A NEW METHOD OF ESTIMATION OF OXALIC ACID IN BIOLOGICAL
MATERIALS AND THE OXALIC ACID CONTENT OF INDIAN
FEEDING STUFFS
By S. K. TALAPATRA, S. C. RAY and K. C. SEN, Indian Dairy Research Institute, Bangalore
(Received for publication on 24 November 1947)
I N majority of the methods published on the estimation of oxalic acid in biological materials
[Dakin, 1907, Arbenz, 1917, Nelson and Mottern, 1931, Pucher et al, 1934, and Kohman, 1939], the
principle involved is to release the acid free from its combination of a salt by treating the sample
with a mineral acid. The free acid is then extracted with ether and from the ethereal extract, oxalic
acid is precipitated as its calcium salt and finally estimated by permanganate titration. Some of
these ether-extraction methods published on or before 1931 have been found not sufficiently accurate.
Incomplete extraction of oxalic acid and the lack of proper control in its precipitation from the ethereal
extract are factors responsible for inaccuarcy in estimation. In later methods, particularly in that
of Pucher et al cited above, these limitations have been successfully overcome and accurate and
reproducible values have been obtained. The accuracy in estimation, however, is achieved only
after introducing laborious and complicated modifications in the technique which are unsatisfactory
for routine work involving at one time a large number of analyses. The ether-extraction method,
moreover, offers a difficulty of considerable magnitude under the Indian conditions. In summer
months, the complete extraction of oxalic acid by ether proves to be an impossible operation. Not
only large amount of ether is wasted, but due also to much reduced number of syphoning, the extrac-
tion period has to be prolonged indefinitely and even then, uniformly satisfactory extraction cannot
be made. Majumdar and De [1938] have attempted to simplify the extraction method by excluding
the use of ether. Because of its simplicity, the method was given careful and repeated trials in this
laboratory. These trials had however, shown that with biological materials, such as, cattle feeds,
fodders and faeces, the method was unworkable. Further, in connection with some metabolism
studies reported elsewhere [Talapatra, Ray and Sen, 1947], it was found essential to know the relative
proportions of 'soluble' and ' insoluble ' forms of oxalicacid occurring in oxalate-rich feeding stuffs.
In the published literature there is no reference in which an attempt has been made to partition the
oxalic acid of a food or other biological sample.
In view of the limitations in existing methods, and as none has scope in carrying out the partition
a new method with the necessary advantage has been evolved. A part of the work reported in this
paper was published in an abridged form by Talapatra et al [1942]; in the present communication
the full details are being given and additional information regarding oxalic acid content in Indian
feeding stuffs, its variation with the progressive maturity of the plant and its distribution in leaf
and stem are also being included.
General principle of the new method. Depending upon the nature of the biological sample, it is
first boiled in distilled water or in 1 per cent solution of caustic soda for half hour. Sufficient con-
centrated sodium carbonate solution is then added to make the final strength approximately 4 per
cent and the boiling is continued for another half hour. During these two cooking processes, the
following changes are effected; (a) the cellular tissues are completely broken down, (b) fatty matters
are saponified and (c) by a process of metathesis, all insoluble oxalate salts are converted into soluble
sodium oxalate. The liquid extract is then filtered and residue washed with hot water till the wash
water does not show any alkaline reaction. The combined filtrate and wash water is then con-
centrated to a small volume and allowed to cool. In the cold, with constant stirring, hydrochloric
acid (1:1) is added drop by drop till the final acid concentration after the neutralization of the alkali
becomes approximately one per cent. At this stage, a heavy precipitate of extraneous matter appears
which is allowed to flocculate. The extract is then carefully filtered. In the clear lightly coloured
filtrate, oxalic acid is precipitated as its calcium salt and is determined in the usual way.
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