100
New Method of Estimation of Oxalic Acid [Vol. XVIII, Part II,
Details of technique. The biological material, such as a feeding stuff, is first dried and then
finely powdered in a milling machine. A quantity varying from 2 to 5 grams of the powdered sample
is taken in a 600 c.c. tall-form pyrex beaker and 40 c.c. of distilled water added. The beaker with
its content is then placed over a sand bath, the top of the beaker being covered by a suitable round
bottom flask filled with cold water to act as a condenser. As soon as the content in the beaker start
boiling, it is given a thorough shaking and the boiling is allowed to continue for another half hour.
At the end of this period keeping the beaker in the sand bath, 10 c.c. of a 20 per cent solution of
sodium carbonate is added. The content of the beaker is thoroughly stirred with a rubber capped
glass rod which is then washed in the beaker. The water in the condensing flask is also changed with
fresh cool water. The cooking is then continued for another half hour. During the interval, the
contents in the beaker are occasionally shaken without removing the condenser flask at the top. When
the cooking is over, the content is filtered hot by suction through a disc of cloth (65 threads to an
inch) fitted to a 7 c.c. Buchner funnel. The residue on the cloth is washed with hot water until the
wash water does not show any alkaline reaction. The filtrate is quantitatively transferred back
to the original beaker and boiled down to 100 to 150 c.c. It is allowed to cool, and then enough
hydrochloric acid (1:1) is added drop by drop and with constant stirring until the final acid con-
centration becomes approximately one per cent. At this stage a flocculent precipitate appears and
the whole thing after vigorous stirring is transferred quantitatively into a 200 c.c. volumetric flask
and the volume is made up to the mark. The precipitate in the flask is allowed to settle and the
clear supernatant liquid is filtered off through a fluted filter paper. An aliquot of the filtrate is taken
in a 400 c.c. beaker, diluted with water to 200 c.c. and then made just ammonical and reacidified
with acetic acid. In the cold medium is added 10 c.c. of a 10 per cent solution of calcium chloride
and the mixture is stirred well to induce the precipitate of calcium oxalate to appear. The preci-
pitate is allowed to settle overnight. The clear supernatant liquid is carefully decanted off through
a Whatman No. 42 filter paper leaving as much of the precipitate as possible undisturbed. The
precipitate is then dissolved in hydrochloric acid (1:1) after first allowing a few c.c. of the acid to
pass through the filter paper and collecting it in the beaker containing the precipitate. From the
hydrochloric acid solution, oxalic acid is reprecipitated and, this time, from a hot solution. The
second precipitation is necessary because, in the first, some colouring matter is liable to be dragged
along with the calcium oxalate precipitate which in subsequent titration with standard permanganate
is likely to introduce an error.
The final calcium oxalate precipitate is dissolved in dilute sulphuric acid and then titrated with
0.05 N potassium permanganate solution. One c.c. of 0.05 N permanganate solution is equivalent
to 0.00225 gram anhydrous oxalic acid. All figures of oxalic acid shown in this paper are expressed
in the anhydrous form.
Estimation of oxalic acid in cattle faeces. Since cattle faeces contain relatively large amount of
silica, it is necessary that this should be removed before oxalic acid is precipitated by calcium chloride.
Besides this additional step, the slight modification in the method required for the purpose may be
described as follows: 5 grams of the dried and finely ground sample of faeces are first cooked in
40 c.c. of one per cent solution of caustic soda for half hour which is followed by another half hour
cooking with the sodium carbonate solution. After the cooking is over, the extract is separated
by filtration through a disc of 7 c.m. Whatman No. 1 filter paper fitted to a Buchner funnel. The
extract is boiled down to about 50 c.c. and allowed to cool. In the cold extract, enough hydrochloric
acid (1:1) is added to make the final acid concentration one per cent. The precipitate formed is
filtered off. A suitable volume of the filtrate is taken in a graduated flask and just sufficient ammonia
added to make it alkaline. The volume is made upto the mark with distilled water and the content
of the flask is thoroughly shaken. The silicic acid precipitate is filtered off and in an aliquot of the
filtrate, oxalic acid is determined.
Estimation of water soluble oxalates. The obvious use of water for the extraction purpose is
precluded by the fact that free organic acids are liberated from the plant material if cooking is carried
out with water alone. The liberated acids are liable to act on some of the salts of calcium and magne-
sium and the cation thus set free will combine with soluble oxalates to form insoluble salts. The