Found, Lost, Found, Lost,
Mg₂P₂O₇ milligrams Mg₂P₂O₇ milligrams
in grams. Mg₂P₂O₇. in grams. Mg₂P₂O₇.
0.10 0.6 0.24 4.0
0.12 0.8 0.25 4.2
0.14 1.2 0.26 4.6
0.15 1.4 0.27 5.0
0.16 1.6 0.28 5.5
0.17 2.4 0.29 6.1
0.18 2.6 0.30 6.8
0.19 3.2 0.31 7.6
0.20 3.5 0.32 8.6
0.21 3.6 0.33 9.6
0.22 3.8 0.34 10.6
When phosphoric acid is to be estimated as pyrophosphate it must always
be first separated as molybdate, even when the original solution
contains no bases capable of forming insoluble phosphates, as otherwise
these corrections will not be applicable.
Using these corrections the estimation of phosphoric acid becomes one
of the most accurate of known analytical methods.
=61. The Color of the Magnesium Pyrophosphate.=—After the final
ignition of the magnesium pyrophosphate, whether secured by the citrate
or the molybdic method, a black or grayish tint is often noticed. This
may be due to traces of organic matter brought down by the precipitate
and especially to a lack of care in the initial ignition. Many devices
have been proposed for the purpose of avoiding this coloration,
although general experiments have shown that there is no appreciable
increase in the weight of the precipitate when colored in this way.
When the precipitation is carried on according to the citrate method,
Neubauer[47] proposes to eliminate this coloration by the use of
ammonium sulfate. About seven cubic centimeters of a saturated
solution of ammonium sulfate should be added to the solution before
the precipitation by the magnesium mixture. With this precaution it is
possible to obtain a perfectly white precipitate after five minutes of
ignition. The lively glowing of the precipitate throughout the whole
mass at the time of changing into pyrophosphate, is much more easily
observed by this treatment than when the mass is gray or black. Even
should the addition of the ammonium sulfate solution to one containing
a large amount of lime produce a precipitate of crystalline calcium
sulfate, it is of no importance inasmuch as the ammonium citrate
immediately dissolves large quantities of the calcium salt.
In this laboratory a white pyrophosphate is easily obtained by treating
the precipitate on the gooch after washing free of chlorids with a drop
or two of ammonium nitrate. The ignition is commenced very gently at
first and afterwards when the mass is white the blast is used.
If the ignited residue be gray it may sometimes be whitened by
moistening with a drop or two of nitric acid, burning at a very low
temperature, followed by the blast. There is no appreciable difference
in weight between a gray and white pyrophosphate.
=62. Determination of Phosphoric Acid and Nitrogen in the Same Solution
by Treatment with Sulfuric Acid and Mercury.=—Fertilizing materials
which contain organic nitrogen and phosphoric acid, such as bones, are
of such a nature that it is often difficult to obtain a fair sample
of them in quantities suited to the direct determination; _viz._,
about one gram. Thus it often becomes important to take a much larger
quantity of the material, to bring it into solution and to take an
aliquot part thereof. It may also often happen that it is important to
determine the phosphoric acid in the same sample which has been used
for the determination of the nitrogen by moist combustion with sulfuric
acid and mercury. In this connection, however, it is somewhat difficult
to avoid the precipitation of some of the mercury with the phosphoric
acid.
The mercuric sulfate which is produced by the Kjeldahl method is
not precipitated in the presence of ammoniacal solution of ammonium
citrate, but there may be small quantities of mercurous salts present
or some finely divided metallic mercury which may contaminate
mechanically the phosphate precipitate. These disturbing influences may
be removed by previous treatment with sodium chlorid. If from fifty
to sixty cubic centimeters of sulfuric acid have been used for the
solution and oxidation and this be made up to half a liter, it will be
sufficiently dilute to permit an almost quantitative separation of the
mercurous chlorid produced by treatment with sodium chlorid.
Neubauer, who has proposed this method, finds that when sodium chlorid
is used previous to the precipitation of the phosphoric acid, a
precipitate of ordinary size contains, at most, only one milligram
of mercury, while without the use of sodium chlorid as much as four
milligrams may be found. The details of the method employed by Neubauer
are as follows:
Ten grams of the fertilizing material are placed in a half liter flask
with from fifty to sixty cubic centimeters of strong sulfuric acid,
two grams of mercury, and a little paraffin to prevent foaming. The
oxidation is carried on as usual in the Kjeldahl method. The liquid,
after cooling, is diluted with water and one cubic centimeter of a
citrate solution of sodium chlorid added, cooled, filled to the mark,
filtered, and fifty cubic centimeters taken for the determination of
the phosphoric acid, according to the citrate method and the same
quantity for the determination of the ammonia by distillation.