with filter of (3) is dissolved in a mixture of about five cubic
centimeters of hydrochloric acid and twenty cubic centimeters of water.
Then filter and make up to 150 cubic centimeters. Take fifty cubic
centimeters for the determination of iron and alumina together by
precipitation with ammonia, after oxidizing the organic matter (filter)
with aqua regia; also fifty cubic centimeters for iron alone; keep fifty
cubic centimeters in reserve. Determine the iron by means of a standard
solution of potassium permanganate after reduction; this latter is done
by evaporating the fifty cubic centimeters almost to dryness with strong
sulfuric acid, adding water and transferring the solution to a flask,
and then reducing by means of pure metallic zinc in the usual way. The
alumina is then determined by difference. This method of determining the
two oxids in their intermixture is in several respects more satisfactory
than the separation with alkaline lye, which, however, has served for
most determinations made, until within the last ten years. It is,
however, much more liable to miscarry in unpracticed hands than the
other.
(5) _Estimation of Lime._—The filtrate A from iron and alumina is
acidified slightly with hydrochloric acid, and if too bulky is
evaporated to about twenty-five cubic centimeters, unless the soil is a
very calcareous one, and the lime is precipitated from it by
neutralizing with ammonia and adding ammonium oxalate. The precipitation
of the lime should be done in the hot solution, as the precipitate
settles much more easily. It is allowed to stand for twelve hours, then
filtered (filtrate B), washed with cold water, and dried. By ignition
the lime precipitate is partially converted into the oxid. It is then
heated with excess of powdered ammonium carbonate, moistened with water,
and exposed to a gentle heat (50°–80°) until all the ammonia is
expelled. It is then dried below red heat and weighed as calcium
carbonate. When the amount of lime is at all considerable, the treatment
with ammonium carbonate must be repeated till a constant weight is
obtained.
(6) _Estimation of Sulfuric Arid._—The filtrate B from the calcium
oxalate is put into a bohemian flask, boiled down over the sand-bath,
and the ammoniacal salts destroyed with aqua regia. From the flask it is
removed to a small beaker and evaporated to dryness with excess of
nitric acid. This process usually occupies four to five hours. The
residue should be crystalline-granular; if white-opaque, ammonium
nitrate remains and must be destroyed by hydrochloric acid. The dry
residue is now moistened with nitric acid, and the floccules of silica
usually present separated by filtration from the filtrate, which should
not amount to more than ten or fifteen cubic centimeters; sulfur trioxid
is then precipitated by treatment with a few drops of barium nitrate,
both the solution and the reagent being heated to boiling. If the
quantity of sulfuric acid is large it may be filtered after the lapse of
four or five hours (filtrate C). If very small let it stand twelve
hours. The precipitate is washed with boiling water, dried, ignited, and
weighed. Care should be taken in adding the barium nitrate to use only
the least possible excess, because in such a small concentrated acid
solution the excess of barium nitrate may crystallize and will not
readily dissolve in hot water. Care must also be taken not to leave in
the beaker the large heavy crystals of barium sulfate, of which a few
sometimes constitute the entire precipitate, rarely exceeding a few
milligrams. Should the ignited precipitate show an alkaline reaction on
moistening with water, it must be treated with a drop of hydrochloric
acid, refiltered and weighed. The use of barium acetate involves
unnecessary trouble in this determination.
(7) _Estimation of Sodium and Potassium._—Filtrate C is now evaporated
to dryness in a platinum dish; the residue is treated with an excess of
crystallized oxalic acid, moistened with water, and exposed to gentle
heat. It is then strongly ignited to change the oxalates to carbonates.
This treatment with oxalic acid must be made in a vessel which can be
kept well covered, otherwise there is danger of loss through spattering.
As little water as possible should be used, as otherwise loss from
evolution of carbon dioxid is difficult to avoid. Spatters on the cover
should not be washed back into the basin until after the excess of
oxalic acid has been volatilized. The ignited mass should have a
slightly blackish tinge to prove the conversion of the nitrates into
carbonates. White portions may be locally retreated with oxalic acid.
