J. Electroanal. Chem., 73 (1976) 295--302 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands 295 ELECTROANALYTICAL EVIDENCES FOR THE INSTABILITY OF HYDROGEN TETRAFLUOROBORATE IN ACETONITRILE GINO BONTEMPELLI and RENATO SEEBER Istituto di Chimica Analitica dell'Universit5 di Padova, Via Marzolo 1, 35100-Padova (Italy) SANDRO ZECCHIN and GILBERTO SCHIAVON Laboratorio di Polarografia ed Elettrochimica Preparativa, C.N.R., P.O. Box 1075, 35100- Padova (Italy) (Received 19th January 1976) ABSTRACT The voltammetric behaviour of tetrafluoroborate salts, acetonitrile solutions, to which hydrogen ions had been added, has been investigated both at platinum and mercury elec- trodes. It has been established that HBF 4 is unstable in this medium and its decomposition gives HF and BF 3. INTRODUCTION Tetrafluoroborate salts have been suggested as supporting electrolytes for anodic investigations in aprotic media owing to the large anodic potential limit which can be achieved with appropriate solvents [1]. It can happen that in an anodic process the release of hydrogen ions occurs if organic depolarizers are investigated [ 2] so that tetrafluoroboric acid is formed. Recently it has been observed that the voltammetric behaviour of acid solu- tions of BF4 ions is not similar to that exhibited by C104 solutions containing hydrogen ions [ 3,4]. Furthermore, in an attempt to prepare hydroxytriphenyl- arsonium tetrafluoroborate (Ph3AsOHBF4) by anodic oxidation of triphenyl- arsine in wet acetonitrile with Et4NBF 4 as supporting electrolyte [ 5], we ob- tained PhaAsOBF 3 instead of the expected compound and, to account for this result, we suggested a loss of hydrogen fluoride from the tetrafluoroborate initially formed. All these unexpected findings suggested to us that BF~- is an unstable ion in acid acetonitrile solutions and hence some serious trouble can rise from its use as supporting electrolyte. The aim of this investigation is to verify this hypothesis and to give account of this instability by voltammetric investigations.