nitrate in one liter of
water, two cubic centimeters of pure nitric acid, and a mixture of about
twenty cubic centimeters of the phosphomolybdic reagent and one and a
half cubic centimeters of a solution of potassium nitrate containing
eighty grams per liter, slightly heated in order to saturate the liquid
with potassium phosphomolybdate. The solution is shaken, allowed to
rest, and the liquid decanted.
For the preparation of the solution in which the potash is to be
estimated, a sample of soil is carefully weighed of such magnitude as to
contain about fifteen milligrams of anhydrous potash. The potash salts
are dissolved by the usual processes and are separated from the largest
part of the calcium, iron, and aluminum salts, and converted into
nitrates. The solution is reduced to a volume of a few cubic centimeters
and slightly acidulated with nitric acid. Four cubic centimeters of the
phosphomolybdic reagent are added for every ten milligrams of anhydrous
potash supposed to be present. The solution is evaporated to dryness at
50° and immediately brought upon very small weighed filters, of which
each one is double, by using sixty cubic centimeters of the washing
liquor mentioned above. The tared filter is likewise washed with the
same liquid at 50° and weighed. The weight multiplied by 0.052 gives the
anhydrous potash. This method for a direct precipitation of the potash
salts does not have the merits of the perchlorate process and both are
inferior in accuracy to the usual platinochlorid procedure.
=353. Russian Method for Estimating Potash in Soils.=[227]—Ten grams of
the soil are digested with 100 cubic centimeters of ten per cent
hydrochloric acid on a steam-bath for twenty-four hours. After adding
five cubic centimeters of nitric acid to the filtrate it is evaporated
to dryness, taken up with dilute hydrochloric acid, filtered, the
filtrate saturated with ammonia, the excess of ammonia driven off, again
filtered, and the lime separated by ammonium oxalate.
The filtrate is treated with a little barium chlorid for the removal of
sulfuric acid and afterwards with ammonium carbonate in excess, and
digested for twenty-four hours. After filtering, the solution is
evaporated in a platinum dish, the excess of ammonia driven off, the
residue taken up with water, filtered, treated with hydrochloric acid,
evaporated to dryness, and ignited at low heat. The residue is again
dissolved in water, filtered, and the potash precipitated with platinum
chlorid and estimated in the usual way.
=354. Potash Method of the Italian Stations.=[228]—The potash in the
soil should be determined in three forms; _viz._,
1. Assimilable potash.
2. Potash soluble in concentrated acid.
3. Total potash.
For determination of the first, 100 grams of earth are put into a retort
holding a liter and digested with dilute nitric acid.
For the analysis, an aliquot portion of the clear liquid is taken or
weighed, and the determination of the potash is made by the common
methods.
For an alternate method, from twenty to fifty grams of earth are put
into a retort of 500 cubic centimeters, moistened with water, and nitric
acid is gradually added. After one or two hours there are added from 200
to 300 cubic centimeters of water; the liquid is poured without
filtering into a retort and the residue washed by decantation.
In the liquid, after the elimination of the other substances with barium
chlorid, ammonium carbonate, etc., the potash is determined by the
ordinary methods.
In the second case, by using warm concentrated acids, a portion of the
insoluble silica is decomposed, but this decomposition is always partial
and the quantity of the potash extracted depends upon the temperature,
upon the concentration, upon the duration of the action, and upon the
nature of the acid.
The method of moistening twenty to fifty grams of earth with water and
adding, thereto, concentrated nitric acid of 1.20 density, in such a
manner that the earth shall be completely saturated, may also be
employed. Then the temperature is kept at 100° during two hours. In the
solution, the potash is determined as usual.
In the third case the soil is to be decomposed by hydrofluoric and
sulfuric acids, or by fusion with alkaline carbonates, and the total
potash determined by one of the standard methods.