1418
Z. D. DRAGANI~, I. G. DRAGANI~, AND M. M. K O S A N I ~
their sincere thanks to Drs. Verner Schomaker and Jule Rabo for many helpful discussions and constructive
criticism, and to Dr. Edith Flanigen for many of the zeolite samples.
Radiolysis of Oxalate Alkaline Solutions in the Presence of Oxygen
by Z. D. Draganid, I. G. Draganit, and M. M. Kosanii: Boris Kidric Institute of Nuclear Sciences, Vinza, Yugoslavia (Received September 94, 106'6)
The radiation yields of carbon dioxide, molecular hydrogen, and hydrogen peroxide were measured over the pH range from 9.5 to 14 in aqueous solutions of various oxalate and oxygen concentrations. The results obtained confirmed the change in the nature of hydroxyl radical with increasing pH. The basic form is replaced by ion radical 0- whose fate is determined in reactions with oxygen molecules and oxalate ions. The rate constant ratio ko-+o,/ko-+C20,2-was calculated from the experimental data. A cube root dependence of the molecular hydrogen yields, measured as a function of the oxalic acid concentration, was observed. The experimental results were quantitatively consistent with a simple reaction scheme which allowed determination of the primary yields in water y radiolysis in the pH region 9.5-14.
lar or in free-radical yields over a pH region from neuIntroduction tral to alkaline. In radiation chemistry of aqueous solutions there Evidence has been accumulating also concerning the is an increasing interest in the pH influence on the nature of the hydroxyl radical in alkaline media. radical yields in alkaline medium. Haissinsky and Hochanadel" has confirmed the existence of the 0his collaborators studied tellurous acid, platinum salts,*potassium i ~ d a t eand , ~ potassium ~ermanganate.~ ion radical in his study of the radiolysis and photolysis of hydrogen peroxide solutions. This species was As suggested in experiments with phosphite^,^ all originally proposed by Hart, et aZ.I2 Its existence was these systems indicate that after pH 12 the radical yields increase while the molecular yields decrease. Dainton and Watt6,' studied aqueous solutions of (1) M .Haissinsky and P . Patigny, J . Chim. Phys., 59, 675 (1962). acrylamide and potassium ferrocyanide and ferricya(2) M. Haissinsky, ibid., 60, 402 (1963). nide, with and without NzO. They found that the (3) M. Haissinsky, J. JovB, and W. Szymanski, ibid., 61, 572 (1964). molecular product yields remained practically un(4) M. Haissinsky and J. Petit, ibid.,62, 222 (1965). changed in alkaline media but that the free-radical (5) M. Haissinsky, ibid., 62, 224, 1141, 1149 (1965). yields increased. Similar results were obtained at (6) F. S. Dainton and W. S. Watt, Nature, 195, 1924 (1962). pH 13 by Hughes and Willjss in the ferroferricyanide(7) F. S. Dainton and W. S. Watt, Proc. Roy. SOC.(London), A275, 447 (1963). methanol system. Hayong recently found in different (8) G . Hughes and C. Willis, Discussions Faraday SOC.,36, 223 alkaline solutions an increase in the primary free(1963). radical yields and a decrease in molecular product (9) E. Hayon, Trans. Faraday SOC.,61, 734 (1965). yields. Cheek and Linnenbom'O studied hypobromite (10) C. Cheek and V. Linnenbom, J . Phys. Chem., 67, 1856 (1963). solutions and found no change either in primary molecu( 1 1 ) C. J. Hochanadel, Radiation Res., 17, 286 (1962). The Journal of Physical Chemistry
RADIOLYSISOF OXALATE ALKALINESOLUTIONS
confirmed by the recent pulse-beam experiments of Czapski and Dorfmanls and Rabani and M a t h e ~ 0 n . l ~ Supporting the existence of this species we also cite the experiments by Hughes and Willis* as well as some of our own observations on the pH influence on decomposition of oxalate ions in absence of oxygen.ls However, only Adams, et ul.,lBhave measured some relative rate constant ratios for the 0- ion radical. Our earlier experiments" showed that radiationinduced decomposition of oxalate ions, in oxygenated aqueous solutions, was practically due only to reaction with hydroxyl radicals. These experiments were carried out a t pH lo. The purpose of this study was to use the simple reaction scheme and the complete material balance for the determination of the primary free-radical and molecular yields in the pH region 9.5-14. The competition data enabled calculation of the relative rate constant ratio for the 0- ion-radical reactions with oxygen molecules and oxalate ions.
