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On the γ-radiolysis of aqueous solution of cerium (III) nitrilotriacetate
Radiat. Phys. Chem. Vol. 24, No. 2, pp. 233-237, 1984 Printed in the U.S.A.
0146-5724/84 $3.00 + .00 Pergamon Press Ltd.
ON THE y-RADIOLYSIS OF AQUEOUS SOLUTION OF CERIUM(III)NITRILOTRIACETATE B. K. SI-LA.R~J" Department of Chemistry, University of Delhi, Delhi 110007, India and R. GUPTA Department of Chemistry, Shyam Lal College, University of Delhi, Delhi 110032, India
(Received for publication 2 December 1983) Abstraet--Radiolysis of aqueous solutions (pH 5.5-10.0) of cerium(III)nitrilotriacetate (C) has been studied both in deoxygenated solutions and in the presence of various scavengersfike t-butanol, N20 and Br-. Both H and OH radicals directly degrade the ligand and H202 oxidises the central metal ion. The radiolytic products iminodiacetic acid (IDA), Ce(IV) and the carbonyl compounds formaldehyde and glyoxalic acid and their yields are consistent with the proposed mechanism.
INTRODUCTION STUDIES on the radiolysis of aqueous solutions of cerium(Ill) chelates with various aminopolycarboxylic acids as ligands are scant, v-4) Ce(III)NTA, (C), is unique because unlike TTHA, EDTA and Citric acid used earlier(~-5)for chelation of Ce(III), the present ligand NTA is such that its radiolytic end-product iminodiacetic acid (IDA) can be quantitatively estimated. Since IDA is formed mainly through OH attack on the ligand, one can really form an unambiguous opinion about the centre of attack (i.e. metal ion or the ligand) of the latter. EXPERIMENTAL Materials Cerium(III)sulphate octahydrate, nitrilotriacetic acid, chromotropic acid and v.11other reagents used during the investigation were of A.R. grade. Triple distilled water was used for preparing all solutions for irradiation. In the oxygenated solutions, pure 02 gas was continuously passed through the solutions being radiolysed, while deoxygenation was carried out by nitrogen gas saturation. Nitrous oxide used was of anesthetic grade.
Irradiation Irradiations were carried out with a ~Co ),-source 0,-cell-220, Atomic Energy of Canada Ltd.). The dose-rate, O.64_+0.01 x 10'7eVmin-lg-l), was determined by Fricke's ferrous sulphate dosimeter. G(Fe3+) was taken as 15.6. (~)
tAuthor to whom all correspondence should be addressed.
Analysis The (1:1) complex Ce3+-NTA was obtained by mixing 2 x 10-3 M solutions ofcerium(III) sulphate and NTA. The pH of the solutions was adjusted with carbonate-free NaOH. Beckmann model DU spectrophotometer was used for O.D. measurements. The radiation-induced decomposition of the chelate was obtained from the O.D. at 274 nm (E = 850). Iminodiacetic acid (IDA) was determined by the ninhydrin method.0) H202 was determined with titanium sulphate reagent(s,9) whereas the carbonyl compounds, formaldehyde (HCHO) and glyoxalic acid (CHO.COOH) were estimated by the standard procedure<10,n) the amount of Ce(IV) formed was determined with ferrous ammonium sulphate and O-phenanthroline.O2) The Ce(IV)NTA complex was prepared by oxidation with H2Ov RESULTS A N D DISCUSSION Radiolysis of Ce(III)NTA was done in the pH range 5.5-10.0; below pH 5.5 and above pH 10.0, the complex was unstable.°3) Figure 1 shows the absorption spectra of free NTA together with those of Ce(III), Ce(IV) and their corresponding NTA complexes. The characteristic absorption wave length and its molar absorbance for each compound is given in Table I. The variation in pH between 5.5-10.0 was found to have no effect on the nature of spectra of the chelate. Figure 2 shows the effect of irradiating a 0.66 x 10-3 M Ce(III)NTA solution in deoxygenated medium at pH 9.5. Curve (a) of Fig. 2 represents the absorption spectra of unirradiated solution of Ce(III)NTA and curves (b) and (c) are those for 233
234
B.K. SHARMAand R. GUPTA
Corlc. ( a ) I x IO-'~M ( b ) I x IO'~M ( c ) 0 . 6 5 x IO'~M
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FIG. 1. Absorption spectra of (a) Ce(III) and (b) NTA at pH 4.0; (c) Ce(III)NTA, (d) Ce(IV) and (e) Ce(IV)NTA at pH 6.5; 1 cm light path.
