-------------------------------------------------------------------
Time Total Removed Removed
of phos. by official by
Substance. treatment. acid. method. digester.
Per cent. Per cent. Per cent.
-------------------------------------------------------------------
½ hour 27.67 10.59 14.52
1 “ 12.21 14.82
2 hours 14.61 17.56
Steamed bone, 3½ “ 16.48 18.53
5 “ 17.94 20.22
7½ “ 18.99 20.25
10 “ 19.73 21.18
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½ hour 13.86 4.43 4.11
1 “ 8.28 6.82
2 hours 10.34 9.76
Marl, 3½ “ 11.00 11.31
5 “ 11.80 11.83
8 “ 12.51 12.64
10 “ 12.58 13.00
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½ hour 19.38 12.09 12.28
1 “ 12.47 12.40
Acidulated bone, 2 hours 12.20 12.43
3½ “ 12.40 12.24
5 “ 12.43 12.26
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Bone, ½ hour 21.40 6.97 8.48
Ammoniated dissolved bone, ½ “ 18.22 9.28 10.63
Cottonseed-meal and
castor pomace, ½ “ 2.52 0.23 0.25
Phospho bone, ¼ “ 16.55 7.06 7.21
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In comparing duplicates, the results from the use of the digester are
found to be subject to less variation than those from the usual method.
=131. Huston’s Mechanical Stirrer.=—The stirring apparatus shown in
Fig. 9 differs from those which have heretofore come into use, in
requiring but a single belt to drive all the stirring rods, and in
having all the parts protected from the laboratory fumes.[119] The
details of the belt system are shown in the small diagram in the lower
central part of the figure. The apparatus is mounted on a substantial
wooden box, 200 centimeters long, thirty centimeters high, and eighteen
centimeters wide. The driving pulleys, ten centimeters in diameter,
are enclosed in the upper part of the case. The shafts on which these
pulleys are mounted extend through the bottom of the enclosing box
and carry a wooden disk, eleven centimeters in diameter, to prevent
particles of foreign matter from falling into the beakers. The shafts
extend two centimeters below these disks, and to the end of the shafts
the bent stirring rods are attached by rubber tubing.
The board forming the support of the driving pulleys is extended two
centimeters in front of the apparatus, and in this extension twelve
notches are cut, in which are held the corks carrying the tubes which
contain the solution to be used in precipitating the material in the
beakers.
[Illustration: FIGURE 9. HUSTON’S MECHANICAL STIRRER.]
The ends of these tubes are drawn out to a fine point so as to deliver
the liquid at the rate of about one drop per second.
The front of the apparatus is hinged and permits the whole to be closed
when not in use, or during the precipitation.
The apparatus has proven extremely satisfactory in the precipitation of
ammonium magnesium phosphate. The precipitate is very crystalline, and
where the stirring is continued for some minutes, after the magnesia
solution has all been added, no amorphous precipitate is observed on
longer standing.
=132. The Citrate Method Applied to Samples with Small Content of
Phosphoric Acid.=—It is well established that the citrate method does
not give satisfactory results when applied to samples containing small
percentages of phosphoric acid, especially when these are of an organic
nature, as for instance, cottonseed cake-meal. In this laboratory
attempts have been made to remedy this defect in the process so as
to render the use of the method possible even in such cases.[120]
Satisfactory results have been obtained by adding to the solution
of the cake-meal a definite volume of a phosphate solution of known
strength. Solutions of ordinary mineral phosphates are preferred for
this purpose. The following example will show the application of the
modified method:
In a sample of cake-meal, (cottonseed cake and castor pomace) the
content of phosphoric acid obtained by the molybdate method, was 2.52
per cent.
Determined directly by the citrate method, the following data were
obtained:
Allowing to stand thirty hours after adding magnesia mixture, 1.08 and
1.53 per cent in duplicates.
Allowing to stand seventy-two hours after adding magnesia mixture, 2.17
and 2.30 per cent in duplicates.
In each case fifty cubic centimeters of the solution were taken,
representing half a gram of the sample.
