Navassa T. S. Gladding[115]{ ½ hour 65 0.00 0.00 2.53
rock, { ½ “ 65 0.733 gm 4.87
{ ½ “ 65 0.733 gm 1.22
South T. S. Gladding, { ½ “ 65 0.00 0.00 1.35
Carolina { ½ “ 65 0.733 “ 2.89
rock, { ½ “ 65 0.733 “ 1.06
Grand T. S. Gladding, { ½ “ 65 0.00 0.00 1.97
connetable, { ½ “ 65 0.733 “ 1.12
{ ½ “ 65 0.733 “ 11.44
Dissolved H. B. McDonnell, { ½ “ 65 0.00 0.00 2.49 11.51
bone-black { ½ “ 65 0.01 “ 2.42
and { ½ “ 65 0.01 “ 2.37
cottonseed-meal,
Ground H. B. McDonnell, { ½ “ 65 0.00 0.00 8.66 26.62
bone, { ½ “ 65 0.01 “ 9.18
{ ½ “ 65 0.01 “ 8.00
Calcined H. B. McDonnell, { ½ “ 65 0.00 0.00 30.61 45.11
Redonda, { ½ “ 65 0.01 “ 29.42
{ ½ “ 65 0.01 “ 32.47
Dissolved H. A. Huston, { ½ “ 65 0.00 0.00 2.24 11.32
bone-black { ½ “ 65 1.00 “ 2.24
and { ½ “ 65 1.00 “ 2.21
cottonseed-meal,
Ground H. A. Huston { ½ “ 65 0.00 0.00 8.78 26.35
bone, and W. J. { ½ “ 65 1.00 “ 13.48
Jones, Jr., { ½ “ 65 1.00 “ 5.35
Calcined H. A. Huston { ½ “ 65 0.00 0.00. 25.54 45.15
Redonda, and W. J. { ½ “ 65 1.00 “ 18.84
Jones, Jr., { ½ “ 65 1.00 “ 35.20
South H. A. Huston { ½ “ 65 0.00 0.00 1.81 27.67
Carolina and W. J. { ½ “ 65 1.00 “ 4.59
rock, Jones, Jr., { ½ “ 65 1.00 “ 0.74
Basic H. A. Huston { ½ “ 65 0.00 0.00 6.98 19.42
slag, and W. J. { ½ “ 65 1.0 “ 10.12
Jones, Jr., { ½ “ 65 1.0 “ 5.49
Pamunky H. A. Huston { 5 h’rs 65 0.00 0.00 11.80 13.84
phosphate, and W. J. { 5 “ 65 1.0 11.79
Jones, Jr., { 5 “ 65 1.0 “ 12.28
Raw bone, H. A. Huston { 2 “ 65 0.00 0.00 8.22 23.58
and W. J. { 2 “ 65 1.0 “ 11.20
Jones, Jr., { 2 “ 65 1.0 “ 4.02
Steamed H. A. Huston { 5 “ 65 0.00 0.00 17.94 27.67
bone, and W. J. { 5 “ 65 1.0 “ 22.55
Jones, Jr., { 5 “ 65 1.0 “ 9.64
Precipitated H. A. Huston { 2 “ 65 0.00 0.00 24.20 33.34
calcium and W. J. { 2 “ 65 1.0 “ 30.70
phosphate Jones, Jr., { 2 “ 65 1.0 “ 20.67
from glue works,
Florida H. A. Huston, { 2 “ 65 0.00 0.00 1.69 19.75
soft rock, and W. J. { 2 “ 65 1.0 “ 3.37
Jones, Jr., { 2 “ 65 1.0 “ 0.72
Calcined H. A. Huston { 5 “ 65 0.00 0.00 40.64 44.30
Redonda, and W. J. { 5 “ 65 1.0 “ 40.05
Jones, Jr., { 5 “ 65 1.0 “ 41.01
In the above tabulations no mention is made of the work of Fresenius,
Neubauer, and Luck, on whose researches the citrate method is based,
but an examination of their original paper shows that the temperature
conditions are not carefully enough controlled to justify us in
tabulating their results.[116] An attempt has been made to include in
the above tables, work made under well-defined conditions, which will
illustrate the various points under consideration. While each authority
of value upon the subject is represented, no attempt has been made
to include all the work done by any of them. One element that seems
to have been generally overlooked in discussing the problem is that
nearly all results have been obtained from one-half hour’s treatment of
the material. This means simply the study of an incomplete reaction,
and one which is interrupted while the solution is very rapidly going
on. This, of course, is only clearly brought out by a comparison of
long-time and short-time work in the various tables. In the opinion
of Huston very much more work will have to be done before it can be
assumed that we have any very clear knowledge of this subject, and
very likely the final result will be that all kinds of goods cannot be
examined by the same method. The fact that half a gram of dicalcium
phosphate is instantly soluble in 100 cubic centimeters of citrate
solution, at ordinary temperatures, while an equal amount of iron and
aluminum phosphate is acted upon very slowly at ordinary temperatures
will probably have to be taken into consideration, as well as the fact
that dicalcium phosphate is less soluble in hot solutions of ammonium
citrate than it is in cold solutions, while the reverse is true of the
precipitated iron and aluminum phosphate.
