S. K. TALAPATRA, S. C. RAY AND K. C. SEN 251
ESTIMATION OF CALCIUM
This mineral is generally estimated by precipitating it as an oxalate salt.
The oxalate is subsequently dissolved in H 2 SO 4 and the oxalic acid liberated is
titrated against the standard KMnO 4 solution. There are, however, several
interfering factors which must be eliminated before calcium can be precipi-
tated quantitatively as oxalate. In soil this mineral is generally estimated
after removing iron, aluminium etc. but in biological samples like fodders and
concentrate, the concentration of iron, aluminium, manganese etc. are so low
that calcium can be precipitated quantitatively even in their presence. The
phosphate-ion on the other hand can hinder complete precipitation of oxalate
if the pH of the precipitating medium is not properly controlled. Numerous
experiments [Britton, 1929] have shown that the medium becomes favourable
for the formation of calcium phosphate whenever its pH exceeds five. This
point should specially be recognised in the case of samples in which the phos-
phate contents are relatively much higher than the calcium content. The next
important point to be considered is that magnesium is sometimes co-precipi-
tated with calcium [Kolthoff and Sandell, 1937]. The pH which is favourable
for the precipitation of calcium oxalate is favourable also for precipitation of
magnesium oxalate when magnesium is present in relatively large amount.
This co-precipitation of magnesium oxalate however can be prevented when the
concentration of NH 4 C1 in the precipitating medium is increased. NH 4 C1
has a great solvent action on magnesium oxalate but practically no such
action is observable on calcium oxalate. Having these theoretical con-
siderations in mind, the following modification of Chapman's method [1928]
has been evolved in this laboratory.
METHOD
A suitable aliquot from the HCl extract is taken in a beaker which is
then diluted to nearly 100 c.c. To this are added 5-10 c.c. of strong HCl.
Generally 5 c.c. of HCl will be enough but to samples which contain a very
large excess of magnesium 10 c.c. of HCl should be added. To this acid
solution 10 c.c. of saturated solution of ammonium oxalate and two or three
drops of alcoholic solution of methyl-red are added. The solution is then tho-
roughly mixed and gradually neutralised with concentrated ammonia added
from a dropping pipette. As soon as the precipitate begins to appear, the
pH of the liquid is next adjusted with dilute ammonia (1:4) until a faint
pink colour appears. The content of the beaker is then boiled for a few minu-
tes in order to allow the precipitate to become granular and is then left over-
night. On the next day, it is filtered through a 9 cm. Whatman No. 40, leav-
ing as much of the precipitate as possible in the beaker. The precipitate can
be filtered four hours after the precipitation. The precipitate in the beaker is
then washed several times. The wash water every time is passed through the
filter paper. After three or four such washings the filter paper is also washed
once with hot water. The filter paper is then unfolded and transferred to the
beaker. 100 c.c. of water are now added to the beaker and 10 c.c. of strong
H 2 SO 4 are carefully poured into the mixture. The solution is then warmed to
70°C and is titrated with N/20 KMnO 4 . During the final stage of titration,
the wall of the beaker is washed with hot water and the end point of the