acid.
Ingredients. 1.00 1.115 1.160
Insoluble residue 71.88 70.53 74.15
Soluble silica 11.38 12.30 9.42
Potash 0.60 0.63 0.48
Soda 0.13 0.09 0.35
Lime 0.27 0.27 0.23
Magnesia 0.45 0.45 0.45
Manganese oxid 0.06 0.06 0.06
Ferric oxid 5.15 5.11 5.04
Alumina 6.84 8.09 6.22
Sulfuric acid 0.02 0.02 0.02
Volatile matter 3.14 3.14 3.14
—————— —————— —————
Total 100.02 100.69 99.29
Amount of soluble matter 24.00 27.02 22.27
„ „ „ bases 13.50 14.70 12.83
From the above table it is seen that the strongest acid exerts the least
soluble effect upon the substances present in the soil, while the
greatest degree of solution was obtained by the acid of 1.115 specific
gravity. This result indicates that while lime and magnesia are probably
present chiefly as carbonates, potash as well as alumina, and to some
extent lime, are present as silicates, and for that reason are not as
fully extracted by acid of low strength as by that of medium
concentration.
In regard to the influence of the time of digestion, the acid of
specific gravity 1.115 being used, the data obtained are given in the
following table:
Number of days digested.
Ingredients. 1. 3. 4. 5. 10.
Insoluble residue 76.97 72.66 71.86 70.53 71.79
Soluble silica 8.60 11.18 11.64 12.30 10.96
Potash 0.35 0.44 0.57 0.63 0.62
Soda 0.06 0.06 0.03 0.09 0.28
Lime 0.26 0.29 0.28 0.27 0.27
Magnesia 0.42 0.44 0.47 0.45 0.44
Manganese oxid 0.04 .06 0.06 0.06 0.06
Ferric oxid 4.77 5.01 5.43 5.11 4.85
Alumina 5.15 7.38 7.07 7.88 7.16
Phosphoric acid 0.21 0.21
Sulfuric acid 0.02 0.02 0.02 0.02 0.02
Volatile matter 3.14 3.14 3.14 3.14 3.14
————— —————— —————— —————— —————
Total 99.63 100.68 100.55 100.69 99.80
Amount of soluble matter 19.67 24.88 25.57 27.02 24.87
„ „ „ bases 11.05 13.68 13.91 14.49 13.68
From this table it appears that the amount of dissolved ingredients
increases up to the fifth day, the increase becoming, however, very slow
as that limit is approached. It is also found that the ingredients
offering the greatest resistance to this action are the same as those
whose amounts were sensibly affected by the strength of the acid;
namely, silica, potash, and alumina.
In regard to lime and magnesia, one day’s digestion not being sufficient
for full extraction, it is evident that they do not exist in the soil as
carbonates or hydric oxids only, as has been supposed, but also as
silicates. A comparison of the results of the five and ten days’
digestion shows that the solvent action of the acid has substantially
ceased at the end of five days, there being no further increase of the
amount of dissolved matter.
=332. Digestion Vessels.=—Hilgard prescribes that the digestion of the
sample of soil with acid be conducted in a small porcelain beaker
covered with a watch-glass.[215] Kedzie, however, prefers beakers of
bohemian glass, and shows that hydrochloric acid attacks the porcelain
with greater energy than the glass.[216] Platinum would be the ideal
material for the digestive vessels, but its great cost would exclude its
general use. In most cases it will be found that the error introduced
into the analysis by the use of porcelain or bohemian glass beakers is
quite small and not likely to affect the quantitative estimation of
soluble soil ingredients to any extent.
In this laboratory some comparative tests made by Mr. W. D. Bigelow have
shown that vessels of hard glass of special manufacture are less soluble
in hot hydrochloric acid of 1.115 specific gravity than porcelain, thus
confirming the observation of Kedzie. Following are the data showing the
weights of material dissolved in fifty hours:
Berlin porcelain 2.8 milligrams
Bohemian glass 1.7 „
Kaehler and Martini glass 1.2 „
In each case twenty-five cubic centimeters of the acid were used. The
vessels all had approximately a capacity of 200 cubic centimeters.
=333. Processes Employed—Hilgard’s Method.=—The sample of soil sifted
through a 0.5 millimeter mesh sieve and thoroughly air-dried, is
conveniently preserved in weighing tubes. The actual content of
hygroscopic and combined moisture may be previously made on a separate
sample of soil.
