ISRAEL LOEFFAND A. J. SWALLOW
2470
of Apl and Apz for experiments with systems in class I1 and class IV. I n addition to facilitating tests of stability for previous experiments, conditions (42) and
(43) may help in choosing suitable ranges of values of the Ap$ in future experiments when preliminary estimates of the D l j are available.
On the Radiation Chemistry of Concentrated Aqueous Solutions
of Sodium Benzoate
by Israel Loeffl and A. J. Swallow Nuclear Technology Laboratory, Department of Chemical Engineering and Chemical Technology, Imperial College, London, S. W.?, England (Received February 26, 1964)
The radiation chemistry of sodium benzoate in aerated aqueous solution a t concentrations up to 2.9 M has been investigated with the help of colorimetric and C14 methods. A hitherto unknown product, a dialdehyde, has been found. All products except carbon dioxide appear to be formed only by an indirect mechanism but carbon dioxide is formed by both indirect and direct mechanisms. The dependence of the product yields on concentration does not fit a simple theory in which energy absorption by the two components is proportional only to outer electron fraction (or electron fraction). It seems likely that the aromatic ring is taking up more of the radiation energy than would be expected on such a basis, as observed previously in purely organic mixtures containing substances possessing a-electrons.
Studies of the radiation chemistry of aqueous systems have so far been confined mainly to dilute solutions.2 This has enabled the decomposition of the water and the subsequent reactions of the active species to be worked out without the possibility of complications arising from interference of the solute with the primary processes. However, any interference of the solute with primary processes is of interest in itself and indeed is of particular interest in connection with the irradiation of biological systems. We have now studied the irradiation of solutions of concentrations up t o nearly 3 M (nearly 40% of solute by weight). Sodium benzoate was chosen as solute because of its high solubility? because its dilute solutions had been studied previously, and because of the possibility of obtaining “energy transfer” effects like those found with purely organic systems containing aromatic compounds. The Journal of Physical Chemistry
Experimental Water was distilled from alkaline permanganate and then distilled again. Sodium benzoate was the material available commercially from Hopkin and Williains and was used without further purification, while the material used for checking and comparison was coniniercial calcium benzoate which had been recrystallized several times. The carboxyl C 14-labeled benzoic acid had been prepared by the Radiocheinical Center, Amersham. Its specific activity was 6.8 mc./mM. Thep H values of solutions were in the region 6.8-8.3. I n the C14 experiments, A.R. grade (1) Department of Physical Chemistry, The Hebrew University, Jerusalem, Israel. (2) A. 0. Allen, “The Radiation Chemistry of Water and Aqueous Solutions,” D. Van Nostrand Co. Ltd., London, 1961.
24'71
RADIATION CHEMISTRY OF SODIUM BENZOATE SOLUTIONS
sodium hydroxide was added to the benzoic acid to bring the pH into this region. For the hydrogen estimations, solutions were deaerated until the pressure mm. of the noncondensible gases was below 1 X Samples were irradiated with y-rays from a kcurie cobalt-60source3except for the hydrogen measurements, where a 200 KVP X-ray machine, working a t 15 ma., 'was used. Dosimetry was performed using the Fricke dosimeter (in 0.1 N acid) assuming G(Fe3+) = 15.5. It was assumed that even for 200 KVP Xrays, Compton scattering was the only significant mode of energy absorption. If for 200 KVP X-rays G(Fe3+) = 14.5, then the hydrogen yields irt the present paper have to be multiplied by 0.93. Dosimetry calculations at the higher concentrations took into account the electron densities of the solutions. Dose rates were 4600-5200 rads/min. for the colorimetric estimations of salicylic acid, dialdehyde, and hydrogen peroxide, 800-900 rads/min. for the CI4 experiments, and 1750-1950 rads/min. for the hydrogen measurements. Salicylic acid was estimated by the cupric-nitrous acid methoda4 The pH of the irradiated solution was pH 3.8 after final adjusted so as to est,ablish 3.5 dilution. The optical density was measured in a 4cm. cell a t 520 mp ( e 2490 f 25). Aldehyde was estimated as the p-nitrophenylhydrazone in aqueous alkali according to B method described e1sewhere.j" Hydrogen peroxide was estimated using the Ti4+method5bwith a 4-cm. cell. All the above-mentioned colorimetric methods necessitated the additions of acid to the irradiated solutions. At the higher concentrations this resulted in precipitation of the benzoic acid. Blanks were carried out (using synthetic salicylic acid and hydrogen peroxide and in the case of the dialdehyde irradiated dilute solutions) to examine any quantitative changes involved in this step. The corrections which had to be applied for the most highly concentrated solutions were quite high in some cases. Hydroxybenzoic acids were measured using C 14. For these experimenis the irradiated aqueous solution was evaporated to dryness and the residue was transferred to chromatographic paper. The system used was butanol saturated with 5 N ammonia.6 The spots were located by their fluorescence under ultraviolet illumination ; the m-hydroxybenzoic acid spots were located only after being sprayed with alkali. The Rf values differed somewhat from the published ones,6 except for the 2,6-dihydroxybenzoic acid, the same sequence was observed. For the latter compound a much lower Rf value was found which brought it into the same region as the p-hydroxybenzoic acid. For
