Potentiometric acid-base titrations in molten salts: The acid character of group (VI) oxides as inferred from their reaction with molten KNO 3

Abstract

The reaction between CrO 3, MoO 3, and WO 3, and fused KNO 3 at 350° has been studied by measuring the variation in the potential of an oxygen electrode relative to an Ag/Ag(I) reference electrode. When the reactions come to an end, the reaction products are determined by acid-base titrations in situ, using Na 2O 2 as titrant. The acid strength of the three oxides decreases in the order CrO 3 > MoO 3 > WO 3. This is evident from the variation of the potential of the oxygen electrode with time, and from the fact that, whereas dichromate is formed in case of CrO 3, only the triomolybdate is obtained from the reaction of MoO 3 with fused KNO 3. WO 3 is stable under the same experimental conditions. Neutralization of all the reaction products with Na 2O 2 leads to the formation of the most basic anion, XO 4 2−. Acid-base equilibrium constants for the reactions: 2 CrO 3 + O 2− → Cr 2O 7 2− CrO 3 + O 2− → CrO 4 2− MoO 3 + O 2− → MoO 4 2− Mo 3O 10 2− + 2O 2− → 3 MoO 4 2− WO 3 + O 2− → WO 4 2− have been calculated. The kinetics of the reaction: 2 CrO 3 + 2 NO 3- → Cr 2O 7 2- + N 2O 5 are established. The reaction is bimolecular with respect to CrO 3 and has an activation energy of 25.3 kcal/mole.