ISRAEL LOEFFAND A. J. SWALLOW
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.
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
the higher concentrations several papers mere needed even for a very small irradiated sample in order to achieve separation. The p- and m-hydroxybenzoic acids were eluted and again developed with the benzenepropionic acid-water 2 : 2 : 1 ~ y s t e m . ~The pure monohydroxy acids were counted in a thin window GM counter from aluminum planchettes after adjusting the amount with inactive material to 5 X mole. A drop of a detergent was necessary to achieve a uniform layer. The paralysis time of the counter was 400 psec. and has been taken into account.8 The counter was calibrated using an aliquot from the known stock solution under the same conditions. The dihydroxybenzoic acids were estimated by adding inactive synthetic dihydroxy acids to the xrradiated solution which was then separated directly using the benzene-propionic acid-water system. Four distinct spots were obtained from the six different isomers: the 3,5-dihydroxy derivative with Rf = 0.13, the 3,4.dihydroxy derivative (Rf = 0.22), the 2,5-dihydroxy derivative (Rf = 0.32), and a fourth spot of a higher Rf value which contained the 2,4dihydroxy acid (Rf = 0.46), the 2,B-isomer (Rr = 0.43), and probably the 2,3-isomer, whose Rf = 0.41 according to Acheson and Hazelwood. For the carbon dioxige measurements the irradiated solution was connected t d a line beyond a known amount of inactive sodium carbonate and before two traps of warm barium hydroxide solution. The carrier carbon dioxide was adjusted so as to reach a barium carbonate thickness of 6 nig./cm.2. Excess of hydrochloric acid was added to the irradiated solution and to the sodium carbonate solution and nitrogen bubbled through. The barium carbonate was filtered off and counted. Results were corrected for self-absorption.* Hydrogen, formed in deaerated solutions, was estimated manometrically with the aid of a JIcLeod gauge.
Results The production of salicylic acid as measured colorimetrically was linear with dose up to about 30,000 rads a t all the concentrations studied. G-Values (cal(3) G. R. Hall and M. Streat, J . Imp. Call. Chem. Eng. SOC.,13, 80 (1961). (4) F. D. Snell and C. T. Snell, "Colorimetric Methods of Analysis," Vol. 111, 3rd Ed., D. Van Nostrand Co., Inc., New York, N. Y., 1953, p. 409. ( 5 ) (a) I. Loeff and G. Stein, J . Chem. Soc., 2623 (1963); (b) G. M . Eisenberg, Ind. Eng. Chem., Anal. Ed., 15, 327 (1943). (6) A. M. Downes, Australian J . Chem., 11, 154 (1958). (7) R. M. Acheson and C. M. Hazelwood, Bioehim. Biophys. A d a , 42, 49 (1960). (8) R. ,+. Faires and R. H. Parks, "Radioisotope Laboratory Techniques, George Newnes, Ltd., London, 1960.
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ISRAEL LOEFFAND A. J. SWALLOW
4$ 0 '4
10-3 2 5 10-2 2 5 10-1 Electron fraction of benzoate ion.
culated for the total dose absorbed by the whole solution) after a dose of 20,000 rads are given in Fig. 1 as a function of the electron fraction of (benzoate)-. The reproducibility is about f.2-3% in dilute solutions but only about +lOyoa t higher concentrations. At the highest concentration the G-value could not be determined accurately because of the high corrections needed, and only a lower limit was obtained. Nevertheless, Fig. 1 shows that the yield of salicylic acid increases with concentration up to about 0.05 electron fraction (0.5 M ) and then decreases to quite low values. The influence of impurities in the sodium benzoate used was checked by comparison with calcium benzoate which had been recrystallized several times. The Gvalues for salicylic acid obtained colorimetrically M and a t a dose rate for a concentration of 5 X of 850 rads/min. were for sodium benzoate G = 0.60 and for calcium benzoate G = 0.57, showing that a t this concentration a t least any impurities had produced a negligible effect. Hydroxylation of the benzoate ion was also studied M soluby the C14 method. Except for the 5 X tions where the experiment was done in triplicate, the results are for single irradiations to a dose of 40,000 M rads (Table I). At the 1 X lopa M and 5 X concentrations the G-values for salicylic acid as measured by the C1*method are lower than the coloriinetric values, perhaps because of the higher doses given. At higher concentrations the G-values are in good agreement. The C14 method could not be used a t the highest concentrations because the products could not be separated by the paper chromatographic methods used. Our results are lower than those obtained by Downes6 and somewhat lower than those of Armstrong, et al.,9 but the experimental conditions varied slightly in all the studies. The ratio of the G-values for the three monohydroxy derivatives is in agreement with the The Journal of Physical Chemistru
G(ortho isomer) C'4
0.515 f 0.03
0.865 0.97 0.56 0.38 0.28
0.875 ... 0.55
Figure 1. Corrected yields of salicylic acid as a function of the electron fract,ion of (benzoate)-. Curve shows calculated results assuming energy to be absorbed in water proportionally to its electron fraction.