TABLE I. SPECTRALDATAFOR Ce(III), Ce(IV) ANt) THEIR NTA COMPLEXES Ions
Ce(III) Ce(IV) Ce(III)NTA Ce(IV)NTA
k m a x nm
M - ~cm- i
252 320 274 274
665 5140 850 5700
220
1
1
1
I
240
2£X)
280
300
320
X(nm)
FIG. 2. Absorption spectra of 0.66 x 10-3 M Ce(III)NTA: Unradiolysed (curve a) and radiolysed to a dose 1.96x 101SeVml-I at pH 6.5 (curve b) and at pH 9.5
(curvec).
o,'-1 '5
solutions irradiated to a dose of 1.96 x 101~eV m l - t at pH 6.5 and 9.5, respectively. As can be seen from these curves, the oxidation of the chelate increases with the increase of pH and curves (b) and (c) resemble curve (e) of Fig. 1 for synthetic Ce(IV)NTA. Hence, during the radiolysis of aqueous solutions of Ce(III)NTA, formation of Ce(IV)NTA takes place. Further, it was observed that on addition of excess alkali, the radiolysed solutions on centrifugation or on keeping deposited a yellow precipitate of hydrous ceric oxide and the centrifugate gave test for IDA only and not for NTA. Since any free IDA formed through degradation of the ligand NTA does not significantly absorb at 274nm, the radiolytic degradation of the chelate could be estimated spectroohotometrically by the decrease in O.D. of the centrifugate (after precipitation of Ce(IV)), at this wave length. The radiolytic end-products IDA, Ce(IV) and the carbonyl compounds were estimated from the radiolysed solutions. Figures 3 and 4 show that the decrease of the complex as well as formation of the
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F1G. 3. Radiolysis of 0.5 x 10-3 M Ce(III)NTA solution at pH 9.5: G(-chelate) and G(IDA) in deoxygenated and N20-saturated medium (deoxygenated: a and b), (N20: a' and b'). radiolytic products are linear with dose up to 4.4 x 10Is eV m l - t in deoxygenated conditions, the G-values, as calculated from the slopes of such curves, under different experimental conditions are given in Table 2. No free H202 could be detected in the radiolysed solution. The primary radical and molecular yields(6~ of
On the ~,-radiolysis of aqueous solution of cedum(III)nitrilotriacetate ]
235
oxidation state, oo In deaerated conditions, the dehydrogenated radical intermediates of (1) and (2) are likely to be stabilised through electron disproportionation amongst themselves either by reactions (3) or (4), leading to the formation of I D A and a carbonyl compound
I0
x
e---I x
(3) 2Ce 3+-R2N(~H'COOH + H20 --*Ce3+-R2NCH2"COOH + Ce3 +-R2NH + CHO,COOH
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(4) 2Ce 3+_R2N(~H.COOH + H20 ~ C e 3+-R2NCH2.COOH + Ce 3+ -R2NH + CO2 + H.CHO
40
Dose x IO-"(eV mC ~I F;o. 4. Formation of Olyoxalic acid (curves a and a') and
Ce(IV) (curve b and b') in the radiolysis of 0.5 x 10-3M Ce(III)NTA at pH 9.5 in deoxygenated and N20-saturated solutions, respectively. water radiolysis in the pH range 3.0-10.0 are GoB = 2.7, G(~.~)= 2.7, G(H)=0.5, G(,2) = 0.45, G(.2o2)= 0.7.
where Ce 3÷-R2NH represents the Ce3÷-IDA molecule. Since glyoxalic acid, and not formaldehyde, is the product formed on radiolysis of Ce3+-NTA, the stabilisation of the intermediate radical occurs through reaction (3) and not (4), and accordingly, both G(-Ce 3+ - N T A ) and G(IDA) should be equal to ½G(,+o,) = 1.6; but our observed G-values (Table 1) are higher and may be due to the occurrence of reaction (5) followed by reactions (2) and (3).
