potash the following points must be carefully considered:
1. The quantity of the earth to be examined.
2. The state of humidity or dryness of the same.
3. The quantity, nature, and concentration of the acid.
4. The quantity of the water.
5. The duration of the treatment.
=355. Method of J. Lawrence Smith for Potash.=—This method, designed
especially for mineral analysis, has been fully approved by the general
experience of analysts.
The principle of the method[229] depends upon the decomposition of
silicates on ignition with calcium carbonate and ammonium chlorid. The
object of this mixture is to bring into contact with the mineral,
caustic lime in a nascent state at a red heat, the caustic lime being
soluble to some extent in calcium chlorid at a high temperature. Pure
calcium carbonate, made by precipitation of marble, should be used.
The ammonium chlorid should be prepared by taking crystals of pure,
sublimed sal ammoniac, dissolving in water, and filtering, and
evaporating the solution until small crystals are deposited, the
solution being well-stirred until one-half or two-thirds of the whole
has crystallized. The mother-liquor is poured off while still hot, and
the crystals dried on an asbestos filter at ordinary room temperature.
A special platinum crucible should be used in the Smith method, but the
common crucible, especially if very deep, can be employed. The special
crucible is of about double the usual length. Smith recommends a
crucible ninety-five millimeters in length, diameter at top twenty-two
millimeters, at bottom sixteen millimeters, and weighing thirty-five to
forty grams. The object of the long crucible is to have the part of the
bottom containing the silicate subjected to a high heat, while the top
of the crucible is at a much lower temperature, thus preventing the loss
of alkalies by volatilization.
_Method of Analysis._—The samples of soil or silicate containing the
alkalies are well pulverized in an agate mortar, and from one-half to
one gram of the finely pulverized material taken for analysis. This is
carefully mixed with the same weight of finely powdered sal ammoniac and
the mineral and sal ammoniac rubbed well together in a mortar. Eight
parts by weight of calcium carbonate are next added in three or four
portions, and the whole intimately mixed after each addition. The
contents of the mortar are emptied on a piece of glazed paper and then
introduced into the crucible, which is tapped gently upon the table
until the contents are well settled. It is then fixed in the furnace
which is used for heating, and a small bunsen burner is placed beneath
the crucible, and the heat applied just about at the top of the mixture
and gradually carried toward the lower part until the sal ammoniac is
completely decomposed, which requires from four to five minutes. The
heat is then applied by means of a blast-lamp and the crucible kept at a
bright red heat for from forty to sixty minutes. The crucible is allowed
to cool, the contents detached and placed in a platinum or porcelain
dish of about 150 cubic centimeters capacity, and sixty to eighty cubic
centimeters of distilled water added. The solution of the flux may be
hastened by heating the water to the boiling point. The crucible and its
cover are also well washed with hot water until all matter adhering to
them is dissolved. After the slaking of the mass it is best to continue
the digestion with hot water for six or eight hours, although this is
not absolutely necessary. The contents of the crucible are filtered and
washed well with about 200 cubic centimeters of water. The filtrate
contains in solution all the alkalies of the mineral, or soil, together
with calcium chlorid and caustic lime. A solution of pure ammonium
carbonate containing about one and one-half grams of the pure salt is
added to the filtrate. This precipitates the lime as carbonate. The dish
containing the material is placed on a water-bath and its contents
evaporated to about forty cubic centimeters. Two additional drops of
ammonium carbonate are added, and a few drops of caustic ammonia, to
precipitate any lime which may be redissolved by the action of the
ammonium chlorid solution on the calcium carbonate. Filter on a small
filter and wash with as little water as possible and collect the
filtrate in a small beaker. The filtrate contains all the alkalies as
chlorids, together with a little ammonium chlorid. Add a drop of
ammonium carbonate solution to be sure all the lime is precipitated,
evaporate on a water-bath in a deep platinum dish, in which the alkalies
are to be weighed. The dish should have from thirty to sixty cubic
centimeters capacity, and during the evaporation should never be more
than two-thirds filled. After the evaporation has been completed the
dish is slowly heated and then gently ignited over a gas-flame to drive
off any ammonium chlorid which may be present. During this process the
platinum dish may be covered with a thin piece of platinum to prevent
any possible loss by the spitting of the salt after the ammonium chlorid
has been driven off. The heat should be gradually increased until it is
brought to a point a little below redness, leaving the cover off. The
platinum dish is again covered, and when sufficiently cooled placed on a
balance and weighed.