G(ortho isomer) oolorimetrically
1.0 1.5 2.0 2.9
G(para isomer) C'P
0.21 0 . 2 7 rfi
0.19 0.22 i
0.005 0.315 0.40
results of Downes6 and Armstrong, et U Z . , ~ and differs from the less accurate values obtained some years ago by Loebl, et a l l 0 The value G(para isomer) = 0.65 obtained recently by Sakumoto and Tsuchihashill for benzoic acid a t a concentration of 2.4 X low2M a t high doses (>IO6 rads) is higher than the other recent values, and the reason for this is not clear. I n a single experiment using the C14 method and long counting periods the following yields for the dihydroxyM solucarboxylic acids were obtained in a 5 X tion: G(3,4- isomer) = 0.048, G(2,5- isomer) = 0.018, G(3,5- isomer) = 0.019, and for the sum of the 2,4-, 2,6-, and 2,3-dihydroxybenzoic acids, G = 0.031. These yields are much lower than those obtained previouslyll a t very high doses. This would indicate that they are mainly or completely secondary products. In fact qualitative results indicate12 that an irradiated solution of salicylic acid yields the 2,3-, 2,4-, and 2,sdihydroxybenzoic acids, the m-hydroxy isomer gives the 2,3-, 3,4-, and 2,5-dihydroxy isomers, while the p-hydroxybenzoic acid gives the 3,4- isomer and an unidentified fluorescing product with Ri = 0.12 in the benzene-propionic acid-water system. An aldehyde, not discovered previously, has been found in aerated solutions irradiated with y-rays. Paper chromatographic analysis showed it to be a dialdehyde, very similar to that found in an aqueous solution of benzene irradiated in the presence of oxygen.13 However, the dialdehyde in the present case could be a much more complicated mixture because of ~~~p
(9) W. A . Armstrong, B. A. Black, and D. 1%'.Grant, J . P h w . Chem., 64, 1415 (1960); W. A. Armstrong, R. A. Facey, D. W. Grant, and W. G. Humphreys, Radiation Res., 19, 120 (1963). (10) H. Loehl, G. Stein, and J. Weiss, J . Chem. SOC.,405 (1951). (11) A. Sakumoto and G, Tsuchihashi, Bull. Chem SOC.J a p a n , 34, 663 (1961). (12) C. Capellos and A. J. Swallow, unpublished work. (13) G. Stein and J. Weiss, J . Chem. Soc., 3265 (1951).
RADIATION CHEMISTRY OF SODIUM BENZOATE SOLUTIONS
the possibility of additional isomerization due to the carboxylic group. Quantitative measurements were made a t a dose of 25,000 rads with the assumption that the molar extinction coefficient a t 390 mp is the same as that found experimentally for the niucondialdehyde ( E 8240).4 The change in G-value with electron fraction is sho.wn in Fig. 2. The relative Gvalues are of more significance than the absolute Gvalues, although if G( -benzoate) a t 5-12 X M == 2.6,'jthe absolute values seem to be quite reasonable. Hydrogen peroxide yields a t a dose of 25,000 rads are shown in Fig. 3. The values in dilute solutions agree very well with those obtained previously using the Is- n z e t h ~ d . ~ Decarboxylation yields a t 42,000 rads are shown in M (G = 0.74) is the same Fig. 4. The yield a t 5 x as that obtained by I)ownes6and by Armstrong, et al.,9 but the yield varies with concentration even in the range studied previously.6 The formation of hydrogen was studied in deaerated solutions irradiated with 200 KVP X-rays. Yields were initially high and leveled off gradually with dose. The reason for this is not known but may be associated with impurities present in the solute, Figure 5 shows the results obtained aSter a dose of approximately 2 >< lo5rads, by which dose the hydrogen yield had become independent of dose.
o.2 (1.10' 10-4
I - I
I I I I I $*H4M)~ I I
1 C S . ~ ~ I~ ~I )/S;I,O~~M,
I (P.?M)l 11 (Z~)C?.l,yl~ I
5 10-4 2 5 io-? z 5 10-2 2 Electron fraction of benzoate ion.
