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In the pulse radiolysis of concentrated phosphoric acid solutions, the direct action of radiation on phosphoric acid gives rise to the phosphate radical. The yields of the phosphate radical and the hydroxyl radical are proportional, respectively, to the electron fractions of phosphoric acid and water, and can be quantified as G('0H + H2PO',) = 0.30fw + 0.33, pmol J-'. The forward and reverse rate constants of the equilibrium 'OH + H, PO, e H 2 0 + H2PO', were evaluated as 4.2 x lo4 and 2.5 x lo3 dm3 mol-' s-' respectively, from which E0(H2P0',, H+/H,PO,) = 2.65 V was derived. The decay kinetics and the reactions of the phosphate radical have also been studied.Few studies have been carried out on the pulse radiolysis of phosphoric acid in aqueous solutions.'*2 Grabner et al. have reported that 'OH and H' are converted into the phosphate radical on reaction with H,PO,, whereas Black and Hayon failed to observe the phosphate radical in the pulse radiolysis of 1 mol dm-3 H,P0,.3 In the pulse radiolysis of H2P0, in aqueous solutions, the phosphate radical has been shown to result from the direct action of radiation on the solute.'*4 Recently, we have studied the pulse radiolysis of concentrated nitric and sulfuric acid in order to understand the direct action of radiation on solutes and the reactions of the hydroxyl radical with the acids. This work extends the previous studies to phosphoric acid, in order to examine the direct action of radiation on phosphoric acid, the reaction of the hydroxyl radical with phosphoric acid and the reactions of the phosphate radical.

The formation of nitrous acid has been studied in the 'OC0-y radiolysis of concentrated nitric acid solutions. The yields of nitrous acid are proportional to the electron fraction of nitric acid, on the basis of which, an additional reaction path, NO; O(3P) + NO; with a primary yield of 0.16 pmol J -' , has been verified for the direct action of radiation on nitric acid.