In another series of determinations twenty-five cubic centimeters of
the sample were mixed with an equal volume of a mineral phosphate
solution, the value of which had been previously determined by both
the molybdic and citrate methods. The fifty cubic centimeters thus
obtained represented a quarter of a gram each of the cake-meal and
mineral phosphates. The filtration followed eighteen hours after adding
the magnesia mixture. The following data show the results of the
determinations:
Per cent Per cent Per cent Per cent
P₂O₅ in P₂O₅ in P₂O₅ P₂O₅ in
mineral organic found in organic
phosphate. sample. mixture × 2. sample.
1 15.37 2.52 17.90 2.53
2 29.16 2.52 31.68 2.52
3 31.37 2.52 33.83 2.45
4 31.58 2.52 34.20 2.62
----
Mean content of P₂O₅ in organic sample 2.53
It is thus demonstrated that the citrate method can be applied with
safety even to the determination of the phosphoric acid in organic
compounds where the quantity present is less than three per cent. It is
further shown that solutions of mineral phosphates varying in content
of phosphoric acid from fifteen to thirty-two per cent may be safely
used for increasing the content of that acid to the proper degree for
complete precipitation. In cases where organic matters are present they
should be destroyed by moist combustion with sulfuric acid as in the
determination of nitrogen to be described in the next part.
=133. Direct Precipitation of the Citrate-Soluble Phosphoric Acid.=—The
direct determination of citrate-soluble phosphoric acid by effecting
the precipitation by means of magnesia mixture in the solution obtained
from the ammonium citrate digestion, has been practiced for many years
by numbers of European chemists, and the process has even obtained
a place in the official methods of some European countries. Various
objections have been urged, however, against the general employment of
this method in fertilizer analysis on account of the inaccuracies in
the results obtained in certain cases, and it has, therefore, been used
to but a very limited extent in this country. Since it is impracticable
to effect the precipitation with ammonium molybdate in the presence of
citric acid the previous elimination or destruction of this substance
has been recognized as essential to the execution of a process
involving the separation of the phosphoric acid as phosphomolybdate.
It is evident from the data cited in the preceding paragraph, that
great accuracy may be secured in this process by adding a sufficient
quantity of a solution of a mineral phosphate and proceeding by the
citrate method.
Ross has also proposed to estimate the acid soluble in ammonium citrate
directly by first destroying the organic matter by moist combustion
with sulfuric acid.[121] He recommends the following process:
After completion of the thirty minutes’ digestion of the sample with
citrate solution, twenty-five cubic centimeters are filtered at once
into a dry vessel. If the liquid be filtered directly into a dry
burette, twenty-five cubic centimeters can be readily transferred
to another vessel without dilution. After cooling, run twenty-five
cubic centimeters of the solution into a digestion flask of 250-300
cubic centimeters capacity, add about fifteen cubic centimeters of
concentrated sulfuric acid and place the flask on a piece of wire gauze
over a moderately brisk flame; in about eight minutes the contents of
the flask commence to darken and foaming begins, but this will occasion
no trouble, if an extremely high, or a very low flame be avoided. In
about twelve minutes the foaming ceases and the liquid in the flask
appears quite black; about one grain of mercuric oxid is now added
and the digestion is continued over a brisk flame. The operation can
be completed in less than half an hour with ease, and in many cases,
twenty-five minutes. After cooling, the contents of the flask are
washed into a beaker, ammonia is added in slight excess, the solution
is acidified with nitric, and after the addition of fifteen grams of
ammonium nitrate, the process is conducted as usual.
In case as large an aliquot as fifty cubic centimeters of the original
filtrate be used, ten cubic centimeters of sulfuric acid are added,
and the digestion is conducted in a flask of 300-500 cubic centimeters
capacity; after the liquid has blackened and foaming has progressed to
a considerable extent, the flask is removed from the flame, fifteen
cubic centimeters more of sulfuric acid are added, and the flask and
contents are heated at a moderate temperature for two or three minutes;
the mercuric oxid is then added and the operation completed as before
described.
Following are some of the advantages offered by the method described:
(1) It dispenses with the necessity of the execution of the frequently
tedious operation of bringing upon the filter and washing the residue
from the ammonium citrate digestion, while the ignition of this residue
together with the subsequent digestion with acid and filtration are
also avoided.