The ignited mass is treated with a small amount of water, which
dissolves the alkaline carbonates and leaves the magnesium carbonate,
manganese protosesquioxid, and the excess of barium carbonate behind.
The alkalies are separated by filtration into a small platinum dish
(filtrate D), and the residue is well but sparingly washed with water on
a small filter. When the filtrate exceeds ten cubic centimeters it may,
on evaporation, show so much turbidity from dissolved earthy carbonates
as to render refiltration on a small filter necessary, since otherwise
the soda percentage will be found too large and magnesia too small. If,
on dissolving the ignited mass, the solution should appear greenish from
the formation of alkaline manganates, add a few drops of alcohol to
reduce the manganese to insoluble dioxid. The residue of barium,
magnesium, and manganese compounds is treated on the filter with
hydrochloric acid, and the platinum dish is washed with warm nitric acid
(not hydrochloric, for the platinum dish may be attacked by chlorin from
the manganese oxid) dissolving any small traces of precipitate that may
have been left behind.
The filtrate D, which should not be more than ten or fifteen cubic
centimeters, containing the carbonates of the alkalies, is evaporated to
dryness and gently fused, so as to render insoluble any magnesium
carbonate that may have gone through; then redissolved and filtered into
a small weighed platinum dish containing a few drops of dilute
hydrochloric acid, to change the carbonates into chlorids; evaporated to
dryness, exposed to a gradually rising temperature (below red heat), by
which the chlorids are thoroughly dried and freed from moisture, so as
to prevent the decrepitation that would otherwise occur on ignition.
Then, holding the platinum basin firmly by forceps grasping the clean
edge, pass it carefully over a very low bunsen flame, so as to cause,
successively, every portion of the scaly or powdery residue to collapse,
without fully fusing. There is thus no loss from volatilization, and no
difficulty in obtaining an accurate, constant weight. The weighed
chlorids are washed by means of a little water into a small beaker or
porcelain dish, treated with a sufficient quantity of platinum chlorid,
and evaporated to dryness over the water-bath. The dried residue is
treated with a mixture of three parts absolute alcohol and one part
ether, leaving the potassium platinochlorid undissolved. This is put on
a filter, and washed with ether-alcohol. When dried, the precipitate and
filter are put into a small platinum crucible and exposed to a heat
sufficiently intense to reduce the platinum chlorid to metallic platinum
and to volatilize the greater part of the potassium chlorid. This is
easily accomplished in a small crucible, which is roughened by being
constantly used for the same purpose (and no other), the spongy metal
causing a ready evolution of the gases. The reduced platinum is now
first washed in the crucible with hot acidulated water, then with pure
water; then all moisture is driven off and it is weighed. From the
weight of the platinum, is calculated the potassium chlorid and the oxid
corresponding; the difference between the weights of the total alkaline
chlorids and potassium chlorid gives the sodium chlorid, from which may
be calculated the sodium oxid. When the heating of the platinum
precipitate has not been sufficient in time or intensity, instead of
being in a solid spongy mass of the color of the crucible itself, small
black particles of metallic platinum will obstinately float on the
surface of the water in the crucible, and it becomes difficult to wash
without loss.
(8) _Estimation of Manganese._—The solution containing the magnesium and
manganese chlorids is freed from barium salts by hot precipitation with
sulfuric acid, and the barium sulfate, after settling a few hours, is
separated by filtration. The filtrate is neutralized with ammonia, any
resulting small precipitate (of iron) is filtered, and the manganese
precipitated with ammonium sulfid, let stand twelve hours and filtered
(filtrate E); wash with cold water, dry, ignite, and weigh as manganese
protosesquioxid, Mn₃O₄. If preferred the manganese may be precipitated
with chlorin or bromin water as dioxid; but the process requires a
rather longer time and may fail in inexpert hands more readily than the
other.
(9) _Estimation of Magnesium._—The filtrate E from the manganese is now
freed from sulfur by acidulating with hydrochloric acid, evaporating, if
necessary, and filtering. From the filtrate the magnesia is precipitated
by adding an equal bulk of ammonia water and then sodium phosphate.
