=86. Classification of Methods.=—The time required for a gravimetric
determination of phosphoric acid has led analysts to try the speedier
if less accurate processes, depending on the use of volumetric methods.
The chief difficulty with these methods has been in securing some
sharp method of distinguishing the end reaction. In most cases it has
been found necessary to remove a portion of the titrated solution and
prepare it for final testing by subsidence or filtration. As is well
known, this method of determining the end reaction is less accurate and
more time-consuming than those processes depending on a change of color
in the whole mass. All the volumetric processes now in general use
may be divided into two classes; _viz._, (1) the direct precipitation
of phosphoric acid and the determination of the end reaction by any
appropriate means, and (2) the previous separation of the phosphoric
acid, usually by means of a citro-magnesium or molybdenum mixture, and
in the latter case the subsequent titration of the yellow ammonium
phosphomolybdate either directly or after reduction to a lower form
of oxidation. In respect of extent of application by far the most
important volumetric method is the one depending on titration by
a uranium salt after previous separation by ammoniacal magnesium
citrate. A promising method after previous separation by molybdenum is
the one proposed by Pemberton, but it has not yet come into general
use. For small quantities of phosphoric acid or of phosphorus, such
as are found in steels and irons, the method of Emmerton, either as
originally proposed or as modified by Dudley and Noyes, is in frequent
use. Where volumetric methods are applied to products separated by
molybdic solution, the essential feature of the analytical work is to
secure a yellow precipitate of constant composition. If this could
be uniformly done such methods would rival the gravimetric processes
in accuracy. Hence it is highly important in these methods that the
yellow precipitate should be secured as far as possible, under constant
conditions of strength of solution, duration of time, and manner of
precipitation. In these cases, and in such only, can the quicker
volumetric methods be depended on for accurate results.
The direct volumetric precipitation of the phosphoric acid by a uranium
salt or otherwise is practiced only when the acid is combined with the
alkalies and when iron and alumina are absent and only small quantities
of lime present. This method has therefore but little practical value
for agricultural purposes. In all volumetric analyses the accuracy of
the burettes, pipettes, and other graduated vessels should be proved by
careful calibration. Many of the disagreements in laboratories where
the analytical work is conducted equally well can be due to no other
cause than the inaccuracy of the graduated vessels which are found in
commerce. Burettes should not only be calibrated for the whole volume
but for at least every five cubic centimeters of the graduation.