If lithium chlorid be present it is necessary to weigh it quickly as the
salt being very deliquescent takes up moisture rapidly. The alkalies may
now be separated in the usual way.
If the sample under examination contains magnesia the residue in the
capsule should be dissolved in a little water and sufficient pure
lime-water added to render the solution alkaline. It should then be
boiled and filtered. The magnesia will, in this way, be completely
separated from the alkalies. The solution which has passed through the
filter is treated with ammonium carbonate in the manner first described,
and the process continued and completed as above mentioned.
If it be suspected that the whole of the alkalies have not been obtained
by the first fusion, the residue upon the filter can be rubbed up in a
mortar with an amount of ammonium chlorid equal to one-half the weight
of the mineral, mixed with fresh portions of calcium carbonate and
treated exactly as in the first instance. Any trace of alkali remaining
from the first fusion is thus recovered in the second one.
_Method of Heating the Crucible._—The apparatus used by Smith for
igniting the crucible is shown in Fig. 67. It consists of an iron
filter-stand HG, a clamp, ED, carrying the muffle NC, attached by the
supports AB, and heated by the lamp F. The muffle NC is a chimney of
sheet iron, eight to nine centimeters long, ten centimeters high, the
width at the bottom being about four centimeters on one side and three
centimeters on the other. It is made with the sides straight for about
four centimeters and then inclining toward the top so as to leave the
opening at the top about one centimeter in width. A piece is cut out of
the front of the chimney of the width of the diameter of the hole in the
iron support and about four centimeters in length, being semi-circular
at the top, fitting over the platinum crucible. Just above this part of
the chimney, is riveted a piece of sheet iron in the form of a flattened
hook, N, which holds the chimney in place by being slipped over the top
of the crucible support; it serves as a protection to the crucible
against the cooling effects of the currents of air.
[Illustration:
FIGURE 67.
SMITH’S MUFFLE FOR DECOMPOSITION OF SILICATES.
]
=356. International Method for Assimilable and Total Potash.=—In the
International Congress of Chemists held in Paris in 1889,[230] the
discrimination between the assimilable and total potash was declared to
be of prime importance. Unfortunately no method is known by which the
potash which is present in the soil in a state suited to the wants of
plants can be determined with approximate accuracy. In general, that
portion which is given up to weak acids may be assumed to be available.
In the treatment of soils with weak acid, as pointed out in the
Congress, it is demonstrable that with a 0.05 to 0.1 per cent nitric
acid solution, the quantity of potash which goes into solution increases
by continued stirring of the mixture with the time of action of the acid
up to a certain maximum which is reached in from three to four hours,
and after that, it is not changed even when the strength of the acid
mixture is increased to two per cent. From this time on, concentrated
acids withdraw from the soil which has already been exhausted by the
weak acid, a new quantity of potash. The soils which have been exhausted
by concentrated acids yield also an additional quantity of potash when
they are treated with hydrofluoric acid, or melted with barium or sodium
carbonate. Potash, therefore, appears to exist in the soil in various
forms.
First. In the form of indecomposable silicates which have,
agriculturally perhaps, very little interest.
Second. In the form of silicates which are more basic than those just
mentioned. These silicates are attacked by strong acids and give up
probably every year a portion of their potash to vegetation.
Third. In a form which is easily soluble in weak acids and consequently
directly assimilable by plants.