Experimental Section Solutions. The water used was triply distilled in a continuous Pyrex system (alkaline permanganate, acid dichromate, and finally without any additive). The oxalic acid (Merck AR) was recrystallized before use. The alkaline solutions were made by means of sodium hydroxide, freshly prepared by dissolving sodium (Carlo Erba AR) in triply distilled water in a nitrogen atmosphere. The pH of the solution was measured by a Beckman GS pH meter with an E2 electrode. The introduction of the given amount of oxygen and sodium hydroxide in deaerated solution was performed by standard techniques. The ampoules were completely filled, leaving no gas space. The solubility of oxygen varied with the concentration of oxalic acid and with the pH of the solution. The initial oxygen concentration in 47 mM solutions was 0.60 f 0.02 mM. In studies of the oxygen concentration influence, it varied from 0.3 to 0.8 mM. In experiments with 14C, the solutions were saturated with atmospheric air. Irradiations. The samples were irradiated in the 2-kcurie irradiation source a t VinEa. The absorbed dose rate, as determined with a Fricke dosimeter [G(Fe3+) = 15.5 3, was 2.1019ev ml-l hr-l. Analyses. The gas products, H2 and COZ,and the Oa initially present were determined on a Perkin-Elmer 154 DG gas chromatograph. Argon or helium was used as carrier gas on a silica gel column.1g Since careful sample preparation did not give reproducible
1419
initial concentrations of C02, we paid special attention to the determination of blank corrections. Generally, ten samples were prepared as a series and three or four unirradiated samples of each series were analyzed for GOz. The observed COz content was considered the blank and was plotted at the zero absorbed dose. The amount of H ~ O Zwas measured spectrophotometrically by the iodide method developed by Ghormley.*O The molar extinction coefficient a t 24" was determined to be 25,810 l. mole-' cm-'. The presence of oxalic acid slowed down the iodide oxidation so that the optical measurements were made about 30 min after the preparation of the solutions. An unirradiated sample of oxalic acid was used as a reference solution. Reference samples were prepared and measured simul-
3
62
1
I
0
"
Y
Y
60
100 [H,C:Od, mM.
Y
150
Figure 1. Influence of oxalate ion concentration on measured yields of HzOZ,COZ,and HZa t pH 12.0; 02 present decreasea from 0.61 to 0.52 mM with oxalate concentration: X, HzOZ;0, COZ; and A, Hz.
(12) E.Hart, S. Gordon, and D. Hutchison, J. Am. Chem. Soc., 7 5 , 6165 (1953). (13) G. Czapski and L. Dorfman, J . Phys. Chem., 68, 1169 (1964). (14) J. Rabani and M. Matheson, J . Am. Chem. SOC., 86, 3176 (1964). (16) I. Draganic, J . Chin. Phys., 56, 18 (1959). (16) G. E.Adams, J. W. Boag, and B. D. Michael, Trana. Furaduy Soc., 61, 492 (1965). (17) Z.D. Dragani6, I. G. Dragani6, and M. M. Kosani6, J. Phy8. Chem., 68, 2086 (1964). (18) B. Radak and I. Draganid, Bull. Inst. Nucl. Sci. Inat. "Boris Kid&'' (Belgrade), 13, 77 (1962). (19) Lj. Petkovid, M. Kosanih, and I. Draganic, ibid., 15, 9 (1964). (20) A. 0.Allen, C. J. Hochanadel, J. A. Ghormley. and T. W. Davis, J . Phus. Chem., 56,675 (1952).
Volume 70, Number 6 M a y 1966
Z.
1420
I
I
I
D.DRAQANI~, I. G.DRAGANI~, AND M. M. KOBANI~
I
I
I
4,
-x 3 -
x-
6 2 -
1 0
I
I
taneously with irradiated ones in order to avoid errors due to the oxidation of iodide by atmospheric oxygen. At pH 13-14, the solutions were first acidified with a 0.6 M oxalic acid solution and then measured in the usual way. In this case the molar extinction coefficient was 22,900 1. mole-' cm-'. The decrease in the extinction coefficient is due mainly to the presence of an excess of oxalate salts. We have noticed that the impurities in potassium iodide of different origin can also contribute to this decrease. In experiments where 14C was used as a radioactive tracer, we used NaH14COaand H14COONa(Amersham). The specific activity was 5 pcuries ml-l. The radioactivity counting was performed on a 27r proportional flow counter.
