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In the preparation of dihydroxy maleic acid from tartaric acid by hydrogen peroxide in the presence of ferrous sulphate, the very small yields are the result of inactivation of the catalyst by the production of the relatively strong dihydroxy maleic acid; this reaction product undergoes such rearrangement to an isomer as prevents quantitative separation. By means of conductivity measurements of the reaction mixture, the course of the reaction is accurately followed.The existence of the tautomeric keto form of dihydroxy maleic acid, first suggested by Nef as an alternative formula, has been clearly indicated.The search for a procedure which will improve the yield of dihydroxy maleic acid is now restricted to the discovery of some compound which will at the same time reduce the hydrogen ion concentration and shift the equilibrium in favor of the enol form.

Hydrogen peroxide-induced oxidation of ascorbic acid was found to be catalysed by several fruit juices. Products exhibiting peroxidatic activity were passion fruit, orange and pineapple juices, papaya and guava purees, liquid coconut products, and bottled (pasteurised) apple juice. A mathematical relationship between logarithm of juice dilution to initial reaction rate was developed. Passion fruit juice exhibited the highest peroxidatic activity on the basis of calculated intercepts and slopes. Several other fresh juices exhibited peroxidatic activity. Although the fruit juices reduce the activity in Fenton's reaction with ascorbic acid as substrate, the oxidising activity of the juices may not be a Fenton reaction. Coconut skim milk showed the highest inhibitory effect and passion fruit juice was second highest. Mechanisms for the inhibitory effect are considered.