(2) It affords a means for the direct estimation of that form of
phosphoric acid which, together with the water-soluble, constitutes the
available phosphoric acid, thus enabling the latter to be determined by
making only two estimations.
(3) In connection with the advantages above mentioned it permits
of a considerable saving of time, as well as of labor required in
manipulation.
In addition to the tests with mercuric oxid, both potassium nitrate and
potassium sulfate were used in the digestion to facilitate oxidation.
With the former, several additions of the salt were necessary to secure
a satisfactory digestion, and even then the time required was longer
than with the mercury or mercuric oxid digestion. With potassium
sulfate, the excessive foaming which took place interfered greatly with
the execution of the digestion process.
=134. Availability of Phosphatic Fertilizers.=—There is perhaps no one
question more frequently put to analysts by practical farmers than
the one relating to the availability of fertilizing materials. The
object of the manufacturer should be to secure each of the valuable
ingredients of his goods in the most useful form. The ideal form in
which phosphoric acid should come to the soil is one soluble in water.
Even in localities where heavy rains may abound, there is not much
danger of loss of soluble acid by percolation. As has before been
indicated, the soluble acid tends to become fixed in all normal soils,
and to remain in a state accessible to the rootlets of plants, and yet
free from danger of leaching. For this reason, by most agronomists, the
water-soluble acid is not regarded as more available than that portion
insoluble in water, yet soluble in ammonium citrate.
In many of the States the statutes, or custom, prescribe that only the
water and citrate-soluble acid shall be reckoned as available, the
insoluble residue being allowed no place in the estimates of value.
In many instances such a custom may lead to considerable error, as
in the case of finely ground bones and some forms of soft and easily
decomposable tricalcium phosphates. There are also, on the markets,
phosphates composed largely of iron and aluminum salts, and these
appear to have an available value often in excess of the quantities
thereof soluble in ammonium citrate.
As a rule the apatites, when reduced to a fine powder and applied to
the soil, are the least available of the natural phosphates. Next in
order come the land rock and pebble phosphates which, in most soils,
have only a limited availability. The soft fine-ground phosphates,
especially in soils rich in humus, have an agricultural value,
almost, if not quite equal to a similar amount of acid in the acid
phosphates. Fine-ground bones also tend to give up their phosphoric
acid with a considerable degree of readiness in most soils. Natural
iron and aluminum phosphates, have also, as a rule, a high degree of
availability. In each case the analyst must consider all the factors of
the case before rendering a decision. Not only the relative solubility
of the different components of the offered fertilizer in different
menstrua must be taken into consideration, but also the character of
the soil to which it is to be applied, the time of application, and the
crop to be grown. By a diligent study of these conditions the analyst
may, in the end, reach an accurate judgment of the merits of the sample.
=135. Direct Weighing of the Molybdenum Precipitate.=—It has already
been stated that many attempts have, been made to determine the
phosphoric acid by direct weighing as well as by titration, as in
the Pemberton method. The point of prime importance in such a direct
determination is to secure an ammonium phosphomolybdate mixture of
constant composition. Unless this can be done no direct method, either
volumetric or gravimetric, can give reliable results. Hanamann[122]
proposes to secure this constant composition by varying somewhat the
composition of the molybdate mixture and precipitating the phosphoric
acid under definite conditions. The molybdate solution employed is
prepared as follows:
Molybdic acid 100 grams.
Ten per cent ammonia 1.0 liter.
Nitric acid (1.246 sp. gr.) 1.5 liters.
The precipitation of the phosphoric acid is conducted in the cold
with constant stirring. It is complete in half an hour. The ammonium
phosphomolybdate is washed with a solution of ammonium nitrate and then
with dilute nitric acid, dried, and ignited at less than a red heat. It
should then have a bluish-black color throughout. Such a body contains
4.018 per cent of phosphoric anhydrid.
Twenty-five cubic centimeters of a sodium phosphate solution
containing fifty milligrams of phosphoric acid, treated as above, gave
a bluish-black precipitate weighing 1.249 grams, which, multiplied
by 0.041018, equaled 50.018 milligrams of phosphorus pentoxid. The
method should be tried on phosphates of various kinds and contents of
phosphorus pentoxid before a definite judgment of its merits is formed.