Alkaline Medium (pH 9.5) During the radiolysis of NTA, °4' m it has been shown that H and OH radicals (and not e~) abstract a hydrogen atom from the carbon atom ~ to the carboxylic group and a similar mechanism can also be assumed in the radiolysis of Ce 3+-NTA. Reactions with H and OH radicals may, then, be represented by equations (1) and (2) leading to the formation of a dehydrogenated radical intermediate (1)
Ce3÷-R2NCH2'COOH + H ---*Ce 3÷-R2N(~H'COOH + H2
(2)
Ce 3+-R2NCH2.COOH + OH ---*Ce3+-R2N(~H'COOH + H20
where Ce 3+-R2NCH2.COOH represents the Ce3 + - N T A molecule. The direct reduction of the central metal ion, Ce 3÷, by H and e~ does not seem to be possible since cerium is not known to exist in the stable + 2
(5)
Ce3+-R2NCH2.COOH + H202 ~Ce4 +-R2NCH2"COOH + OH + O H -
Since reaction (5) generates an equivalent amount of OH radicals, G(-Ce 3+-NTA) and G(IDA) should be i equal to ~G(H+ oH + ,2o:)= 1.95 which now well agrees with the observed G-values. The occurrence of reaction (5) is supported by the observed value of. G(Ce 4+) - 0 . 7 5 , which is equal to G(,:o:) and seems to suggest the oxidation of Ce 3+ by H202 and not by OH radicals. In the light of the observed results, e~ might disappear through a fast °7) recombination process. In view of the redox potentials of Ce(III)/Ce(IV) and O H - / O H couples, 08) one would expect oxidation of Ce(III) by OH radicals, and in fact pulse radiolytic studies ~9) report the rate constant for this reaction as 7.2 x 1 0 7 m - i s -I. It, therefore, may be suggested that OH radicals should oxidise Ce(III) of the chelate "C" (reaction 6). However, our observed G(Ce 4+)
T^aL£ 2. TrIE OeSEgVEDG-VALUr.SIN THE RADIOLYSISOF Ce(III)NTA Conditions
pH
G(-Ce3+-NTA)
G(IDA)
G(Ce4+)
G(CHO.COOH)
Deoxygenated Nitrous Oxide Bromide ions t-butanol Deoxygenated
9.5 9.5 9.5 9.5 6.5
2.1 3.2 0.6 0.5 1.8
2.0 3.1 0 0 1.9
0.75 0:7 0.6 0.6 0.3
0.5 2.0 0 0 1.1
236
B.K. SHARMAand R. GUPTA
and G(IDA) values do not support this viewpoint. (6)
Ce(III)NTA + O H - , C e ( I V ) N T A + OH -
The absence of reaction (6) could be explained as due to the higher probability of reaction (2), since the rate constant of the reaction of OH radicals with free NTA (pH 9.0) is much higher, i.e. 2.5 x 1 0 9 M - ~s - 1,(2o)than that for OH radicals reacting with Ce(III). Moreover, the rate constant of the reaction of OH radicals with the ligand is expected to be further increased on chelation as has also been observed by Bhattacharya et al. (21,22)in the radiolysis of NTA chelates with Co(II) and Ni(II). Thus, in view of the values of these rate constants of reactions of OH radicals on the two possible, alternate sites of attack of the chelate "C", H-abstraction by OH from the ligand site (reaction 2) is preferred over the oxidation of the metal ion by OH radical (reaction 6). One need not invoke the idea of shielding (or steric hindrance) of the metal ion by the ligand, as has been suggested by some workers in the radiolysis of Ce(III) chelates with EDTA 1,3'4)and TTHA, (2) to account for non-occurrence of reaction (6).
Effect of nitrous oxide When radiolysis was done in the presence of nitrous oxide, an e~ scavenger, °3) an increase of OH radicals took place due to reaction (7) (7)
e~ + N20 H2O N2 + OH + O H - .
This was actually found to be so since no detectable IDA or carbonyl compounds could be found (since G(H) at this pH is only 0.5), when a deoxygenated, aqueous solution of Ce3+-NTA (0.5 x 10 -3 M) was radiolysed in the presence of 1 x 1 0 - 2 M bromide ions. G (Ce 4 +) and G (-Ce 3+-NTA) were almost similar, as expected, to G(H2o2)=0.7. Since all OH radicals are being scavenged by bromide ions, Ce 3+ - N T A complex is degraded only by molecular H202 through attack on the central metal ion (reaction 5). The fact that no carbonyl compounds could be detected in the radiolysed solutions further indicated that the oxidation of Ce(III) by H202 was probably occurring through heterolytic dissociation of the latter into OH ÷ and O H - and it was this OH ÷ which was in fact responsible for Ce(IV) production. The OH radicals produced during this process were subsequently being removed by B r - as was shown by the absence of IDA and carbonyl compounds in the radiolysed solutions.