In view of the fact that it would be of interest to chemists and
agronomists to establish certain methods of investigation so as to be
able to obtain comparative results, it was decided to adopt the original
method recommended by Gasparin for the estimation of the potash
decomposable by concentrated acids. This method consists in the
treatment of the soil with boiling aqua regia until the sand which is
not decomposed, is white.
_Determination of the Fineness of the Earth which is Used for
Analysis._—For the estimation of potash, the soil should be divided as
finely as possible, and passed through a sieve of thirty meshes to the
centimeter. The decomposition is then completed in two hours, while if a
sieve of only ten perforations per centimeter is used, the acid must be
allowed to work for twelve hours.
The determination of the potash after solution, is accomplished by any
of the standard methods.
=357. Method of Tatlock as Used by Dyer.=—Attention was called in
paragraph =328= to the estimation of the total plant food in the soil by
extraction of the sample with citric acid. Dyer first determines the
total potash by Tatlock’s method which is as follows:
To determine potash ten grams of fine dry soil are treated with ten
cubic centimeters of hydrochloric acid and evaporated to dryness on the
water-bath, the residue taken up with another ten cubic centimeters of
acid, warmed, diluted with water, boiled, filtered, and washed. The
filtrate and washings are concentrated and gently incinerated to get rid
of organic matter, and the residue redissolved in hydrochloric acid, and
evaporated slowly with a considerable quantity of platinum chlorid. If
the evaporation be conducted slowly, the potassium platinochlorid
settles out well, despite the iron, aluminum, and calcium salts, and is
easily washed with some more platinum chlorid solution, followed by
alcohol. The application of this modification of the platinum chlorid
process to solutions containing comparatively minute quantities of
potash amid an overwhelming excess of iron, aluminum, and calcium salts
is probably new to many chemists. It works admirably, and obviates the
necessity for removing iron, aluminum, calcium, magnesium, etc., with
the necessary use of ammonia, and the tedious processes of concentration
and final volatilization of the ammonium salts; but, of course, the
process cannot be employed if soda also is to be determined.
The potash, soluble in hydrochloric acid, having been thus determined,
the undissolved siliceous matter is incinerated, weighed, and finely
ground in an agate mortar. A weighed portion of it is then, as in the
Smith method, mixed with a large bulk of pure calcium carbonate and a
little ammonium chlorid and heated, beginning with a low temperature,
rising slowly to bright redness. The mass is then boiled with water,
washed, incinerated, reground, mixed with some more ammonium chlorid,
and again heated, boiled, and washed. The process is repeated and the
filtrates from all the treatments concentrated, the calcium being
removed as carbonate, and the potash determined in the filtrate, after
evaporation and incineration at a low temperature, by means of platinum
chlorid.
Five hundred cubic centimeters of the citric acid solution of the soil,
made as described in paragraph =328=, corresponding to fifty grams of
soil, are evaporated to dryness in a platinum dish and ignited at a low
temperature. The residue is dissolved in hydrochloric acid filtered and
washed, and the filtrate again evaporated to dryness and treated again
as just described. The potash is then determined as above.
=358. Estimation of Total Alkalies and Alkaline Earths.=—To properly
determine the exact amount of these substances in a sample of soil it is
necessary first to remove the silica. This is accomplished in the
process of Berthelot and André[231] by intimately incorporating with the
sample, in a state of very fine powder, four or five times its weight of
ammonium fluorid. The mixture, in a platinum dish, is moistened with
strong sulfuric acid and allowed to stand for a few hours. It is then
gently heated until all fumes of hydrofluosilicic acid have disappeared,
but the mass is not raised to a red heat. If there is any doubt about
the complete decomposition of the silica the treatment is repeated.
At the end of the operation there remain only sulfates without excess of
sulfuric acid. The sulfates likely to be present are of potash, soda,
lime, magnesia, alumina, and iron. The separation of these bodies is
conducted in the ordinary manner.