Results General. As in our previous work at pH 13. The experimental scatter of C02 results is a consequence of the relatively low yields and the high blank corrections. At pH 12-13 the correction was of the same order of magnitude as the effect measured (-3 X 10I6 molecules ml-l). At pH 14 this ratio is so unfavorable that measurement of G(C02) was considered meaningless and therefore not done. Products in Gas Phase. The concentrations of the gas products formed increased linearly with dose, as seen in Figure 3. The radiation yields decrease from 3.9 to 1.6 when the initial pH increases from 9.5 to 13. By adding different amounts of C02 before irradiation, the carbon dioxide content was varied in some cases severalfold. No visible influence was observed on the yields measured. The hydrogen yield in 47 mM oxalate solutions was found to be 0.4, independent of the changes in pH value in the studied region. However, as seen in Figure 4, there is a cube-root dependence on the initial oxalate concentration. It allowed us to derive from the intercept the value 0.44as the initial yield. Products in Liquid Phase. The linear part of the
0.2
I
0
0.1
0.2
0.3
0.4
0.5
0.6
[&CZO~]~/:.
Figure 4. Cube root plot: measured molecular hydrogen yields a t yI3 12, as a function of oxalate ion concentration. Oxygen present: 0.61-0.52 mM.
H20zus. dose curve was found to be considerably shorter in alkaline than in neutral and acid media. After an absorbed dose of 0.7 X lo1*ev rnl-l, the measured yields decrease with increasing dose. In calculating the initial yields we have used the corrected values, obtained as G(H202)readings at zero dose on diagrams where H202 yields were plotted against dose. As can be seen in Figure 2, the hydrogen peroxide yields are pH independent and, within experimental error, equal to 3.10. We have obtained in our preceding work” somewhat lower G(H202) values for pH 9.5 and 9.8. The reason for this was that in calculation we used the uncorrected values obtained by measurements at doses higher than 0.7 X lo1*ev rnl-l. We attempted to prove the presence of ozone in Volume 70,Number 6 M a y 1986
Z. D. DRAGANI~, I. G. DRAGANI~, AND M. M. KOSANI~
1422
irradiated solutions at pH 12.0, 12.5, and 13.0. These measurements were made according to Taube and Bray's methodlZ15 or 10 sec after irradiation. No ozone could be measured. Figure 5 shows data from measurements of the radioactivity of oxalate precipitated after irradiation. In oxalic acid solutions of different pH values, carbon dioxide or sodium formate labeled with 14C were used as the source of carboxyl radicals. It is seen that no activity higher than the background was measured. It was easy, however, to follow the formation of labeled oxalic acid in the absence of oxygen.22 The radioactivity of the samples was a function of the dose and of the other relevant parameters such as the pH value of the solution and the source of the carboxyl radical. Possible sources considered were carbon dioxide and formate ion.
Discussion Mechanism. These results, as well as earlier studies
7
+ OH
OH-
+ C02 + COO-
(1)
We have neglected the relatively slow reaction with short-lived reducing species, evident from the small G(H2) decrease shown in Figure 4. Observations in acid medium, where this decrease was much more pronounced, have shown that this reaction may be neglected. An intermediate, probably HOOC-C(OH)2, is formed which reacts with oxygen, producing oxalate and 0 2 - , the net effect being as if oxygen has reacted directly with short-lived reducing species. The data for G(C02), presented in Figure 2, show that the decomposition of oxalate ions decreases strongly with increasing pH. The re-formation of oxalic acid is not likely to be the reason for this decrease, as confirmed by experiments with carbon-14 as tracerZ2(Figure 5 ) . Also, no change in the nature of oxalate ion should occur as the oxalic acid is completely dissociated after pH 6. Hence, the decrease observed could be explained only by the change in the nature of oxidizing short-lived species in reaction 1. The scavenging of OH radicals by OH- ions
9
10
11
12
13
PH. Figure 5. Radioactivity of oxalate precipitates : pH influence. 0,Hl'COONa; 0,W O n ; solid symbols denote blanks.
all pH values studied indicate its formation by the following reactions. 0 2
at pH 9 and given in the preceding work’? at pH
