Electrochemistry in fused ethylammonium nitrate 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 dissolve organic or inorganic compounds and its intrinsic conductance make it suitable for electrochemical analysis, The purpose of this article is to present some physical and electrochemical properties of this solvent. From Che variations of the interfacial tension with the applied voltage, the potential of zero charge (E-pen) of the NEA/mercury interface has been inferred: the study of the electrocapillary curve shows that the negative charge born by the electrode at E<E-pen is less in NEA than in an aqueous electrolyte containing 1 M sodium nitrate, This has been attributed to the size of the ethylammonium ion. The electrochemical window of the NEA on Pt, C and Hg electrode material is comparable to that of water and exhibits a 60 mV per decade variation with pH. The standard potentials for the Ag+/Ag and Hg-2(2+)/Hg systems have been determined by classical potentiometry using an aqueous calomel electrode as reference electrode. Cd2+, Pb2+ and Zi(2+) but not Cu2+, can be reduced at the dropping mercury electrode. Well defined polarographic waves are obtained and only the Zn2+/Zn,Hg system has been found 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 electrode surface at potential below E-pen. Some common organic of inorganic redox systems have been investigated in NEA at 25 degrees C by mean of cyclic voltammetry and linear sweep voltammetry at the Pt rotating electrode. The standard rare constant for the hexacyanoferrate(III)/hexacyanoferrate(II) redox system. determined by cyclic voltammetry has been found to be two order of magnitude less in NEA than in an dilute aqueous electrolyte, but only one order less than in a concentrated (1 M) aqueous electrolyte. This effect is attributed to the Helmoltz compact layer formed at the electrode/electrolyte interface in the liquid salt. This study emphases the duality of the NEA which exhibits toward solutes the solvating properties of hydrogen bounded solvents as well 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 formation 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 been established from the present electrochemical measurements and compared Co the corresponding scale in water using the ferrocene/ferricinium hypothesis.