In determining the amount of material to be employed for the general
analysis regard must be had to the nature of the soil. This is necessary
because of the impracticability of handling successfully such large
precipitates of alumina as would result from the employment of as much
as five grams in the case of calcareous clay soils; while in the case of
very sandy soils even that quantity might require to be doubled in order
to obtain weighable amounts of certain ingredients. For soils in which
the insoluble portion ranges from sixty to eighty per cent, two and a
half to three grams are about the right measure for general analysis,
while for the phosphoric acid determination not less than three grams
should be employed in any case. It has been alleged that larger
quantities must be taken for analysis in order to secure average
results. It is difficult to see why this should be true for soils and
not for ores, in which the results affect directly the money value,
while in the case of soils the interpretation of results allows much
wider limits in the percentages. Correct sampling must be presupposed to
make any analysis useful; but with modern balances and methods it is
difficult to see why five grams should be employed instead of half that
amount, which in some cases is still too much for convenient
manipulation of certain precipitates.
The weighed quantity, usually of two to two and a half grams, is brought
into a small porcelain beaker, covered with a watch-glass, treated with
eight to ten times its bulk of hydrochloric acid of 1.115 specific
gravity, and two or three drops of nitric acid, and digested for five
days over the laboratory steam-bath. At the end of this time it is
evaporated to dryness, first on the water-bath and then on the
sand-bath. By this treatment all the silica set free is rendered
insoluble.
=334. Provisional Method of the Official Agricultural Chemists.=—Place
ten grams of the air-dried soil in a round bottom 150 to 200 cubic
centimeter bohemian flask, add 100 cubic centimeters of pure
hydrochloric acid of specific gravity 1.115, insert the stopper, wire it
securely, place in a steam-bath, and digest for thirty-six hours at the
temperature of boiling water. Pour the contents of the flask into a
small beaker, wash with distilled water, add the washings to the
contents of the beaker and filter through a washed filter. The residue
is the amount insoluble in hydrochloric acid. Add a few drops of nitric
acid to the filtrate, and evaporate to dryness on the water-bath; take
up with hot water and a few drops of hydrochloric acid, and again
evaporate to complete dryness. Take up as before, and filter into a
liter-flask, washing with hot water. Cool and make up to the mark. This
is solution A. The residue represents the silica originally dissolved.
In comparing the two preceding methods it is found that the former;
_viz._, digestion in flasks covered only with a watch-glass gives a
larger quantity of dissolved matter in five days than the digestion
under pressure does in thirty-six hours. In comparative tests in this
laboratory made by Mr. W. D. Bigelow the respective quantities of
soluble and insoluble matter obtained by the two methods in two soils
are as follows:
Soil No. 1. Soil No. 2.
Per cent. Per cent.
Method of Digestion. Insoluble. Soluble. Insoluble. Soluble.
Open flask 75.62 24.38 79.62 20.38
Closed flask 76.81 23.19 80.48 19.52
=335. The German Station Method.=—The method recommended by the German
Stations[217] is greatly different from that described above, both in
temperature and time of digestion. To one part of the soil are added two
parts by volume of a twenty-five per cent hydrochloric acid solution,
the quantity being increased to correspond to any excess of carbonates.
The mixture is left for forty-eight hours with frequent shaking. As an
alternate method, one part of soil is treated with two parts by volume
of ten per cent hydrochloric acid, and heated on the water-bath, with
frequent shaking, for three hours.
The soluble materials are determined in the filtrate by some of the
methods usually employed.
=336. The Gembloux Method.=—The method of making the acid extract of the
soil at the Gembloux Station does not differ greatly from some of those
already described.
The quantity of air-dried material taken is such that it may weigh
exactly 300 grams exclusive of the moisture which it contains. It is
dried at 150° for at least six hours. The drying is necessary in order
to obtain an extract in hydrochloric acid of exactly 1.18 specific
gravity. The dry earth is placed in a flask of two or three liters
capacity to which one liter of hydrochloric acid of 1.18 specific
gravity is added, being careful to take precautions to prevent frothing
if much carbonate be present. The acid is allowed to act for twenty-four
hours, it being frequently shaken meanwhile. After settling it is
decanted and filtered upon a double folded filter, the apex of which
rests upon a small funnel covered with a plain filter of strong paper.
Five hundred cubic centimeters of the filtrate are taken for the
estimation, and in this filtrate are estimated the silica, phosphoric
and sulfuric acids, potash, soda, iron, alumina, lime, and magnesia.