At present, the only conclusion that can be safely drawn from the work,
is that it would be unsafe to make any generalization upon the subject
until more facts are at hand, except that the present methods are
unscientific and, unsatisfactory. As the work progresses, new features
present themselves, and in such a way as to show that they must be
given careful consideration before drawing any final conclusions in the
matter.
=127. Arbitrary Determination of Reverted Phosphoric Acid.=—The
so-called reverted phosphoric acid, that is, the acid insoluble in
water and soluble in a solution of ammonium citrate, is the most
difficult constituent of commercial fertilizers from the point of
view of the scientific analyst. A review of all the standard methods
which have been given in the preceding pages for its determination
must convince every careful observer that, as a rule, each process is
based on arbitrary standards, and can give only concordant results when
carried out under strictly unvarying conditions. For this reason there
can be no just comparison between the results obtained by different
methods, which vary from each other only in slight particulars.
When, on the other hand, the processes are radically different, the
deviations in data become more pronounced.
In such a condition of affairs the analyst is left to choose between
methods. He must be guided in his choice not only by what seems to
be the most scientific and accurate process, but also, to a certain
extent, by the general practice of his professional brethren. For this
country, therefore, it is strongly urged that the methods adopted by
the Association of Official Agricultural Chemists, be followed in every
detail.
By the phrase “reverted phosphoric acid,” was originally meant an acid
once soluble in water, as CaH₄(PO₄)₂, and afterwards changed to a form
insoluble in water, but soluble in ammonium citrate as Ca₂H₂(PO₄)₂. But
in practice this has never been the true signification of the term.
In the manufacture of acid and superphosphates there is formed, more
or less of the dicalcium phosphate, either directly or after a time,
and this salt which, in no sense can be called reverted, is entirely
soluble in ammonium citrate. The iron and aluminum phosphates are
also, to a certain degree, soluble in the same reagent. When an acid
phosphate, containing various forms of calcium phosphate, is applied to
a soil containing iron and alumina, the soluble parts of the compound
tend to become fixed by union with those bases, or by precipitation as
Ca₂H₂(PO₄)₂. But it is not alone reverted phosphate formed in this way,
which the analyst is called on to determine in a fertilizer, although
he may have occasion to treat it in soil analysis.
The expression “reverted phosphoric acid,” therefore, in practice not
only includes a dicalcium phosphate, which once may have been the
monocalcium salt, but also all of that salt originally existing in
the superphosphate, and formed directly during its manufacture, as
well as any iron and aluminum phosphates present which are soluble in
ammonium citrate. The expression “citrate-soluble” is, therefore, to be
preferred to “reverted” phosphoric acid.
In the reversion of the phosphoric acid in superphosphates the iron
plays a far more important role than the aluminum sulfate. It was
formerly supposed that the reversion took place as indicated in the
following formula: 2CaH₄(PO₄)₂ + Fe₂O₃ = 2(CaHPO₄, FePO₄) + 3H₂O, while
Wagner affirms that the reverted acid compounds consist of varying
quantities of ferric oxid, aluminum oxid, phosphorus pentoxid, and
calcium oxid, in various states of combination.[117] The more probable
reaction is the following: 3CaH₄(PO₄)₂ + Fe₂(SO₄)₃ + 4H₂O = 2(FePO₄,
2H₃PO₄, 2H₂O) + 3CaSO₄. This reaction can be demonstrated by adding
to a superphosphate solution one of a ferric salt. In addition to
free phosphoric acid, iron phosphate is separated, which gradually
passes into an insoluble form by the abstraction of water due to the
crystallization of the gypsum. The alumina present in a superphosphate
seems to have no direct influence on the process of reversion. Its
phosphate salt is not acted on by the acid calcium phosphate. Even when
a superphosphate solution is treated with alum no precipitation is
produced, except on warming, and this disappears when the mass is again
cold.