After standing at least twenty-four hours, the magnesium salt may be
filtered, washed with ammoniacal water, dried, ignited, and weighed as
magnesium pyrophosphate.
=343. Examination of Acid Extract by the Methods of
Petermann.=—_Estimation of the Silica._—The Gembloux method of
estimating silica consists in taking up the dry extract obtained from
the treatment of the earth, in the manner described in paragraph =336=,
with water and a few drops of hydrochloric acid, heating for a short
time on a sand-bath to facilitate the solution, and filtering, washing,
drying, igniting, and weighing the residue obtained as silica.
_Estimation of the Sulfuric Acid._—The method employed consists in
heating the filtrate obtained in the estimation of silica for half an
hour with a few drops of nitric acid and making the volume up to 500
cubic centimeters. One hundred cubic centimeters of this are
precipitated with barium chlorid, diluted to double its volume, heated
for some time, the precipitate of barium sulfate collected and weighed,
and the quantity of sulfuric acid calculated therefrom.
_Potash and Soda._—Potash and soda are estimated at the Gembloux Station
by heating the filtrate obtained in the estimation of the sulfuric acid
and precipitating the excess of barium in the hot solution after the
addition of ammonia by ammonium oxalate and carbonate. The whole is
allowed to digest for six hours at a gentle heat and then allowed to
remain at rest for twenty-four hours, filtered, washed, and the filtrate
evaporated to dryness in a large platinum dish and the ammoniacal salts
driven off at a low temperature. At the end, the temperature is carried
a little higher until it reaches low redness. The residue is taken up by
distilled water, filtered into a weighed platinum dish, a few drops of
hydrochloric acid added, evaporated, dried, heated with great care and
the sodium and potassium chlorids obtained weighed together. The
respective quantities of potash and soda in the earth are estimated in
the usual way by precipitating the potash with platinum chlorid.
_Estimation of the Iron and Aluminum Oxids._—The iron and aluminum oxids
are estimated by taking twenty-five cubic centimeters of the primitive
solution obtained with hydrochloric acid and adding ammonium carbonate
almost to complete neutralization, that is to say until the precipitate
formed is just redissolved in the feeble excess of hydrochloric acid
which remains. Dilute with distilled water and precipitate with a little
excess of ammonium acetate, and boil for a moment; after boiling, the
basic iron and aluminum acetate and the small quantity of iron and
aluminum phosphate present are easily deposited, and the supernatant
liquid should be completely limpid and colorless. Wash the precipitate
by decantation, boiling each time, filter, wash the filter with boiling
water to which a little ammonium acetate has been added, dry, ignite,
and weigh. The material obtained consists of ferric oxid, aluminum oxid,
and iron and aluminum phosphates. Deduct from the whole, the phosphoric
acid determined in another portion. The residue will be the sum of the
iron and aluminum oxids.
_Estimation of the Lime._—The filtrate from the portion used for the
estimation of the iron and alumina is treated with ammonium oxalate. The
mixture is kept at a low temperature for at least twelve hours, after
which it is filtered, washed with hot water, dried, and ignited over a
blast-lamp to constant weight and weighed as calcium oxid.
_Estimation of the Magnesia._—For the estimation of the magnesia the
filtrate obtained in the estimation of lime is evaporated to dryness in
a platinum dish, the ammoniacal salts driven off, the residue taken up
with water slightly acidified with hydrochloric acid, filtered, the
filtrate saturated with ammonia and heated some time to the boiling
point to precipitate any traces of iron and alumina which may have
remained in solution. Filter, wash, allow to cool and precipitate the
magnesia by the addition of sodium phosphate. It is then allowed to
stand for twelve hours, collected on a filter, ignited, and weighed as
pyrophosphate, and the quantity of magnesia calculated from the weight
of salt obtained.
_Estimation of the Phosphoric Acid._—The phosphoric acid is estimated by
taking 100 cubic centimeters of the original solution obtained by the
treatment of the soil with hydrochloric acid and evaporating it to
dryness on the water-bath. The residue is taken up with water to which a
few drops of nitric acid have been added and filtered. The total
phosphoric acid is then obtained by precipitation with ammonium
molybdate in the usual way.
=344. Analysis of the Insoluble Residue.=—The insoluble residue left
after digestion with hydrochloric acid is not without interest from an
agricultural and analytical point of view. While it is true that the
plant food, therein contained, is not immediately available, yet it must
not be forgotten that the method of the chemist may not fix a limit to
nature’s method of collecting nutriment for plants. In however
refractory a state they may exist, it is possible that all nutritive
elements may eventually become available for assimilation. For the
completion of an estimate of the total nutritive power of a soil,
therefore a further examination of the insoluble residue should be made.
The methods of securing this are essentially those of making a bulk
analysis of the soil.
The principle of the method depends on the reduction of the sample to an
impalpable powder and the subsequent decomposition of the insoluble
portions by treatment with hydrofluoric and sulfuric acids or by fusion
with the alkalies.
=345. Method of Wolff for Treating Residue Insoluble in Hot Acid.=—The
well-washed residue is dried with the filter, then separated therefrom,
the filter burned and the ash weighed with the whole of the residue.
About eight grams of the residue are ignited and serve for the
estimation of the insoluble mineral matter. Another portion of ten grams
of the dried, but not ignited, residue is boiled with a concentrated
solution of sodium carbonate with the addition of caustic soda, and the
quantity of dissolved silicic acid estimated. A third portion of about
fifteen grams is treated with about five times its weight of pure
concentrated sulfuric acid, and is evaporated until the mass has taken
the form of a dry powder. After moistening with concentrated
hydrochloric acid the mass is boiled with water, filtered, and the
filtrate examined according to the ordinary methods for silicic acid,
alumina, iron, lime, magnesia, and alkalies. The residue after treatment
with concentrated sulfuric acid is dried, but not ignited, and boiled
with a concentrated solution of sodium carbonate with the addition of a
little caustic soda, filtered, heated, and the silicic acid separated
from the solution. After thorough washing, the residue, after ignition,
is weighed and represents the material insoluble in concentrated
hydrochloric and sulfuric acids. The silicic acid found as before,
together with the small quantity dissolved in the hydrochloric acid
extract, gives, in connection with the alumina contained in the sulfuric
acid extract, approximately the quantity of pure water-free clay
contained in the soil.
In six samples of soils of very different compositions which were
examined by the above process, it was found that the clay had the
following mean composition: Silicic acid, 55.1 to 61.5 per cent,
alumina, 38.6 to 44.9 per cent; as a mean 58.05 per cent silicic acid
and 41.95 per cent alumina.
Finally, four or five grams of the residue, after treatment with
sulfuric acid and sodium carbonate, are rubbed up in an agate mortar and
completely separated into silt by water. The silt mass is dried, lightly
ignited, and three grams of it spread in a flat platinum dish moistened
with sulfuric acid, and subjected to the action of hydrofluoric acid in
a lead oven at 60°, until a complete decomposition of the material is
accomplished. In the solution all the different bases can be determined.
=346. Method of the Belgian Chemists.=—The method employed by
Petermann[222] at the Gembloux Station in the examination of the part of
the soil insoluble in hydrochloric acid consists in washing the
insoluble portion by decantation with distilled water until all acid
reaction is removed. Place the contents of the flask and of the filter
in a porcelain dish and dry. After a careful mixing of the mass take out
about fifty grams and wash upon the filter until all reaction for
chlorin has disappeared, dry, detach the mass from the filter, and
incinerate. Place in a platinum crucible two grams of the ground and
ignited residue and mix it, using a platinum stirring rod, with twelve
grams of ammonium fluorid; heat slightly over a bunsen burner in a
muffle with a good draught and regulate the flame in such a way that the
operation shall continue for about one hour. After complete
decomposition add about two cubic centimeters of sulfuric acid in such a
way as to moisten completely the residue, drive off the sulfuric acid
carefully at a low red heat and take up the residue with water slightly
acidulated with hydrochloric acid and wash the whole into a flask of 500
cubic centimeters capacity. Oxidize by heating for an hour with nitric
acid, make up to the mark and filter. The percentages of potash, soda,
lime, magnesia, and the silicates are determined exactly as in the
hydrochloric acid extract.
=347. Bulk Analysis.=—It is frequently desirable to determine the total
composition of a soil sample as well as the nature of that part of it
soluble in any of the solvents usually employed. The latest methods for
this purpose have been well studied by Packard[223] who finds that the
variations which occur between duplicates are probably due to the small
quantities of material taken for analysis, it being difficult to obtain
average samples of a material which is not very finely powdered when
small quantities are taken. Moreover, as it is likely to become of
importance to know whether the proportions of lime and magnesia vary by
as much as one-tenth per cent, and such small variations are within the
limits of error of an analysis, and as the total proportion of lime and
magnesia in highly siliceous soils, probably does not exceed one-tenth
per cent, it is deemed best to take a large quantity of soil for the
bulk analysis in each case. The amount adopted for the highly siliceous
soils containing much quartz is ten grams. This quantity, taken after
quartering down the entire sample, is ground to an impalpable powder and
used for the determination of the lime, magnesia, and alkalies, the
silica, iron oxid, alumina, and loss on ignition, being determined in
one gram samples. The ten grams are decomposed by hydrofluoric and
sulfuric acids in a large platinum dish, the solution evaporated, at
first on the water-bath until all water is removed and then at a higher
temperature until all the free sulfuric acid is driven off, when the
residue is heated in a muffle at a low red heat for several hours. At
this temperature the sulfuric acid combined with the iron oxid and
alumina is driven off, leaving the remaining sulfates unchanged and the
iron oxid and alumina are in the form of a powder of no great volume
which is easily and quickly washed. This operation is usually successful
at first but in some cases the decomposition is not complete as is shown
by the appearance of a precipitate on adding ammonia to the filtrate
from the aluminum and iron oxids. In such cases the precipitate is
dissolved in hydrochloric acid, reprecipitated by ammonia and removed by
filtration. In the filtrate from the thoroughly washed aluminum and iron
oxids, lime is precipitated as oxalate and separated by filtration; the
filtrate is evaporated to dryness and the ammonia salts driven off by
heat; the magnesia in the unfiltered watery extract of this residue is
precipitated by baryta water, which also removes the sulfuric acid with
which the bases had been combined. In the filtrate from this precipitate
baryta is precipitated by ammonium carbonate and removed by filtration,
leaving the alkalies to be determined in the usual way after conversion
into chlorids. The mixed precipitate of magnesia and barium sulfate is
treated with hydrochloric acid, filtered, the baryta present removed as
sulfate, and the magnesia precipitated in the filtrate from the latter
as phosphate. The advantages of this method are that the large quantity
of material employed gives some assurance that an average sample has
been operated on, and all the bases present in small proportions are
estimated in the same sample. The objection to it is the time consumed
both in grinding the samples and in determining all the bases in one
solution. As a small quantity of material is generally used for
determining the silica, iron oxid, alumina, and loss by ignition, and a
larger quantity for the remaining bases, slight differences in the
unground samples are unavoidable, especially when the quartz grains are
somewhat large, it being practically impossible to take two small
samples of such a soil which would have the same number of quartz
grains. Consequently tedious grinding of large quantities of the soils
for the bulk analysis is necessary. This objection does not apply to the
official analysis or assay of soils in which considerable quantities are
extracted by acid and the solution analyzed, and silica is not
determined. In any case, it may be said, when it becomes an object to
know whether a soil contains a total of 0.1 or 0.2 per cent of lime or
magnesia, of 0.7 or 0.5 per cent of potash, one analysis even of the
large quantity of ten grams would be insufficient to decide the point,
and at least the mean of two determinations should be taken.
SPECIAL METHODS OF DETERMINATION OF SOIL CONSTITUENTS.
=348. Preliminary Considerations.=—In the foregoing paragraphs the
general outline of the chemical methods of soil examination have been
given. There are often occasions, however, which demand a special study
of some particular soil constituent. It has been thought proper,
therefore, to add here some of the best approved methods of special
determinations which have been approved in this and other countries.
In the main, the final determination of any particular element of the
soil, and its previous separation from accompanying elements, are based
on the general processes already given. The variations in many
instances, however, seem to require special mention.