Effect of t-butanol Another effective OH scavenger, ~25) t-butanol was also used for confirming the results obtained earlier in the presence of B r - . The alcohol concentration was kept at 1 x 10-3M and that of the chelate solution at 0.5 x 10 -3 M. On radiolysis of the mixture under deoxygenated conditions, a competition ensued between reactions (2) and (8); although, as expected, (8)
The observed G (Ce4+) = 0.7 in presence of N20 was identical with that in the deaerated system. Had OH radicals been responsible for the oxidation of Ce 3÷, G(Ce *+) would have increased in the presence of N~O. However, as expected, G(-Ce3+-NTA) increased as also the radiolytic yields of IDA and CHO.COOH (Table 1) due to enhanced degradation of the ligand by OH radicals generated through reaction (7). Hence G (--Ce3+-NTA) --- G (IDA) ~--- I G ( H +OH + e ~ + H202) ----"3.3.
Effect of Bromide ions To further confirm the mechanism proposed, it was decided to use a OH scavenger which can completely remove all these radicals instead of increasing their amount as is done by N20. Bromide ions happen to be very efficient scavenger (24) of OH radicals. Radiolysis of Ce 3+-NTA, therefore, in the presence of B r should not show any appreciable ligand degradation.
CH3-C(CH3)2-OH + OH --)(~H2-C(CH3)2-OH + H20
no detectable IDA and/or carbonyl compounds were formed (due to scavenging of OH radicals and G(H) being small). Ce(IV) was formed with G(Ce TM)= 0.6 and the chelate was degraded, G(-chelate) = 0.5. Thus, while radiolysis of Ce(III)NTA in the presence of N20 reveals that OH radicals react with the ligand NTA, experiments conducted in the presence of B r - and t-butanol show that the oxidation of the Ce(III) by H202 occurs, most probably, via heterolytic, and not homolytic dissociation of the oxidant H202. Had dissociation of H202 been homolytic resulting in the formation of OH radicals, no Ce(IV) formation would have taken place in the presence of OH scavengers (Br- and t-butanol). Such mode of oxidations by H202 were also suggested by Edwards (26)in the case of many other inorganic ions. The radiolysis of Ce(III)NTA in oxygenated medium at pH 9.5 has been described elsewhere(4) in a different context and the results again point to the mechanism mentioned before. Since the radiolysis of
On the ~,-radiolysis of aqueous solution of cerium(III)nitrilotriacetate C e ( I I I ) N T A leads to Ce(IV) production which is p H dependent, t27) experiments on radiolysis of C e ( I I I ) N T A were done at p H 6.5 also. The results (Table 2) of radiolysis of C e ( I I I ) N T A in deoxygenated medium at this p H support the proposed mechanism. The reduced G(Ce TM)value of 0.3 at this p H points to only partial consumption of H202 towards oxidation of " C " . Hence both G(--C) and G ( I D A ) would be equal to ½G(H + O H + H202) or to 1.95 and the observed G-values are in good agreement with this mechanism. The comparatively low G ( C H O - C O O H ) = 1.1 as observed by us could not be quantitatively correlated with the primary yields. REFERENCES 1. M. B. HAFEZ, H. ROUSADVand J. HAFEZ,J. Radioanal. Chem. 1978, 43~ 121. 2. M. B. HAFEZ, W. H1GAZland N. HA~z, J. Radioanal. Chem. 1979, 49(1), 45. 3. B. K. SHARMAand R. GUPTA, Radiat. Eft. Lett. 1980, 57(5), 149. 4. B, K, SHARlVtA.and R. GUI'TA, Radiat. Eft. Lett. 1982, 67(5), 147. 5. M. B. HAFEZ and B. M. AI~OELKHAIR,Radiochem. Radioanal. Lett. 1980, 53(6), 375. 6. I. G. DRAGNlC and Z. D. DRAGN1C, The Radiation Chemistry of Water, p. 218. Academic Press, New York 1971. 7. S. MOOREand W. H. STEIN, J. BioL Chem. 1948, 176, 367. 8. G. M. EISENaERG,Ind. Engng Chem., Anal. Ed. 1943, 15, 327. 9. R. M. SELLERS,Analyst 1980, 105, 950. 10. C. E. BRICKERand H. R. JOHNSON,Ind. Engng Chem. Analyt. Ed. 1945, 17, 400.
