The oxidation of hydrazines. I.—The volumetric determination of carbonyl compounds with 2 : 4‐dinitrophenylhydrazine

Abstract

The iodate oxidation of 2 : 4-dinitrophenylhydrazine has been investigated and procedures, based on this reaction, are now suggested for the semi-micro volumetric determination of carbonyl compounds. Some observations on the completeness of precipitation of 2 : 4-dinitrophenylhydrazones are included. IntrodnctionMethods which are generally applicable to the volumetric determination of carbonyl compounds have been limited by the reactivity of reagents which give derivatives with the > CO group. Most procedures are dependent upon reaction with either hydroxylamine1,P or phenylhydrazine.F6The hydroxylamine methods, which are probabIy the most widely used, suffer from several disadvantages, chief of which a r e i h e slowness of the reaction between the reagent and the carbonyl compound, and the somewhat difficult end-point. A recent modification6 claims good reproducibility, but the colour matching of solutions involved would still seem to be tedious and requiring of more skill than the detection of the visual end-point of a conventional titration.Some methods based on phenylhydrazine depend on the oxidation of the reagent with Fehling's solution, and have been criticized by Britton & Clissold,' who have ascribed variations in results given by different methods to the varying extents of two simultaneously occurring oxidations:Nevertheless, under carefully controlled conditions, these workers have obtained reproducible results in the potentiometric titration of phenylhydrazine with Fehling's solution. A more satisfactory volumetric method for phenylhydrazine, which allows use of a visual endpoint, is that using potassium iodate as standard oxidantas However, extehsion of any method involving the determination of residual phenylhydrazine to the estimation of carbonyl compounds is limited by the degree of completeness of precipitation and/or reaction. Similarly, although semicarbazide has been determined volumetncally with iodate,% 8, B the formation of semicarbazones may require prolonged refluxing and even so the precipitation may not be quantitative.2 : 4-Dinitrophenylhydrazine, on the other hand, does form a well-defined series of derivatives with carbonyl compounds and is widely used for their qualitative detection and identification. 10-l2 Many of these 2 : 4-dinitrophenylhydrazones have been shown to be sufficiently insoluble to permit their gravimetric determination under controlled conditions of precipitation. 1% l4The purpose of the present work is, firstly, to extend to 2 : 4-dinitrophenylhydrazine the method of estimation of phenylhydrazine by titration with iodate, and secondly, to obtain a volumetric procedure for the determination of carbonyl compounds based upon the iodate titration of the residual 2 : 4-dinitrophenylhydrazine after precipitation of the 2 : 4-dinitrophenylhydrazones.