Figure 4. Yields of carbon dioxide as a function of the electron fraction of (benzoate)-.
I lps;hf),gI~tpk5M)(ZqM\I I I I L
A 1 X 10W3M solution of benzoic acid (A.R. grade) in 0.8 N HzS04after a dose of approximately 50,000 rads gave G(Hz) = 0.78, compared to the result of Armstrong, el al., of 0.62 under similar condition^.^ I I(6.,.rp*l
10-9 :! 5 10-2 2 5 Electron fraction of benzoate ion.
Figure 2. Corrected yields of dialdehyde as a function of the electron fraction of (benzoate)-. Curve shows calculated results assuming energy to be a'bsorbed in water proportionally to its electron fraction.
5 10-3 2 5 10-2 z 5 Electron fraction of benzoate ion.
Figure 3. Corrected yields of hydrogen peroxide as a function of the electron fraction of (benzoate)-. Curve shows calculated results assuming energy t o be absorbed in water proportionally t o its electron fraction.
Discussion It seems to be established that the primary radiationinduced attack on an aromatic ring in dilute aerated aqueous solution is by addition of OH radi~a1s.I~The detailed mechanisin for hydroxylation is still uncertain, but one new fact which must be taken into consideration is that the dialdehyde formed is a major primary p r o d u ~ t . ~ "With regard to the decarboxylation in dilute solution, the mechanism previously proposeds which consists of electron transfer from the benzoate anion to a hydroxyl radical seems to be unlikely because of the very high rate constant for the addition of OH to the aromatic ring,14 and because no pH effcct on the decarboxylation process could be found.g A more plausible explanation seems to be that more OH radicals are added a t the ortho position than one would guess froin the yield of salicylic acid. The reactions of such a radical (14) L. M. Dorfman, 36, 3051 (1962).
I. A. Taub, and R.E. Btihler, J . Chem. Phus.,
Volume 68, Number 9
ISRAEL LOEFFAND A. J. SWALLOW
would then lead to decarboxylation. The effect of oxygeng could then be explained by addition of oxygen to the carboxylate-substituted carbon
or a t the ortho position on a ineta OH-added radical
OH H and their subsequent decomposition to give carbon dioxide. If such an explanation is correct then the electrophilic character of the hydroxyl radical cannot be established from the ratio of the hydroxylated isomers formed by the action of OH radicals on aromatic compounds16: the ratio must be amended to take into account loss of the functional group and dialdehyde formation. Although some controversy still exists with regard to the nature of the reducing radical in neutral solutions,I6 the radical and molecular yields in dilute neutral solution do not seem to be very different from those previously accepted.” The product yields reported here for dilute solutions (e.g., 5 X 10-3 M ) are in fair agreement with these values if we assume that all OH radicals add to the benzene ring giving radicals which add on oxygen. Some of these radicals decompose unimolecularly to give hydroxybenzoates and HOz(O2-), and some decompose to give phenol, Con, and HO,. The rest of the radicals react with HOz to give the dialdehyde, water, and oxygen. All hydrogen atoms and hydrated electrons add to oxygen to give HO2. HOz radicals disproportionate to hydrogen peroxide and oxygen. On this mechanism the sum of Ghydroxybmzoates‘, Gdialdehyde, and GCO, is equal to GOHI and the sum of G d l a l d e h y d e and twice GH~O,minus Ghydroxyhonzoates,GcOz and the molecular yield of hydrogen peroxide should be equal to GH + electron. GoH = 1.20
+ 0.5 + 0.75 = 2.4518
GH + electron = 0.5 5.12 - 1.20 - 0.75 - 0.8 = 2.87 These values compare with 2.05 and 2.75 as reported by Hochanadel and Lind.J7 This correspondence, The Journal of Physical Chemistry
