The chief objection to this method of Wagner lies in the failure to
control the temperature at which the digestion with citrate solution
is made. Huston has shown, as will be described further on, that the
temperature exercises a great influence in digestion with citrate.
Since the laboratory temperature, especially in this country, may vary
between 10° and 35°, it is evident that on the same sample the Wagner
method would give very discordant results at different seasons of the
year.
=82. Solutions Employed in the Wagner Method.=—1. _Ammonium
Citrate._—In one liter there should be exactly 150 grams of citric
acid and 27.93 grams of ammonia, equivalent to twenty-three grams of
nitrogen. The following example illustrates the preparation of ten
liters of the solution: In two liters of water and three and a half
liters of eight per cent ammonia, 1,500 grams of citric acid are
dissolved and the cooled solution made up exactly to eight liters.
Dilute twenty-five cubic centimeters of this solution to 250 cubic
centimeters and treat twenty-five cubic centimeters of this with three
grams of calcined magnesia and distill into forty cubic centimeters
of half normal sulfuric acid. Suppose the ammonia nitrogen found
correspond to twenty cubic centimeters of fourth normal soda-lye. Then
in the eight liters are contained
(20.0 × 0.0035 × 8000)
---------------------- = 224 grams
2.5
of ammonia nitrogen. Then in order to secure in the ten liters the
proper quantity of ammonia there must be added two liters of water
containing 230 - 224 = six grams of nitrogen or seven and three-tenths
grams ammonia; _viz._, ninety-four cubic centimeters of 0.967 specific
gravity.
2. _Molybdate Solution._—Dissolve 125 grams of molybdic acid in dilute
two and five-tenths per cent ammonia, avoiding a large excess of the
solvent. Add 400 grams of ammonium nitrate, dilute with water to one
liter and pour the solution into one liter of nitric acid of 1.19
specific gravity. Allow the preparation to stand for twenty-four hours
at 35° and filter.
3. _Magnesia Mixture._—Dissolve 110 grams of pure crystallized
magnesium chlorid and 140 grams of ammonium chlorid in 700 cubic
centimeters of eight per cent ammonia and 130 cubic centimeters of
water. Allow to stand several days and filter.
=83. Estimation of Lime.=—When the lime is to be determined in basic
slags some difficulty may be experienced by reason of danger of
contamination of the oxalate precipitate with iron and especially
manganese, which is often present in slags.
Holleman[68] proposes to estimate the lime in basic slag by a
modification of the methods of Classen and Jones. The manipulation is
as follows: Fifty cubic centimeters of the solution of slag, equivalent
to one gram of substance, are evaporated to a small volume, twenty
cubic centimeters of neutral ammonium oxalate solution (one to three)
added to the residue and heated on a water-bath with frequent stirring,
until the precipitate is pure white and free from lumps. The time
required is usually about ten minutes. The precipitate is collected
on a filter and washed with hot water until the filtrate contains no
oxalic acid. The precipitated calcium oxalate must be snow-white. The
filter is broken and the calcium oxalate washed through, first with
water and finally with warm, dilute hydrochloric acid (one to one).
The calcium oxalate is dissolved by adding fifteen cubic centimeters
of concentrated hydrochloric acid, the solution evaporated to a volume
of about twenty-five cubic centimeters and ten cubic centimeters of
dilute sulfuric acid (one to five), and 150 cubic centimeters of
ninety-six per cent alcohol added. After standing three hours or more
the precipitate is separated by filtration and washed with ninety-six
per cent alcohol until the washings show no acid reaction with methyl
orange. The calcium sulfate precipitated is dried to constant weight.
This method gives a pure precipitate of calcium sulfate, containing
only traces of manganese.
=84. Estimation of Caustic Lime.=—The lime mechanically present in
basic slags is likely to be found as oxid or hydroxid, especially when
the sample is of recent manufacture. In the form of oxid the lime may
be determined by solution in sugar. In this process one gram of the
fine slag meal is shaken for some time with a solution of sugar, as
suggested by Stone and Scheuch.[69] The dissolved lime is separated
as oxalate by treatment of the solution with the ammonium salt. The
calcium oxalate may be determined by ignition in the usual way or
volumetrically by solution in sulfuric acid and titration of the
free oxalic acid with potassium permanganate solutions. The standard
solution of permanganate should be of such a strength as to have one
cubic centimeter equivalent to about 0.01 gram of iron. The iron value
of the permanganate used multiplied by 0.5 will give the quantity of
calcium oxid found.
=85. Detection of Adulteration of Phosphatic Slags.=—The high
agricultural value of phosphatic slags has led to their adulteration
and even to the substitution of other bodies. Several patents have also
been granted for the manufacture of artificial slags of a value said to
be an approximation to that of the by-products of the basic pig iron
process.