The filtrate is evaporated to dryness in a porcelain capsule, a few
drops of nitric acid added and the liquid kept well stirred. The residue
should be taken up with water, and if not perfectly bright a second and
even a third evaporation with nitric acid should take place, until all
the organic matter is destroyed, which will be indicated by the clear
yellow or reddish-yellow color of the liquid, caused by the iron oxid.
After the last evaporation the material is dried in a drying oven one
hour at 110°.
=337. Treatment with Cold Hydrochloric Acid.=—According to the digestion
method of Wolff[218] the soil sample is treated with cold concentrated
hydrochloric acid. The process is as follows:
Four hundred and fifty grams of the soil dried at 100° are placed in a
glass flask and treated with 1,500 cubic centimeters of hydrochloric
acid of 1.15 specific gravity, corresponding to thirty per cent of
gaseous hydrochloric acid. For every five per cent of calcium carbonate
which the soil may contain, an additional fifty cubic centimeters of
hydrochloric acid are added. With frequent stirring, the soil is left in
contact with the acid for forty-eight hours and then 1,000 cubic
centimeters of liquid, as clear as possible, are poured off, which
corresponds to 300 grams of the soil. After dilution with water it is
filtered and the filtrate treated with a few drops of nitric acid and
evaporated to dryness. After the separation of the silica the solution
is again made up with water to 1,000 cubic centimeters.
Two hundred cubic centimeters of this solution, corresponding to sixty
grams of the soil, are taken for the estimation of iron, alumina, lime,
manganese, and magnesia.
Four hundred cubic centimeters of the solution, corresponding to 120
grams of the soil, are left for the estimation of sulfuric acid and
alkalies. This method gives from five to six times less alkalies and a
much smaller quantity of iron than the treatment with hot acid. In the
use of hot acid, therefore, Wolff reduces the quantity of soil acted on
to 150 grams.
=338. Treatment with Nitric Acid.=—For the purpose of estimating
phosphoric acid Grandeau[219] directs that the soil be extracted with
nitric acid. For this purpose 100 grams of the air-dried fine earth are
placed in a bohemian flask and treated cautiously with nitric acid in
small quantities at a time. If the soil be calcareous in its nature it
should be previously moistened with water, and the acid so added as to
avoid undue effervescence, the flask being inclined during the
operation. Sufficient acid is added to strongly saturate the sample and
it is then digested on the sand-bath for two hours; or at least until
the organic matters are destroyed, which will be indicated by the
cessation of evolution of nitrous vapors. When the supernatant liquid
has become clear it is decanted. The residue is washed with distilled
water and separated on a filter, and washed until the wash-water is
colorless. The decanted portion is united with the filtrate and the
whole made up to a volume of one liter. The determinations are made in
portions of 200 cubic centimeters each.
=339. Digestion with Hydrofluoric and Sulfuric Acids.=—When a complete
disintegration of the siliceous substances in soils is desired as in
analysis in bulk, the decomposition is easily accomplished by digestion
with the above named acids in a platinum dish. The fine earth is
saturated with a concentrated aqueous solution of hydrofluoric acid to
which a few drops of sulfuric acid are added. It is then digested until
nearly dry. If any undecomposed particles remain, the treatment is
continued until complete decomposition is secured. The silica is thus
all volatilized as hydrofluosilicic acid and the bases pre-existing in
the soil are left as sulfates. This method of treatment is especially
recommended when it is desired to estimate the whole quantity of any of
the soil constituents with the exception of silica. The silica may,
however, be determined in the distillate. Instead of using the solution
of hydrofluoric acid, ammonium fluorid may be employed. In this process
the sample of earth reduced to an impalpable powder by grinding in an
agate mortar is mixed with four or five times its weight of the ammonium
fluorid in a platinum dish and thoroughly moistened with sulfuric acid
and allowed to stand at room temperature for several hours. It is then
gently heated until all fumes of hydrofluosilicic acid have been driven
off, but is not raised to a red heat. If any undecomposed particles
remain, the above treatment is repeated.
DETERMINATION OF THE QUANTITY OF DISSOLVED MATTER.
