E. Desimoni

Solubility and detection (down to 30 p.p.b.) of oxygen in molten alkali nitrates

Summary The solubility of oxygen in (Na, K)NO 3 eutectic melt has been determined in the temperature range 520–600 K. The relevant Henrys constant has been found to be K H =4.8×10 −6 (mol kg −1 atm. −1 ) at 520 K while the enthalpy of the process is 4.2 kcal mol −1 at 10 −5 molal concentration levels. The reversible reduction of oxygen (to superoxide and peroxide) and the process O 2 +2H 2 O+4 e =4 OH − , both occurring at a platinum electrode, can be used to detect oxygen in dry and partially wet melts, respectively. The stoichiometry of the last electrode reaction has been established in the course of the present work by massive controlled-potential electrolyses. The relevant RDE limiting current obeyed the Levich relation and permitted precise detection of oxygen down to 30 p.p.b. On the basis of the results the diffusion coefficient for oxygen (at 550 K) was calculated to be D O 2 =3.1×10 −4 cm 2 s −1 and its Arrhenius coefficient 5.0 kcal in the range 525–575 K.

Nucleation phenomena in the electrodeposition of lead onto glassy carbon electrodes

The electrodeposition of lead onto a glassy carbon electrode from 0·1m HCl solution has been studied mainly by potential step techniques. The early stages of metal deposition are shown to be controlled by a three-dimensional nucleation followed by hemispherical growth. The deposit morphology has been confirmed by scanning electron microscopy (SEM). Results obtained by linear sweep voltammetry are also reported and qualitatively interpreted according to the above-mentioned nucleation mechanism.

Catalytic currents in fused salts: Discharge mechanism of nitrite in molten alkali nitrates

Abstract The voltammetric behaviour of the redox system NO2/NO2− has been investigated on platinum electrodes in fused (sodium, potassium) nitrates, pure or containing small amounts of water (10−6 ≤[H2O] ≤ 10−2 m) at the temperature of 520 K. While in a perfectly dry melt the oxidation of nitrite occurs according to the one-electron reversible electrode reaction NO 2 − = NO 2 + e ( a ) NO2−1=NO2+e in the presence of traces of water the disproportionation of nitrogen dioxide occurs to some extent, leading to the “catalytic” mechanism Download : Download full-size image which becomes competitive with the simple electrode process (a). In the presence of a “water excess” the over-all reaction (c) tends to predominate and the oxidation of nitrite switches towards a net balance of two electrons. At nitrite concentrations around 10−3 m, the entire spectrum of the electrochemical situations (from one to two electrons per nitrite ion) could be observed just on changing the water content in the range mentioned.

Oxygen electrodes in fused salts: Potentiometric behaviour of the system CO2, O2/CO32− in molten alkali nitrates

Abstract A potentiometric study on the carbonate electrode (Pt or Au) CO 2 , O 2 /CO 3 2− was performed at 523 and 623 K in a (Na−K)NO 3 equimolar mixture containing carbonate in the concentration range 10 −5 3 2− ] −2 M and maintained under flux of a mixture of oxygen and carbon dioxide at variable partial pressures ( p O 2 + p CO 2 =1 atm.). While the overal electrode reaction for a carbonate electrode can be expressed by the equation 1 2 O 2 + C O 2 + 2 e = C O 3 2 − ( a ) the present study has shown that the potential taken up by an “indifferent electrode” can be described by the equation E = const. + ( R T / 2 F ) 1n { [ C O 2 ] / [ C O 3 2 − ] } ( b ) The apparent potentiometrically irreversible behaviour of the carbonate electrode under the given experimental conditions has been interpreted on the basis of a mechanistic model involving, in the potential-determining step, the oxidized surface of the noble metals. For example Download full-size image

Hydrogen half-cells in molten salts. A potentiometric investigation of the systems (Pt or Au) H2O/H2, OH− in fused alkali nitrates

AbstractThe potentiometric behaviour of the ‘hydrogen electrodes’ (Pt or Au) H2O-H2, OH−has been investigated in molten (Na0·5, K0·5)NO3 at 503 K. In both cases the potential of the ‘indifferent electrode’ could be expressed by the general equation

Oxygen electrodes in fused salts: Investigation and mechanistic remarks on the system (Ni)CO2, O2/CO32− in molten (Na, K)NO3

E = K + \frac{{RT}}{F}1{\text{n}}\frac{{[{\text{H}}_{\text{2}} {\text{O]}}}}{{[{\text{H}}_{\text{2}} ][{\text{OH}}^{\text{ - }} ]}}

Peroxide and Superoxide as Catalysts. a Model Based on Molten Salts Experiments

[H2O]/[H2] [OH−] which is different from the one expected on the basis of a Nernstian behaviour of the theoretical overall system 2H2O+2e=H2+2OH−.The experimental findings are discussed in terms of mechanistic models involving the actual electrode surface and the standard potential for the theoretical (reversible) hydrogen electrode is calculated: