Effect of Temperature on the Neutralization of Hydronium Ions in the Radiolysis of Water Vapour

Abstract

IT has been suggested^1,2 that the yield of hydrogen atoms from the decomposition of water vapour induced by radiation is dependent on temperature. The main evidence in support of this is the observation² that the yield of hydrogen gas from water + cyclohexane (5 mole per cent, total pressure ∼ 1.5 atm.) increases by an amount corresponding to G ∼ 3.6 over the temperature range 240° to 300° C (where G is the yield in molecules/100 eV of energy absorbed). In the ranges 100–240° and 300–350° C, G(H₂) was reported to be essentially independent of temperature. The marked increase in G(H₂) over a relatively narrow temperature range was explained in terms of the effect of the extent of hydration of the hydronium ion on neutralization. Thus it was assumed that at temperatures below ∼ 240° C recombination of electrons with a hydrated ion gives one hydrogen atom per ion whereas, at temperatures above 300° C, neutralization of the unhydrated ion (or a low molecular weight hydrate) gives two hydrogen atoms per ion The postulated change in the extent of hydration of the hydrogen ion with temperature is consistent with the enthalpy of hydration obtained from mass spectrometric studies³.