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

Abstract

The solubility of oxygen in (Na, K)NO 3 eutectic melt has been determined in the temperature range 520–600 K. The relevant Henry's 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.