=340. Substances in Solution.=—By treatment with solvents as indicated
in the preceding paragraphs, greater or less quantities of the original
constituents of soil are brought into solution. The total quantity of
dissolved matters is determined by drying and weighing the insoluble
residue and the percentages of soluble and insoluble matters should be
noted; and each portion saved for further examination. In this country
the common practice of soil analysis is to digest the sample with
hydrochloric acid. The following paragraphs, therefore, will be devoted
to the general methods of determining the matters dissolved by that
treatment, leaving for later consideration the special methods of
analysis. The fundamental principle on which the treatment with
hydrochloric acid rests is based on the belief that such treatment
practically extracts from the soil all those elements which are likely
to become, immediately or in the near future, available for plant food.
=341. Provisional Methods of the Official Agricultural
Chemists.=[220]—(1) _The Analytical Operations_ are conducted with
solution A, paragraph =334=.
(2) _Ferric Oxid, Alumina, and Phosphoric Acid._—To 100 or 200 cubic
centimeters, according to the probable amount of iron present, of the
solution A, add ammonium hydroxid to alkaline reaction to precipitate
ferric and aluminum oxids and phosphates. Expel the excess of ammonia by
boiling, allow to settle, decant the clear solution through a filter;
add to the flask fifty cubic centimeters of hot distilled water, boil,
settle, and decant as before. After pouring off all the clear solution
possible, dissolve the residue with a few drops of warm hydrochloric
acid and add just enough ammonium hydroxid to precipitate the oxids.
Wash by decantation with fifty cubic centimeters of distilled water, and
then transfer all the precipitate to the filter and wash with hot
distilled water till the filtrate becomes free from chlorids. Save the
filtrate and washings which form solution B. Dry the filter and
precipitate at 110°, transfer the precipitate to a tared platinum
crucible, burn the filter and add the ash to the precipitate, heat the
whole red hot, cool in a desiccator, and weigh. The increase of weight,
minus the ash of filter and the phosphoric acid (found in a separate
process), represents the weight of the ferric and aluminum oxids.
(3) _Ferric Oxid._—Precipitate 100 cubic centimeters of solution A, as
under (2), except that only one precipitation is made; wash with hot
water; dissolve in dilute sulfuric acid; reduce with zinc and estimate
as ferrous oxid by a standard solution of potassium permanganate.
To prepare the potassium permanganate solution, dissolve 3.156 grams of
pure crystallized potassium permanganate in 1,000 cubic centimeters of
distilled water, and preserve in a glass-stoppered bottle, shielded from
the light. Standardize this solution with pure ferrous sulfate, ammonium
ferrous sulfate or oxalic acid.
(4) _Alumina._—The calculated weight of ferric oxid deducted from that
of ferric oxid and alumina with corrections for filter ash and
phosphoric acid, will give the weight of alumina in two grams of
air-dried soil.
(5) _Phosphoric Acid._—This may be estimated in the above iron solution,
if the soil is sufficiently rich, by the molybdate method, given under
fertilizers; or if the quantity of soil represented in the iron solution
is not sufficient, a fresh portion of solution A may be taken, and the
phosphoric acid determined directly by the molybdate method.
(6) _Manganese._—Concentrate the filtrate and washings (solution B) to
200 cubic centimeters or less; add ammonium hydroxid to alkalinity; add
bromin water and heat to boiling, keeping the beaker covered with a
watch-glass; as the bromin escapes, the beaker is allowed to cool
somewhat, ammonia and bromin water again added, and heated as before.
This process is continued until the manganese is completely
precipitated, which requires from thirty to sixty minutes, and the
solution filtered while still warm; the precipitate is washed, dried,
ignited and weighed; estimate as manganese protosesquioxid.
(7) _Lime._—If no manganese is precipitated, add to solution B, or the
filtrate and washings from (6) twenty cubic centimeters of a strong
solution of ammonium chlorid and forty cubic centimeters of saturated
solution of ammonium oxalate to completely precipitate all the lime as
oxalate and convert the magnesia into soluble magnesium oxalate. Heat to
boiling and let stand for six hours till the calcium oxalate settles
clear, decant the clear solution on a filter, pour fifty cubic
centimeters of hot distilled water on the precipitate and again decant
the clear solution on the filter, transfer the precipitate to the
filter, and wash it free from all traces of oxalates and chlorids. Dry
and ignite the precipitate over the blast-lamp until it ceases to lose
weight, weigh and estimate as calcium oxid; carefully moisten with
sulfuric acid, heat the inclined covered crucible gently to avoid loss,
then intensely, and weigh as calcium sulfate.
