Mr Fenton, Oxidation in Presence of Iron. 359 



Dioxymaleic Acid. 

 OH.C.COOH 



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 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 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. 