Fusing the soil with potash does not give reliable results but it can be
used in certain cases for the rapid estimation of alumina and iron. In
this case after the separation of the silica in the ordinary way the
iron can be determined as ferric oxid.
The iron can also be directly determined by reducing to the ferrous
state and titrating with potassium permanganate.
_Comparison of Fluorin Method with Common Methods._—To establish the
difference in the data obtained by the old and new processes samples of
the same earth were treated by Berthelot and André by different methods
with the following results:
By the By the cold By the By
fluorin dilute concentrated incineration
method. hydrochloric hydrochloric and
acid method. acid method. subsequent
treatment
with boiling
hydrochloric
acid method.
Per cent. Per cent. Per cent. Per cent.
Potash 0.886 0.021 0.149 0.176
Soda 0.211 0.024 0.033 0.042
Magnesia 0.087 0.033 0.033 0.067
Lime 1.160 0.879 1.120 1.060
Alumina 3.950 0.102 1.009 2.631
Ferric oxid 2.150 0.296 1.401 1.678
The impossibility of getting all the alkalies and oxids into solution by
even the prolonged action of a boiling acid is clearly set forth in the
above table. Boiling sulfuric acid might do a little better but would
not give correct results. Lime alone of the elements in the soil can be
correctly determined by solution in boiling hydrochloric acid, a
circumstance due to the fact that lime is found chiefly as carbonate,
sulfate, and phosphate in the soil, and these compounds are easily
soluble in hot hydrochloric acid with the exception of the sulfate. Even
lime could not be thus determined in soils containing silicates rich in
lime. The other mineral elements cannot be determined by the wet method.
This is due to the forms in which they occur, being mostly silicates of
different composition, with excess of silica.
As to the silicates they may be divided into two groups. The first of
these are the hydrated silicates, resembling the zeolites, capable of
being completely decomposed by boiling acids. The first group of
silicates is doubtless of greater importance to vegetable life than the
second since it would, doubtless, give up its alkalies with greater
ease. This distinction is, however, arbitrary. It is, in fact,
impossible to place on one side the soluble and on the other the
insoluble silicates. This distinction represents only the unequal
degrees in the speed of decomposition of the different silicates
contained in the primitive rocks under the influence of atmospheric
agents, the soil being nothing more than the products of the
decomposition of these rocks with vegetable mold. The second group is
insoluble in acids.
That part of the silicates least decomposed at any given moment will be
attacked more easily by acids, while that portion whose decomposition
has been pushed furthest will be more slowly attacked. The action of the
acid will grow more feeble as the time of contact is prolonged, and
after a time a point is apparently reached where the results are nearly
constant. But it is evident that this distinction is purely conventional
and bears no necessary or even probable connection with the quantity of
alkali really assimilable by plants.
Vegetables, moreover, exert on a soil, for the extraction of its
alkalies and other matters, chemical reactions peculiar to themselves,
altogether distinct from the tardy action of atmospheric agents and
still more distinct from the rapid action of mineral acids.
It is well known with what energy, it ought to be said with what
admirable instinct, plants take from the soil the least traces of
phosphorus, of sulfur, of potash, of iron, and other substances
necessary to their sustenance.
These specific actions of vegetables on the soil merit, in the highest
degree, the attention of analysts and agronomists. Their intervention
plays a most important part in the restitution to the soil, by means of
complementary fertilizers, the mineral elements removed by vegetable
growth.
=359. Estimation of Lime by the French Method.=—The quantity of lime
contained in the soil varies within wide limits. Sometimes this base is
entirely absent to such a degree that it is even impossible to discover
feeble traces of it. Sometimes it composes almost the whole of the
earthy mass. Lime is found in the soil principally in the state of
carbonate. It is also found combined with organic matter under the form
of humates, with sulfuric acid, etc. It is customary to estimate the
lime as a whole, without distinguishing between the different states in
which it exists. The quantity of material which is used in the French
method[232] varies in proportion to the amount of calcareous matter
contained in it. For a soil which contains a large amount of lime, one
or two grams would be sufficient for the analysis. For a soil which is
poor in calcareous matter ten or even twenty grams must be taken. The
quantity of lime dissolved differs according to the strength of the
acids employed and length of contact of the acid with the soil. The
calcium carbonate, the sulfate, the nitrate, and the humate rapidly pass
into solution when treated with acid as above, but this is not the case
with calcium silicates which are attacked much more slowly. Sometimes
the silicates give only an insignificant increase in the amount of lime,
and in this case it is immaterial what process of solution is employed.
For simplicity it is best to adopt the method of solution in boiling
concentrated nitric acid, prolonging the boiling for a period of five
hours. This method of operation is sufficient to bring into solution at
one treatment, not only the lime, but also the potash and magnesia.
After having heated with acid for the necessary time there are added in
the capsule in which the solution took place ten cubic centimeters of
nitric acid and fifty cubic centimeters of water. The mixture is heated,
collected upon a filter and the residue washed. To the filtrate, the
volume of which should be from 400 to 500 cubic centimeters, a
sufficient quantity of ammonia is added to render it slightly alkaline.
There is formed a precipitate of alumina and of iron oxid containing
phosphoric acid and also sometimes a trace of the lime combined with the
same acid. In order to keep the whole of the lime in solution it is
necessary to add a little acetic acid, about ten cubic centimeters more
than is necessary to neutralize the ammonia which has been added in
excess. If the liquid is turbid on account of the presence of the iron
and aluminum phosphates it is necessary to filter it. There is
afterwards added a slight excess of ammonium oxalate in solution, and
the whole is left for twenty-four hours in order that the calcium
oxalate may deposit. Indeed, the complete precipitation is not always
immediate, and especially in the presence of magnesia it takes place
with slowness. The calcium oxalate is collected upon a filter and washed
with hot water. To determine the quantity of the lime the best procedure
consists in transforming the oxalate into carbonate by a careful
ignition, and afterwards heating in a Schloesing or Leclerc furnace for
four or five minutes. The oxalate for this purpose should be contained
in a covered platinum crucible. By this method the calcium carbonate is
transformed into calcium oxid, in which form it is weighed rapidly to
avoid absorption of moisture.
In laboratories which have no means of securing so high a temperature as
is mentioned before, the lime may be weighed as sulfate. For this
purpose the calcium oxalate is transformed into carbonate by ignition in
a platinum crucible. Afterwards it is treated with nitric acid until the
carbon dioxid is completely driven off. The platinum crucible is now
covered with a funnel which is afterwards washed in order to bring back
into the dish the small drops which have been projected in the process
of boiling. An excess of sulfuric acid is added and evaporated to
dryness on a sand-bath. Afterwards, in a muffle, the temperature is
carried to a feeble redness until the vapors of sulfuric acid are all
driven off. The lime is weighed in the form of sulfate, and the weight
multiplied by 0.412 gives the lime contained in the quantity of earth
analyzed.
In special researches in which it is desired to avoid attacking the
siliceous pebbles of the soil, the concentrated nitric acid is replaced
by dilute nitric acid in slight excess, and heated for a few moments
only. The calcium carbonate is then dissolved with the other calcareous
salts not combined with silica in the rock products. The analysis is
continued in other respects as just described.
=360. Estimation of the Actual Calcium Carbonate.=—The lime which is
found in the state of carbonate plays one of the most important rôles in
the chemical phenomena which take place in the soil. It is often of
great importance to determine it. The most certain process is to
estimate the carbon dioxid which is disengaged from the carbonate under
the influence of an acid and to receive this gas in a jar graduated to
measure it by volume. The flask recommended by the French Commission for
this purpose contains about 300 cubic centimeters. The neck of the flask
is connected with a condensing tube of about one centimeter interior
diameter, which is cooled by a current of water.