237
11. G. R. A. JOHNSONand G. SCI-IOLES,Analyst 1954, 79, 217. 12. L. GOROON and A. I. FEIeUSH, AnaL Chem. 1955, 27, 1050. 13. G. L, VARLAMOVA,N. D. MITROFANOVA,L. I. MARa'YNE~rKO, N. I. PECHUROVA and V. G. VARLAMOV, Russian J. Inorg. Chem. 1970, 15(5), 636. 14. S. N. BHATTACHARYAand E. V. SRXSANKAR,Radiat. Phys. Chem. 1976, 8, 667. 15. R. GUPTA, Ph.D. Thesis, p. !10. University of Delhi, Delhi, India, 1981. 16. F. A. CoTtoN and G. WILKINSON,Advanced Inorganic Chemistry, 3rd Edn, p. 1058. Wiley Eastern, New Delhi, 1979. 17. M. ANB^R and P. NETA, Int. J. AppL Radiat. Isotope 1967, 18, 493. 18. J. W. T. SPINKSand R. J. WOODS, An Introduction to Radiation Chemistry, p. 281. Wiley Interscience, New York, 1976. 19. L. F. DORFMANand G. E. ADAMS, Reactivity of the Hydroxyl Radical in Aqueous Solutions, NSRDS-NBS, p. 46. U.S. Department of Commerce, National Bureau Standards, Washington D.C., 1973. 20. J. LATI and D. MEYER.STEIN,J. Chem. Soc. (Dalton Trans.) 1978, 2, i105. 21. S. N. BHATTACHARYAand E. V. SRISANKARA,J. C. S. (Faraday I), 1979, 2089. 22. E. V. SRISANKARand S. N. BHATT^CHARYA,J. Chem. Soc. (Dalton Trans.), 1980, 4, 675. 23. F. S. DAINTONand D. B. P~TERSON,Nature (London) 1960, 186, 878. 24. T. J. SWORSKI,J. Am. Chem. Soc. 1954, 76, 4687. 25. G. SCHOLES,P. SHAW, R. L. WmSON and M. EaERT, Pulse Radiolysis (Edited by J. H. Baxendale, M. Ebert, J. P. Keane and A. J. Swallow), p. 131. Academic Press, New York, 1965. 26. J. D. EDWARDS,Chem. Rev. 1952, 50, 455. 27. R. GUPTA, Ph.D. Thesis, p. 115. University of Delhi, Delhi, India, 1981.
0146-5724/84 $3.00 + .00 Pergamon Press Ltd.
ON THE y-RADIOLYSIS OF AQUEOUS SOLUTION OF CERIUM(III)NITRILOTRIACETATE B. K. SI-LA.R~J" Department of Chemistry, University of Delhi, Delhi 110007, India and R. GUPTA Department of Chemistry, Shyam Lal College, University of Delhi, Delhi 110032, India
(Received for publication 2 December 1983) Abstraet--Radiolysis of aqueous solutions (pH 5.5-10.0) of cerium(III)nitrilotriacetate (C) has been studied both in deoxygenated solutions and in the presence of various scavengersfike t-butanol, N20 and Br-. Both H and OH radicals directly degrade the ligand and H202 oxidises the central metal ion. The radiolytic products iminodiacetic acid (IDA), Ce(IV) and the carbonyl compounds formaldehyde and glyoxalic acid and their yields are consistent with the proposed mechanism.
INTRODUCTION STUDIES on the radiolysis of aqueous solutions of cerium(Ill) chelates with various aminopolycarboxylic acids as ligands are scant, v-4) Ce(III)NTA, (C), is unique because unlike TTHA, EDTA and Citric acid used earlier(~-5)for chelation of Ce(III), the present ligand NTA is such that its radiolytic end-product iminodiacetic acid (IDA) can be quantitatively estimated. Since IDA is formed mainly through OH attack on the ligand, one can really form an unambiguous opinion about the centre of attack (i.e. metal ion or the ligand) of the latter. EXPERIMENTAL Materials Cerium(III)sulphate octahydrate, nitrilotriacetic acid, chromotropic acid and v.11other reagents used during the investigation were of A.R. grade. Triple distilled water was used for preparing all solutions for irradiation. In the oxygenated solutions, pure 02 gas was continuously passed through the solutions being radiolysed, while deoxygenation was carried out by nitrogen gas saturation. Nitrous oxide used was of anesthetic grade.