It is therefore not necessary in the process of manufacture to separate
the alumina by digestion with a hot soda-lye before treating the mass
with sulfuric acid.
In order to avoid the reversion of the phosphoric acid several plans
have been proposed. One of the best is to use a little excess of
sulfuric acid in the manufacture. This tends to hold the phosphoric
acid in soluble form but is objectionable on account of drying,
handling, and shipping the fertilizer. During the digestion, moreover,
it is important that the temperature does not rise above 120°. Another
method consists in adding to the dissolved rock a quantity of common
salt chemically equivalent to its iron content. Ammonium sulfate also
helps to hold the phosphoric acid water-soluble.
=128. Influence of Movement.=—The influence of time and temperature
of digestion, and of variations in the composition of the ammonium
citrate on the quantity of phosphoric acid dissolved by that reagent
has been pointed out. Of great importance also in the process is
the character of the movement to which the materials are subjected
during the digestion. For this reason various mechanical devices have
been constructed to secure uniformity of solution. Inasmuch as the
temperature factor must also be faithfully observed, the best of these
devices are so arranged as to admit of a uniform motion within a bath
of water kept at the desired temperature which, by the Association
method, is 65°.
[Illustration: FIGURE 8. HUSTON’S DIGESTING APPARATUS.]
=129. Digestion Apparatus for Reverted Phosphates.=—The digestion
apparatus used by Huston consists of two wheels twenty-five centimeters
in diameter, mounted on the same axis, having a clear space of four and
one-half centimeters between them.[118] In the periphery of each wheel
are cut twelve notches, which are to receive the posts bearing the
rings through which the necks of the flasks pass. The posts are held in
place by nuts which are screwed down on the faces of the wheel. Should
it become necessary to take the apparatus apart, it is only necessary
to loosen the nuts and the set screw holding one wheel to the shaft and
all the parts can at once be removed. The posts extend ten centimeters
beyond the face of the wheels, and the rings are four centimeters in
internal diameter. Perforated plates, bearing a cross-bar, and held in
place by strong spiral springs attached to the plate and the base of
the posts, serve to hold the flasks in place. Each plate has a number
stenciled through it for convenience in identifying the flasks when it
is time to remove them. Attached to the outside of each post, close
to the outer end, is a heavy wire which passes entirely around the
apparatus, serving to keep the plates in place after they are removed
from the flasks.
The apparatus is mounted on a substantial framework, thirty-six
centimeters high and thirty centimeters wide at the base. The space
in which the wheel revolves is fourteen centimeters wide. The base
bars connecting the two sides are extended seven centimeters beyond
one side, and serve for the attachment of lateral bracing. At the top
of the framework, at one side, is attached a heavy bar forty-five
centimeters long, which serves to carry the cog gearing which transmits
the power. The upright shaft carries a cone pulley to provide for
varying the speed. The usual speed is two revolutions a minute for the
wheel carrying the flasks. The entire apparatus is made of brass. The
details of construction are shown in Fig. 8. Round-bottomed flasks are
used, and rubber stoppers are held in place by tying or by a special
clamp shown at the lower right-hand of the figure.
When high temperatures are used, the plates and flasks are handled by
the hooks shown at the left and right-hand upper corners of the figure.
When any other than room temperature is desired, the whole apparatus is
immersed in water contained in the large galvanized tank forming the
back-ground of the figures. The tank is seventy-five centimeters long,
seventy-five centimeters high, and thirty centimeters wide. At one end,
near the top, is an extension to provide space for heating the fluid in
the flasks before introducing the solid in such cases as may be desired.
The apparatus is held in place by angle irons soldered to the bottom
of the tank and a brace resting against the upright bar bearing the
gear-wheels.
The water in the tank is heated by injecting steam, or by burners under
the tank. As the tank holds about 300 pounds of water the work is not
subject to sudden changes of temperature, and little trouble has been
experienced in raising and maintaining the temperature of the water,
especially when steam is used.
An electric motor, or a small water-motor with only a very moderate
head of water, will furnish ample power.
=130. Comparison of Results.=—The following data show the results
obtained by the digester as compared with those furnished by the
official method, temperature and time of digestion being the same in
each instance.