(1) _Method of Blum._—One of the earliest methods of examining
basic slag for adulterations is the method of Blum.[70] This method
rests upon the principle of the determination of the carbon dioxid
in the sample. The basic phosphatic slag is supposed to contain no
carbon dioxid. This is true only in case it is freshly prepared. The
tetrabasic phosphate, after being kept for some time, gradually absorbs
carbon dioxid from the air. As high as nineteen per cent of carbon
dioxid have been found in slags which have been kept for a long while.
When the slag has absorbed so much of carbon dioxid and water from the
air as to be no longer profitable for market, it can be restored to its
original condition by ignition.
(2) _Method of Richter-Forster._—One of the common adulterants of
tetrabasic phosphate is aluminum phosphate. The method of detecting
this when mixed with the slag is described by Richter-Forster.[71]
The method depends on the fact that soda-lye dissolves the aluminum
phosphate, although it does not dissolve any calcium phosphoric acid
from the slag. Two grams of the sample to be tested are treated with
ten cubic centimeters of soda-lye of from 7° to 8° C. in a small vessel
with frequent shaking for a few hours at room temperature. After
filtration the filtrate is made acid with hydrochloric and afterwards
slightly alkaline with ammonia. With pure basic slag there is a small
trace of precipitate produced, but this is due to a little silica which
can be dissolved in a slight excess of acetic acid. If, however, the
basic slag contain aluminum phosphate, a dense jelly-like precipitate
of aluminum phosphate is produced.
(3) _Method of Jensch._—Edmund Jensch[72] determines the tetrabasic
phosphate in slags by solution in organic acids, and prefers citric
acid for this purpose. This method was also recommended by Blum[73].
It is well known that the tetrabasic phosphate in slags is completely
soluble in citric acid while the tribasic phosphate is only slightly,
if at all, attacked. The neutral ammonium salts of organic acids do
not at first attack the tribasic phosphate at all, and they do not
completely dissolve the tetrabasic phosphate. The solution used by
Jensch is made as follows: Fifty grams of crystallized citric acid are
dissolved in one liter of water. A weaker acid dissolves the tetrabasic
phosphate too slowly and a stronger one attacks the tribasic phosphate
present.
Schucht recommends the following method of procedure:[74] One gram
of the slag, finely ground, is treated in a beaker glass with about
150 cubic centimeters of Jensen’s citric acid solution and warmed for
twelve hours in an air-bath at from 50° to 70° with frequent shaking.
Afterwards it is diluted with 100 cubic centimeters of water, boiled
for one minute and filtered. The filter is washed thoroughly with hot
water and the phosphoric acid is estimated in the filtrate in the usual
way. With artificial mixtures of basic slags and other phosphates the
quantity of basic slag can be determined by the above method.
(4) _Method of Wrampelmeyer._—According to Wrampelmeyer the most
convenient method for discovering the adulteration of basic slag is the
use of the microscope.[75] All finely ground natural phosphates are
light colored and with a strong magnification appear as rounded masses.
In basic slags the particles are mostly black but there are often found
red-colored fragments having sharp angles which refract their light
in a peculiar way so that, with a very little experience, they can be
recognized as being distinctive marks of pure basic slag.
In artificial mixtures of these two phosphates, which we have made in
our laboratory, we have been able to detect with certainty as little as
one per cent of added mineral phosphate.
One form of adulterating natural mineral phosphates has been mixing
them with finely pulverized charcoal or soot to give them the black
appearance characteristic of the basic slags. This form of adulteration
is at once disclosed by simple ignition or by microscopic examination.
(5) _Loss on Ignition._—If all doubts cannot be removed by the use
of the microscope, the loss on ignition should be estimated. Natural
phosphates all give a high loss on ignition, ranging from eight to
twenty-four per cent, while a basic slag gives only a very slight loss
on ignition, especially when fresh. A basic slag which has stood for a
long while and absorbed carbon dioxid and moisture, may give a loss on
ignition approximating, in a maximum case, the minimum loss on ignition
from a natural phosphate.
In experiments made in this laboratory in testing for loss on ignition,
we have uniformly found that natural mineral phosphates will lose from
nearly one to two and one-half times as much on ignition as a basic
slag which has been kept for two years. A basic slag in the laboratory
more than two years old gave, as loss on ignition, 4.12 per cent.
Several samples of finely ground Florida phosphates gave the following
percentages of loss on ignition, as compared with a sample of slag.
Odorless phosphate 4.12.