Irradiation Irradiations were carried out with a ~Co ),-source 0,-cell-220, Atomic Energy of Canada Ltd.). The dose-rate, O.64_+0.01 x 10'7eVmin-lg-l), was determined by Fricke's ferrous sulphate dosimeter. G(Fe3+) was taken as 15.6. (~)
tAuthor to whom all correspondence should be addressed.
Analysis The (1:1) complex Ce3+-NTA was obtained by mixing 2 x 10-3 M solutions ofcerium(III) sulphate and NTA. The pH of the solutions was adjusted with carbonate-free NaOH. Beckmann model DU spectrophotometer was used for O.D. measurements. The radiation-induced decomposition of the chelate was obtained from the O.D. at 274 nm (E = 850). Iminodiacetic acid (IDA) was determined by the ninhydrin method.0) H202 was determined with titanium sulphate reagent(s,9) whereas the carbonyl compounds, formaldehyde (HCHO) and glyoxalic acid (CHO.COOH) were estimated by the standard procedure<10,n) the amount of Ce(IV) formed was determined with ferrous ammonium sulphate and O-phenanthroline.O2) The Ce(IV)NTA complex was prepared by oxidation with H2Ov RESULTS A N D DISCUSSION Radiolysis of Ce(III)NTA was done in the pH range 5.5-10.0; below pH 5.5 and above pH 10.0, the complex was unstable.°3) Figure 1 shows the absorption spectra of free NTA together with those of Ce(III), Ce(IV) and their corresponding NTA complexes. The characteristic absorption wave length and its molar absorbance for each compound is given in Table I. The variation in pH between 5.5-10.0 was found to have no effect on the nature of spectra of the chelate. Figure 2 shows the effect of irradiating a 0.66 x 10-3 M Ce(III)NTA solution in deoxygenated medium at pH 9.5. Curve (a) of Fig. 2 represents the absorption spectra of unirradiated solution of Ce(III)NTA and curves (b) and (c) are those for 233
234
B.K. SHARMAand R. GUPTA
Corlc. ( a ) I x IO-'~M ( b ) I x IO'~M ( c ) 0 . 6 5 x IO'~M
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FIG. 1. Absorption spectra of (a) Ce(III) and (b) NTA at pH 4.0; (c) Ce(III)NTA, (d) Ce(IV) and (e) Ce(IV)NTA at pH 6.5; 1 cm light path.
TABLE I. SPECTRALDATAFOR Ce(III), Ce(IV) ANt) THEIR NTA COMPLEXES Ions
Ce(III) Ce(IV) Ce(III)NTA Ce(IV)NTA
k m a x nm
M - ~cm- i
252 320 274 274
665 5140 850 5700
220
1
1
1
I
240
2£X)
280
300
320
X(nm)
FIG. 2. Absorption spectra of 0.66 x 10-3 M Ce(III)NTA: Unradiolysed (curve a) and radiolysed to a dose 1.96x 101SeVml-I at pH 6.5 (curve b) and at pH 9.5
(curvec).