although not perfect, shows that the yield of unknown products cannot be considerable. The results a t higher concentrations show that no hydroxylation takes place from energy absorbed in the solute. Figure 1 shows the strong decrease in salicylic acid yields a t higher concentrations compared to a theoretical curve calculated on the assumption that hydroxylation takes place only from the energy absorbed in water and that this energy is as often assumed proportional to the electron fraction of water in the solution. The theoretical values have been calculated by multiplying the electron fractions by the niaxiinum experimental values, divided by the electron fraction of water at this maximum, assuming that only a t this highest value there is full radical scavenging leading to the particular product. Actually, outer electron fraction seems to provide a more correct estimate of energy partition in the general case,l9 but the difference between the two fractions is a t most 10% in the present instance. It can be seen that the results do not fit the simple theory and are in fact lower than the theoretical curve. Any interference with radical recombination in the spurs would be expected to lead to yields which are higher than theoretical rather than lower so that the lowering in yields in the concentrated solutions is all the more striking. The dialdehyde yields are given in Fig. 2, compared again to a theoretical curve calculated on the same assumptions. I n this case also a considerable deviation of the experimental results from the theoretical curve can be noted. The hydrogen peroxide yields represent a superposition of molecular yields as diminished with concentration and the hydrogen peroxide formation accompanying the formation of other products. Decarboxylation a t higher concentrations is doubtless the only process studied in this system which is produced by a direct effect. However, contrary to direct effect decarboxylations in the aliphatic acids, 2o the increase in carbon dioxide yields is not linear with concentration. The fact that in solid benzoic acidz1 _
(15) R. 0. C. Norman and G. K. Radda, Proc. Chem. Soc., 138 (1962). (16) J. T. Allan and C. Scholes, Nature, 187, 218 (1960); G. Ceapski and A. 0. Allen, J . Phys. Chem., 66, 262 (1962). and others. (17) C. J. Hochanadel and S. C. Lind, A n n . Rev. Phys. Chem., 7, 91 (1956) (18) 1.20 = the sum of hydroxybenaoates, from Table I, normalized to the value obtained for o-hydroxybenzoates by the colorimetric method. (19) A. J. Swallow, Proceedings of the International Conference of Radiation Research, Natick, Mass.. Jan., 1963, p. 49. (20) W. M. Garrison, et al.. J . Am. Chem. SOC.,77, 2720 (1955). (21) B. M.Tolbert and R. M. Lemmon, Radiation Res., 3, 52 (1955).
RADIATION CHEMISTRY OF SODIUM BENZOATE SOLUTIONS
G(C02) is only 0.29 is not surprising as similar phenomena have been observed in other systems.22 The decrease in hydrogen yields with increasing concentration represents mainly the scavenging of precursors of molecular hydrogen by the solute possibly together with some (other effects which cannot easily be isolated. Finally, the disagreement between the observed and the calculated results for the salicylic acid and dialdehyde yields can be explained by the occurrence of a more complex energy-absorption mechanism than that assumed, provided that any possible impurities in the benzoate do not play an important role even at higher concentrations. The fact that these yields decrease strongly a t high concentrations and the steep increase in direct effect carbon dioxide formation tempt one to assume tentatively that some “energy transfer” from the solvent to the aromatic solute is possible even in this aqueous system. However, it cannot be decided from the present experiments whether this takes the
form of some kind of preferential uptake of radiation energy by the n - e l e c t r o n ~or~ ~of~ some ~ ~ other mechanism. The fact that such a n “energy transfer” (or “protective effect”) could not be found in aqueous solutions of phenolz4 is not surprising as these authors did not use concentrations higher than 0.5 M .
Acknowledgment. The authors wish to thank the International Atomic Energy Agency, Vienna, for B fellowship and the Friends of the Hebrew University, London, for financial assistance for I. L. Thanks are also due to Mr. C . Capellos for his help and advice in the C1* measurements and to Prof. G. R. Hall for his interest in this work. (22) M. A. Proskurnin and Y . M. Kolotyrkin, Proc. Intern. Conf. Peaceful Uses At. Energy, Pnd, Geneua, 29, 52 (1958). (23) J. Lamborn and A. J. Swallow, J . Phys. Chem., 65, 920 (1961); M. Inokuti, R. L. Plateman, and A. J. Swallow, work in progress. (24) K. C. Kurien, P. V. Phung, and M . Burton, Radiation Res., 11, 283 (1959).
Volume 68, Number 9