Sulphuric acid may, if desired, be estimated in the filtrate, but in that
case care must be taken that the magnesia mixture used was free from it.
Mauricheau-Beaupre has elaborated a volumetric method for the estimation
of the phosphine in crude acetylene depending on its decomposition by a
known volume of excess of centinormal solution of iodine, addition of
excess of standard solution of sodium thiosulphate, and titrating back
with decinormal solution of iodine with a few drops of starch solution as
an indicator. One c.c. of centinormal solution of iodine is equivalent to
0.0035 c.c. of phosphine. This method of estimation is quickly carried
out and is sufficiently accurate for most technical purposes.
In carrying out these analytical operations many precautions have to be
taken with which the competent analyst is familiar, and they cannot be
given in detail in this work, which is primarily intended for ordinary
users of acetylene, and not for the guidance of analysts. It may,
however, be pointed out that many useful tests in connexion with
acetylene supply can be conducted by a trained analyst, which are not of
a character to be serviceable to the untrained experimentalist.
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