_Reduction Apparatus._—The reduction of the molybdic acid to molybdenum
trioxid is accomplished in a tube first proposed by Jones. The
apparatus is shown in Figure 7. A piece of moderately heavy glass
tubing thirty-five centimeters long with an internal diameter of
two centimeters is drawn out at the lower end so as to pass into
the stopper of a flask. A circular piece of perforated platinum or
porcelain rests on the constricted portion of the tube and this is
covered with an asbestos felt. The tube is then nearly filled with
powdered zinc which is washed, before using, with dilute sulfuric acid
(1: 20). A, B, C represent different methods of filtering the molybdic
solution. In A a platinum cone is placed in the constricted portion of
the tube and the asbestos felt placed thereon and the tube then filled
with the granulated zinc. In B there is first inserted a perforated
disk then some very fine sand and this is covered with another disk. In
C there is a perforated disk which is covered with asbestos felt. The
filtering arrangement should be such as to prevent any zinc particles
from reaching the flask and yet permitting the filtration to go on
without much difficulty. A blank determination is first made by adding
to 180 cubic centimeters of water, twelve of 0.96 ammonia and ten of
strong sulfuric acid. This is poured through the reducing tube and
followed with 200 cubic centimeters of water taking care that no air
enter the apparatus. Hydrogen peroxid is formed if air enter. Even
after standing for a few moments the tube should be washed with dilute
sulfuric acid before again using it. The filtrate should be titrated
with the permanganate solution and the amount required deducted from
the following amounts obtained with the molybdic salt.
_Calculations._—The calculations of the amount of phosphorus in a given
sample of iron or steel are made according to the following data: In
a given case let it be supposed that the permanganate solution is set
with a solution of piano wire and it is found that one cubic centimeter
of permanganate liquor is equal to 0.003466 gram of metallic iron. It
is found that 90.76 parts of molybdic acid will produce the same effect
on permanganate as 100 parts of iron. Hence one cubic centimeter of
permanganate solution is equivalent to 0.003466 × 0.9076 = 0.003145
gram of molybdic acid. In the yellow precipitate formed, in the
conditions named for the analysis it is found that the phosphorus is
one and nine-tenths per cent of the molybdic acid present. Therefore
one cubic centimeter of permanganate liquor is equal to 0.003145 ×
0.019 = 0.0000597 gram of phosphorus. If then, for example, in a sample
of iron or steel eight and six-tenths cubic centimeters of permanganate
solution, after correction, be found necessary to oxidize the molybdic
solution after passing through the Jones’ reducing tube, the amount of
phosphorus found is 0.0000597 × 8.6 = 0.051 per cent.
=113. The Silver Method.=—The separation of the phosphoric acid by
silver according to the method of Perrot has been investigated by
Spencer, who found the process unreliable.[96] By a modification of the
process, however, Spencer obtained fairly satisfactory results. The
principle of this method depends on the separation of the phosphoric
acid by silver carbonate and the subsequent titration thereof with
standard uranium solution after the removal of the excess of silver.
The operation is conducted as follows: The fertilizer is first ignited
until all organic matter and residual carbon are destroyed. Solution
is then accomplished by means of nitric acid and the volume completed
to a definite quantity. An aliquot part is taken, after filtration,
varying with the supposed strength of the solution so as to contain
about 100 milligrams of phosphorus pentoxid. In the slightly nitric
acid solution add freshly prepared silver carbonate in excess, that
is, sufficient to saturate any free acid present and also to combine
with all the phosphoric acid. Wash thoroughly with hot water and then
dissolve the mixed phosphate and silver carbonate in nitric acid and
remove the silver from the solution with sodium chlorid. The phosphoric
acid is determined in the filtrate by means of a standard solution of
uranium nitrate in the manner already described. Spencer found that the
separation of the phosphoric acid by the silver method was more exact
than by the Joulie magnesium citrate process. With practice on the part
of the analyst in determining the end reaction the process is both
rapid and accurate. The method is also inexpensive, as both the silver
and uranium are easily recovered from the waste.
=114. Volumetric Silver Method.=—Holleman has proposed a modification
of the silver method for the volumetric determination of phosphoric
acid, which is conducted in the following manner:[97]
In a flask of 200 cubic centimeters capacity, are placed fifty cubic
centimeters of the liquid to be analyzed, which should not contain
more than two-tenths gram of phosphoric acid. The solution is treated
with ten cubic centimeters of a normal solution of sodium acetate and
afterwards with a slight excess of decinormal silver solution, four
and five-tenths cubic centimeters for each 0.01 gram of phosphoric
acid. The solution is then neutralized with tenth normal sodium
hydroxid, the amount required having been previously determined by
titrating ten cubic centimeters of the liquid to be analyzed, using
phenolphthalein as an indicator. Five times the quantity required for
the neutralization of the ten cubic centimeters is added, less one-half
cubic centimeter. By this treatment the phosphoric acid in the presence
of sodium acetate is completely precipitated as silver phosphate. The
excess of silver is determined by diluting the mixture to 200 cubic
centimeters, filtering, and titrating 100 cubic centimeters of the
filtrate with ammonium thiocyanate. The presence of sulfuric and nitric
acids does not interfere with the reaction, but of course hydrochloric
acid must be absent. Alkalies and alkaline earth metals may be present,
but not the heavy metals.
When iron and aluminum are present 100 cubic centimeters of the
solution are precipitated with thirty cubic centimeters of normal
sodium acetate, the phosphoric acid is determined in fifty cubic
centimeters of the filtrate, and the precipitate of iron and aluminum
phosphates is ignited and weighed, and its weight multiplied by 2.225
is added to the phosphoric anhydrid found volumetrically. If ammonia
be present it must be removed by boiling, as otherwise it affects the
titration with phenolphthalein.
For agricultural purposes this method can have but little value
inasmuch as the phosphates to be examined almost always have a certain
proportion of iron and aluminum. Inasmuch as the amount of these bases
has to be determined gravimetrically, there would be no gain in time
and no simplification of the processes by the use of the volumetric
method as proposed.