244 Analysis of Mineral Constituents in Biological Materials
While the above recommendation was in circulation, we had already under-
taken an investigation to determine which of the methods in the literature
were suitable for our requirements. We have now completed a comparative
study of the analysis of agricultural food-stuffs by several methods in use in
various places, including those given in the above-mentioned bulletins, and we
have come to the conclusion that a number of modifications are necessary
before we can get good results under our local conditions. The object of this
communication is to give an account of this investigation and suggest a scheme
for the analysis of some of the commoner mineral constituents of feeding
stuffs. It may be stated that these methods are not in any sense new
but we have introduced modifications to some of the existing methods in
order to increase the accuracy of the results.
PRELIMINARY CONSIDERATIONS
After the material has been properly sampled, the first step in the mineral
analysis of a feeding stuff is the ashing of the material in order to destroy the
organic matter. Although this appears to be a simple procedure, a large
amount of work has been carried out in various laboratories to determine the
best method of ashing without any appreciable loss of the mineral to be esti-
mated. For example, decarbonising at a high temperature leads to a loss of
chlorine and some lime is therefore usually added to the material before ashing
in order to fix the chlorine. Ashton [1936] found that at 800°C losses of phos-
phates occur in the absence of magnesium nitrate. In the case of grass, the
material can be ashed without loss of phosphoric acid if the temperature is kept
below 600°C. In the Rowett method, calcium acetate is used as the fixing
material for phosphate. Husband [1937] recommends the use of calcium car-
bonate as a fixing material for both phosphoric acid and chlorine. In view of
these observations, we have carried out a fairly large number of experiments to
determine whether a fixing agent should be used, or whether the material may
be ashed as such at a comparatively low temperature and we have reached the
conclusion that with phosphorus-rich substances, where the percentage of lime
and magnesia is comparatively low, it is advisable to use a fixing agent, and
we have found precipitated lime (CaO) to be a suitable fixing agent for both
P 2 0 5 and chlorine. For the determination of Ca, Mg, Na and K, however,
no fixing material is required so that when an analysis of these constituents is
also required, it is necessary to have two samples of the same food-stuffs ashed
for the two groups of analysis.
ESTIMATION OF PHOSPHORIC ACID
There are numerous methods, both gravimetric and volumetric given in
the literature, for the determination of phosphorus in biological materials, but
for routine work, the volumetric method is found most suitable.
The original volumetric method of Pemberton [1893, 1894] has been con-
siderably modified from time to time. The various factors that have been
taken into consideration are :—
(a) The temperature of precipitation of the phosphate by the addition
of the ammonium molybdate solution.
(b) The medium in which the precipitation is effected.
(c) Washing of the precipitate.