Physics
Scientific paper
Jan 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998jcp....95...25b&link_type=abstract
Journal de Chimie Physique et de Physico-Chimie Biologique, Volume 95, Issue 1, January 1998, pp.25-44
Physics
Nitrate D'Éthylammonium, Sel Fondu, Solvant Non Aqueux, Échelle De Potentiel Redox
Scientific paper
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 the variations of the interfacial tension with the applied voltage, the potential of zero charge (Epcn) 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 < Epcn 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 Hg2 2+/Hg systems have been determined by classical potentiometry using an aqueous calomel electrode as reference electrode. Cd2+, Pb2+ and Zn2+, 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 Epcn. Some common organic or inorganic redox systems have been investigated in NEA at 25 °C by mean of cyclic voltammetry and linear sweep voltammetry at the Pt rotating electrode. The standard rate 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 NEA 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 °C in NEA has been established from the present electrochemical measurements and compared to the corresponding scale in water using the ferrocene/ferricinium hypothesis.
Le nitrate d'éthylammonium (NEA) est un sel fondu à température ordinaire, miscible à l'eau et à certains solvants organiques comme le méthanol. Ses propriétés solubilisantes vis à vis des composés organiques ou minéraux et sa conductivité intrinsèque en font un milieu intéressant pour des études électrochimiques. Le but de cet article est de présenter quelques propriétés physiques et électrochimiques de ce solvant. Le potentiel de charge nulle (E{pcn}) de l'électrode à goutte de mercure a été déterminé par mesure de la tension interfaciale. L'analyse de la courbe électrocapillaire montre que la charge négative de l'électrode à E < Epcn est plus faible dans le NEA que dans l'électrolyte aqueux NaNO3 (1 M). Cet effet a été attribué à la taille de l'ion éthylammonium adsorbé à l'interface. Le domaine d'électroactivité du NEA a été déterminé sur électrode de platine, carbone et mercure. Quelques systèmes redox courants, minéraux et organiques, ont été étudiés par les méthodes électrochimiques usuelles : potentiométrie, polarographie, voltammétrie à l'électrode tournante et voltammétrie cyclique. L'étude électrochimique de ces systèmes dans le NEA montre la dualité de ce solvant qui se comporte tantôt comme un solvant protique (comme l'eau ou les alcools), tantôt comme un solvant dipolaire aprotique (comme le DMF ou l'acétonitrile). La détermination des potentiels standard des couples étudiés a permis d'établir une échelle d'oxydo-réduction qui a été comparée à l'échelle aqueuse.
Benhlima N.
Lemordant D.
Letellier P.
Naejus R.
Turmine M.
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