(8) _Magnesia._—Concentrate the filtrate and washings (from 7) to 200
cubic centimeters, place in a half-liter erlenmeyer, add thirty cubic
centimeters of a saturated solution of sodium phosphate and twenty cubic
centimeters of concentrated ammonium hydroxid, cork the flask, and shake
violently at intervals of a few minutes till crystals form, then set the
flask in a cool place for twelve hours. Filter the clear liquid through
a tared gooch, transfer the precipitate to the filter, and wash with
dilute ammonium hydroxid (1 : 3) till the filtrate is free from
phosphates; dry and ignite the crucible, at first gently and then
intensely, to form magnesium pyrophosphate. The increase of weight ×
0.36024 = MgO. By using an erlenmeyer free from scratches and marks, and
shaking violently instead of stirring with a glass rod, the danger is
almost entirely avoided of crystals adhering to the sides of the vessel;
but if crystals do adhere they are readily removed by a rubber-tipped
glass rod.
(9) _Sulfuric Acid._—Evaporate 200 cubic centimeters of solution A (1)
nearly to dryness on a water-bath to expel the excess of acid; then add
100 cubic centimeters of distilled water, heat to boiling and add ten
cubic centimeters of a solution of barium chlorid, and continue the
boiling for five minutes. When the precipitate has settled, pour the
clear liquid on a tared gooch, treat the precipitate with fifty cubic
centimeters of boiling water, and transfer the precipitate to the filter
and wash with boiling water till the filtrate is free from chlorids. Dry
the filter and ignite strongly. The increase in weight is barium
sulfate, which multiplied by 0.34331 = SO₃ in two grams of air-dried
soil.
(10) _Potash and Soda._—To another portion of 200 cubic centimeters of
solution A, add barium chlorid in slight excess, and make alkaline with
ammonia to precipitate sulfuric and phosphoric acids, ferric oxid, etc.
Then precipitate the calcium and barium by ammonium oxalate. Evaporate
the filtrate and washings to dryness, heat to a low red heat to
decompose oxalates and expel ammonia salts, dissolve in twenty-five
cubic centimeters of distilled water, filter and wash the precipitate;
add to the filtrate and washings ten cubic centimeters of baryta water,
and digest for an hour. Filter and wash the precipitate, add ammonium
carbonate to the filtrate to complete precipitation of baryta, filter
and wash this precipitate. Evaporate the filtrate and washings in a
tared platinum dish, gently ignite the residue to expel ammonia salts,
cool and weigh. The increase of weight represents the potassium and
sodium chlorids in two grams of air-dried soil.
=342. Hilgard’s Methods.=[221]—(1) _Soluble Silica._— The acid filtrate
obtained by the process given in paragraph =333= is employed for the
following determinations. After the solution obtained has been
evaporated to dryness to render silica insoluble, it is moistened with
strong hydrochloric acid and two or three drops of nitric acid. The mass
is warmed, and after allowing to stand for a few hours on a steam-bath
is taken up with distilled water. After clearing, it is filtered from
the insoluble residue, which is strongly ignited and weighed. If the
filtrate should be turbid the insoluble residue which has gone through
the filter can be recovered in the iron and alumina determination.
The insoluble residue is next boiled for fifteen or twenty minutes in a
concentrated solution of sodium carbonate, to which a few drops of
caustic lye should then be added to prevent reprecipitation of the
dissolved silica. The solution must be filtered hot. The difference
between the weight of the total residue and that of undissolved sand and
mineral powder is recorded as soluble silica, being the aggregate of
that set free by the acid treatment and that previously existing in the
soil. The latter, however, rarely reaches five per cent.
(2) _Destruction of Organic Matter._—The acid filtrate from the total
insoluble residue is evaporated to a convenient bulk. In case the
filtrate should indicate by its color, the presence of any organic
matter, it should be oxidized by aqua regia, otherwise there will be
difficulty in separating alumina.
(3) _Precipitation of Iron and Alumina._—The filtrate thus prepared is
now brought to boiling and treated sparingly with ammonia, whereby iron
and alumina are precipitated. It is kept boiling until the excess of
ammonia is driven off, and then filtered hot. (Filtrate A.) The previous
addition of ammonium chlorid is usually unnecessary. If the boiling is
continued too long, filtration becomes very difficult and a part of the
precipitate may redissolve in washing. Filtration may be begun as soon
as the nose fails to note the presence of free ammonia; test paper is
too delicate. Failure to boil long enough involves the contamination of
the iron-alumina precipitate with lime and manganese.