J. Electroanal. Chem., 68 (1976) 237--242 237 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands POLAROGRAPHIC ACTIVITY OF THE ARYLHYDRAZONES OF a-CYANOKETONES AS A CRITERION OF THEIR ACIDITY M.H. ELNAGDI, H.M. FAHMY and M.A. MORSI Chemistry Department, Faculty of Science, Cairo University, Giza (A.R. Egypt) (Received 23rd May 1975; in revised form 25th July 1975) ABSTRACT The polarographic behaviour of phenylhydrazonomesoxalonitrile (2) and ethyl phenyl- hydrazonomesoxalonitrile (3) as well as pK' and pK a values axe reported for a series of arylhydrazones of a-cyanoketones. INTRODUCTION In a previous investigation [1] it has been shown that a-arylhydrazono- nitriles (la--c), unlike other arylhydrazones, are polarographically active in alkaline media. This unusual behaviour was rationalized by assuming that these compounds are much more acidic than aldehyde and ketone arylhydra- zones and that the species undergoing reduction in alkaline media is a reso- nance stabilized anion. In connection with this work we report here the polarographic behaviour of phenylhydrazonomesoxalonitrile (2) and ethyl phenylhydrazonomesoxalonitrile (3) as well as the results of potentiometric titrations of compounds (la--c), (2) and (3). R (NC)2C=N • NHC6H 5 I NC--C=N • NHC6H 5 (3) (la) R = C6H5CO (lb) R = CHACO (lc) R = CH3C=NH CO2C2H5 I NC--C= N" NHC6H 5 (2) EXPERIMENTAL Polarographic curves were recorded using a LP 60 polarograph (Labora- torni Pristroge, Prague). The capillary possessed the following characteristics