Florida phosphates 8.06, 6.90, 9.58, 6.40, 10.38, and 10.67
respectively.
There are some mineral phosphates, however, which are ignited before
being sent to the market. We have one such sample in our laboratory
from Florida which gave, on ignition, a loss of only one and
four-tenths per cent. In this case it is seen that the application of
the process of ignition would not discriminate between a basic slag and
a mineral phosphate.
It may often be of interest to know what part of the loss, on ignition,
is due to water in form of moisture. In such cases the sample should
first be dried to constant weight in a steam-bath and then ignited. In
the following data are found the results obtained here with samples
treated as above indicated and also ignited directly. Number one is a
basic slag two years old and the others Florida phosphates.
Heated to 100° C. then ignited. Ignited directly.
Loss at Loss on Total Loss on
100° C. ignition. loss. ignition.
No. 1 (Slag) 2.57 1.77 4.34 4.12
No. 2 (Rock) 2.61 5.19 7.80 8.06
No. 3 “ 1.09 5.77 6.86 6.90
No. 4 “ 0.42 9.20 9.62 9.58
No. 5 “ 1.81 4.83 6.64 6.40
No. 6 “ 4.36 6.52 10.88 10.83
No. 7 “ 3.31 7.01 10.32 10.67
(6) _Presence of Sulfids._—Another point noticed in this laboratory is
that the basic slags uniformly contain sulfids which are decomposed
upon the addition of an acid with an evolution of hydrogen sulfid.
(7) _Presence of Fluorin._—In applying the test for fluorin, it has
been uniformly found here that the mineral phosphates respond to the
fluorin test while the basic slags, on the contrary, respond to the
hydrogen sulfid test. This test, however, was applied only to the few
samples we have had and may not be a uniform property.
The absence of fluorin might not prove the absence of adulteration,
but its presence would, I believe, certainly prove the fact of the
adulteration in that particular sample.
The fluorin test is applied by Böttcher in the following manner:[76]
From ten to fifteen grams of the slag are placed in a beaker ten
centimeters high and from five to six centimeters in diameter, with
fifteen cubic centimeters of concentrated sulfuric acid, stirred with a
glass rod, and covered with a watch-glass on the under side of which a
drop of water hangs. If there be formed upon the drop of water a white
murky rim, it is proof that a mineral phosphate containing fluorin has
been added. After from five to ten minutes you can notice on the clean
watch-glass the etching produced by the hydrofluoric acid. According to
Böttcher an adulteration of ten per cent of raw phosphate in slag can
be detected by this method.
(8) _Solubility in Water._—Solubility in water is also a good
indication, natural phosphates being totally insoluble in water, while
a considerable quantity of the basic slag will be dissolved in water on
account of the calcium oxid or hydroxid which it contains. If the loss
on ignition is low, and the volume-weight and water-solubility high,
the analyst may be certain that the sample is a pure slag.
In comparative tests made in our laboratory with a sample of basic slag
and seven samples of Florida phosphate, the percentages of material
dissolved by water and by a five per cent solution of citric acid were
found to be as follows:
Sol. in five per cent
Water-soluble. citric acid.
Per cent. Per cent.
Odorless phosphate 0.97 16.10
Florida phosphate 0.01 4.15
“ “ 0.09 4.66
“ “ 0.02 3.43
“ “ 0.08 3.61
“ “ 0.02 3.79
“ “ 0.05 4.46
“ “ 0.02 4.24
From the above data it is seen that the solvent action of water
especially would be of value inasmuch as it dissolves only a mere trace
of the mineral phosphates, approximating one per cent of the amount
dissolved from basic slag. In the case of the citric acid it is found
that the amount of materials soluble in this solvent for basic slag
is fully four times as great as for the mineral phosphates. Both of
these processes, therefore, have considerable value for discriminating
between the pure and adulterated article of basic slag.
(9) _Specific Gravity._—The estimation of the volume specific gravity
is also a good indication for judging of the purity of the slag. This
is best done by weighing directly a given volume. Basic slag will have
a volume-weight of about one and nine-tenths, while natural phosphates
will have about one and six-tenths.
(10) _Conclusions._—From the above résumé of the standard methods which
are in use for determining the adulteration of basic slag, it is seen
that there are many cases in which grave doubt might exist even after
the careful application of all the methods mentioned. If we had only
to consider the adulteration of basic slag with certain of the mineral
phosphates, that is, tricalcium phosphate, the problem would be an easy
one, but when we add to this the fact that iron and aluminum phosphates
are employed in the adulteration, and that artificial slags may be so
used, the question becomes more involved.
In doubtful cases one after another of the methods should be applied
until there is no doubt whatever of the judgment which should be
rendered.