OXALIC ACID. 
551 
§ 730.] 
precipitate washed by subsidence ; it is lastly dried over the water-bath 
in a tared porcelain dish, and its weight taken. The substance is then 
identified by testing the dried powder as follows :— 
(a) It is whitish in colour, and on ignition in a platinum dish leaves 
a grey carbonate of lime. All other organic salts of lime—viz. citrate, 
tartrate, etc.—on ignition become coal-black. 
(b) A portion suspended in water, to which is added some 
sulphuric acid, destroys the colour of permanganate of potash— 
the reaction being similar to that on p. 542—a reaction by which, 
as is well known, oxalic acid or an oxalate may be conveniently 
titrated. This reaction is so peculiar to oxalic acid, that there is 
no substance with which it can be confounded. It is true that 
uric acid in an acid solution equally decolorises permanganate, but 
it does so in a different way ; the reaction between oxalic acid and 
permanganate being at first slow, and afterwards rapid, while the 
reaction with uric acid is just the reverse—at first quick, and towards 
the end of the process extremely slow. 
(c) A portion placed in a test tube, and warmed with concentrated 
sulphuric acid, develops on warming carbon monoxide and carbon 
dioxide ; the presence of the latter is easily shown by adapting a cork 
and bent tube to the test tube, and leading the evolved gases through 
baryta water. 
Alexander Gunn 1 has described a new method of both detecting and 
estimating oxalic acid ; it is based on the fact that a small trace of oxalic 
acid, added to an acid solution of ferrous phosphate, strikes a persistent 
lemon-yellow colour, the depth of colour being proportionate to the 
amount of oxalic acid. 
The reagents necessary for both quantitative and qualitative testing 
are as follows :—A standard solution of oxalic acid, of which 100 c.c. 
equal 1 grm., and a solution of ferrous phosphate, containing about 12*5 
per cent, of Fe 3 2P0 4 , with excess of phosphoric acid. 
Into each of two Nessler graduated glasses 7-5 c.c. of the ferrous 
phosphate solution are run and made up to 50 c.c. with distilled water ; 
both solutions should be colourless. 1, 2, or more c.c. of the solution 
to be tested are then run into one of the Nessler glasses ; if oxalic acid 
be present, a more or less deep tint is produced ; this must be imitated 
by running the standard solution of oxalic acid into the second Nessler 
cylinder—the calculation is the same as in other colorimetric estima¬ 
tions. It does not appear to be reliable quantitatively, if alum is present; 
and it is self-evident that the solution to be tested must be fairly free 
from colour. 
§ 730. Oxalate of Lime in the Urine. —This well-known urinary 
sediment occurs chiefly as octahedra, but hour-glass contracted or 
1 Pharm. Journal, 1893, p. 408. 
