ELECTROCHEMISTRY IN FUSED ETHYLAMMONIUM N ITRATE AT 298 K - DETERMINATION OF A REDOX POTENTIAL SCALE

Ethylammonium nitrate (NEA) is a room temperature fused salt, miscible with water and some organic solvents like methanol, Its ability to di ssolve organic or inorganic compounds and its intrinsic conductance ma ke it suitable for electrochemical analysis, The purpose of this artic le is to present some physical and electrochemical properties of this solvent. From Che variations of the interfacial tension with the appli ed voltage, the potential of zero charge (E-pen) of the NEA/mercury in terface has been inferred: the study of the electrocapillary curve sho ws that the negative charge born by the electrode at E<E-pen is less i n NEA than in an aqueous electrolyte containing 1 M sodium nitrate, Th is has been attributed to the size of the ethylammonium ion. The elect rochemical window of the NEA on Pt, C and Hg electrode material is com parable to that of water and exhibits a 60 mV per decade variation wit h pH. The standard potentials for the Ag+/Ag and Hg-2(2+)/Hg systems h ave been determined by classical potentiometry using an aqueous calome l electrode as reference electrode. Cd2+, Pb2+ and Zi(2+) but not Cu2, can be reduced at the dropping mercury electrode. Well defined polar ographic waves are obtained and only the Zn2+/Zn,Hg system has been fo und to be less reversible in NEA than in water as a consequence of the Frumkin effect due to the adsorption of ethylammonium ions on the ele ctrode surface at potential below E-pen. Some common organic of inorga nic redox systems have been investigated in NEA at 25 degrees C by mea n of cyclic voltammetry and linear sweep voltammetry at the Pt rotatin g electrode. The standard rare constant for the hexacyanoferrate(III)/ hexacyanoferrate(II) redox system. determined by cyclic voltammetry ha s been found to be two order of magnitude less in NEA than in an dilut e aqueous electrolyte, but only one order less than in a concentrated (1 M) aqueous electrolyte. This effect is attributed to the Helmoltz c ompact layer formed at the electrode/electrolyte interface in the liqu id salt. This study emphases the duality of the NEA which exhibits tow ard solutes the solvating properties of hydrogen bounded solvents as w ell as those of dipolar aprotic solvents. As an example, the stability of semiquinone of the methylviologen is MEA has been found to be the same as in water but less than in dipolar aprotic solvents like DMF or acetonitrile. On the reverse, reduction of iodine occurs via the form ation of the stable triiodide ion occurs in NEA as in dipolar aprotic solvents, A scale of standard potentials at 25 degrees C in NEA has be en established from the present electrochemical measurements and compa red Co the corresponding scale in water using the ferrocene/ferriciniu m hypothesis.