Use of Nitrous Oxide to discriminate between Various Forms of Hydrogen Atoms existing in Aqueous Solutions of Potassium Iodide irradiated with Ultra-violet Light

Abstract

DEAERATED, acidified, aqueous solutions of potassium iodide decompose to form equimolar amounts of hydrogen and iodine under the influence of light of wave-length less than 2600 Å. which is absorbed by the hydrated iodide ion (I⁻ _aq.). The quantum yield is small, but increases with increasing iodide and hydrion concentration. A postulated cause¹ of the effect of pH on the quantum yield is that hydrogen atoms found in the primary act can be protonated to form the hydrogen molecule ion, H₂ ⁺, which will oxidize iodide ions in the reaction (1), whereas hydrogen atoms serve only to reduce iodine atoms and molecules. In the preceding communication² it was shown that nitrous oxide is inert to H₂ ⁺, but reacts rapidly with hydrogen atoms most probably according to equation (3), and that pK _a (H₂ ⁺) ≃ 2.3: Therefore, if solutions of potassium iodide saturated with nitrous oxide are irradiated with ultra-violet light it would be expected that little or no nitrogen would be produced until the pH of the solution exceeded 2, when reaction (3) becomes possible and ϕ(N₂) should increase steeply to a limiting value which would be the true primary quantum yield of the iodide photolysis. The hydroxyl radicals liberated in reaction (3) will oxidize iodide ions very rapidly according to equation (4):