Mr Fenton , Oxidation in Presence of Iron. 
359 
Dioxymaleic Acid. 
OH. C. COOH 
I! 
OH. C. COOH. 
[Trans. Chem. Soc. 1894. 899, 1896. 546, Proc. Chem. Soc. 
1898, etc.] 
This acid is prepared by slowly adding strong hydrogen 
dioxide to a nearly saturated solution of tartaric acid containing 
a little ferrous tartrate, or other ferrous salt, and the separation of 
the product is effected by the cautious addition of fuming sul- 
phuric acid or other dehydrating agent. The operations must be 
conducted at a low temperature and the mixture kept in motion 
by means of a current of air or other device. After standing for 
a short time the acid begins to separate in lustrous, transparent, 
diamond- shaped plates. These, the dihydrate C 4 H 4 0 G . 2H 2 0, 
when isolated, are remarkably stable at the ordinary temperature 
and lose their water of crystallization at 80 — 90°. They are very 
sparingly soluble in cold water and the aqueous solution is un- 
stable, decomposing slowly at the ordinary temperature and very 
rapidly at 50 — 60°. The aqueous solution acts as a powerful 
reducing agent, it gives with ferric salts a blackish colour which 
is changed to an intense and beautiful violet by addition of 
caustic alkalis; and with phenylhydrazine acetate it gives the 
salt C 4 H 4 0 6 . 2N 2 H 3 Ph which separates in brilliant silver-like 
plates. 
The di-methyl and di-ethyl esters are crystalline solids and 
both exist in two well-marked distinct crystalline forms. The di- 
ethyl ester has the remarkable property of becoming liquid when 
kept in dry air although it is permanent in moist air or in a 
vacuum desiccator ; this property has recently been explained in a 
short communication to the Society [Fenton and Ryffel, Proc. 
Camb. Phil. Soc. 1901. 109]. 
The molecular weight of the acid was arrived at by the 
vapour pressure method of Will and Bredig and was confirmed by 
the freezing-point method in case of the methyl ester. The 
constitution of the acid follows from various considerations, 
among which may be mentioned the formation of di-acetyl or 
di-benzoyl derivatives both of the acid and esters, the di-basicity 
of the acid, the formation of salts only by the actions of phenyl- 
hydrazine and hydroxylamine, and the colour reactions with ferric 
chloride. 