o,'-1 '5
solutions irradiated to a dose of 1.96 x 101~eV m l - t at pH 6.5 and 9.5, respectively. As can be seen from these curves, the oxidation of the chelate increases with the increase of pH and curves (b) and (c) resemble curve (e) of Fig. 1 for synthetic Ce(IV)NTA. Hence, during the radiolysis of aqueous solutions of Ce(III)NTA, formation of Ce(IV)NTA takes place. Further, it was observed that on addition of excess alkali, the radiolysed solutions on centrifugation or on keeping deposited a yellow precipitate of hydrous ceric oxide and the centrifugate gave test for IDA only and not for NTA. Since any free IDA formed through degradation of the ligand NTA does not significantly absorb at 274nm, the radiolytic degradation of the chelate could be estimated spectroohotometrically by the decrease in O.D. of the centrifugate (after precipitation of Ce(IV)), at this wave length. The radiolytic end-products IDA, Ce(IV) and the carbonyl compounds were estimated from the radiolysed solutions. Figures 3 and 4 show that the decrease of the complex as well as formation of the
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-- I0 c~
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24
32
40
Dose x IO'~r(eVmt'~)
F1G. 3. Radiolysis of 0.5 x 10-3 M Ce(III)NTA solution at pH 9.5: G(-chelate) and G(IDA) in deoxygenated and N20-saturated medium (deoxygenated: a and b), (N20: a' and b'). radiolytic products are linear with dose up to 4.4 x 10Is eV m l - t in deoxygenated conditions, the G-values, as calculated from the slopes of such curves, under different experimental conditions are given in Table 2. No free H202 could be detected in the radiolysed solution. The primary radical and molecular yields(6~ of
On the ~,-radiolysis of aqueous solution of cedum(III)nitrilotriacetate ]
235
oxidation state, oo In deaerated conditions, the dehydrogenated radical intermediates of (1) and (2) are likely to be stabilised through electron disproportionation amongst themselves either by reactions (3) or (4), leading to the formation of I D A and a carbonyl compound
I0
x
e---I x
(3) 2Ce 3+-R2N(~H'COOH + H20 --*Ce3+-R2NCH2"COOH + Ce3 +-R2NH + CHO,COOH
o
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~ I
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I
16
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24
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32
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(4) 2Ce 3+_R2N(~H.COOH + H20 ~ C e 3+-R2NCH2.COOH + Ce 3+ -R2NH + CO2 + H.CHO
40
Dose x IO-"(eV mC ~I F;o. 4. Formation of Olyoxalic acid (curves a and a') and
Ce(IV) (curve b and b') in the radiolysis of 0.5 x 10-3M Ce(III)NTA at pH 9.5 in deoxygenated and N20-saturated solutions, respectively. water radiolysis in the pH range 3.0-10.0 are GoB = 2.7, G(~.~)= 2.7, G(H)=0.5, G(,2) = 0.45, G(.2o2)= 0.7.
where Ce 3÷-R2NH represents the Ce3÷-IDA molecule. Since glyoxalic acid, and not formaldehyde, is the product formed on radiolysis of Ce3+-NTA, the stabilisation of the intermediate radical occurs through reaction (3) and not (4), and accordingly, both G(-Ce 3+ - N T A ) and G(IDA) should be equal to ½G(,+o,) = 1.6; but our observed G-values (Table 1) are higher and may be due to the occurrence of reaction (5) followed by reactions (2) and (3).
Alkaline Medium (pH 9.5) During the radiolysis of NTA, °4' m it has been shown that H and OH radicals (and not e~) abstract a hydrogen atom from the carbon atom ~ to the carboxylic group and a similar mechanism can also be assumed in the radiolysis of Ce 3+-NTA. Reactions with H and OH radicals may, then, be represented by equations (1) and (2) leading to the formation of a dehydrogenated radical intermediate (1)
Ce3÷-R2NCH2'COOH + H ---*Ce 3÷-R2N(~H'COOH + H2
(2)
Ce 3+-R2NCH2.COOH + OH ---*Ce3+-R2N(~H'COOH + H20
where Ce 3+-R2NCH2.COOH represents the Ce3 + - N T A molecule. The direct reduction of the central metal ion, Ce 3÷, by H and e~ does not seem to be possible since cerium is not known to exist in the stable + 2
(5)
Ce3+-R2NCH2.COOH + H202 ~Ce4 +-R2NCH2"COOH + OH + O H -
Since reaction (5) generates an equivalent amount of OH radicals, G(-Ce 3+-NTA) and G(IDA) should be i equal to ~G(H+ oH + ,2o:)= 1.95 which now well agrees with the observed G-values. The occurrence of reaction (5) is supported by the observed value of. G(Ce 4+) - 0 . 7 5 , which is equal to G(,:o:) and seems to suggest the oxidation of Ce 3+ by H202 and not by OH radicals. In the light of the observed results, e~ might disappear through a fast °7) recombination process. In view of the redox potentials of Ce(III)/Ce(IV) and O H - / O H couples, 08) one would expect oxidation of Ce(III) by OH radicals, and in fact pulse radiolytic studies ~9) report the rate constant for this reaction as 7.2 x 1 0 7 m - i s -I. It, therefore, may be suggested that OH radicals should oxidise Ce(III) of the chelate "C" (reaction 6). However, our observed G(Ce 4+)
T^aL£ 2. TrIE OeSEgVEDG-VALUr.SIN THE RADIOLYSISOF Ce(III)NTA Conditions
pH
G(-Ce3+-NTA)
G(IDA)
G(Ce4+)
G(CHO.COOH)
Deoxygenated Nitrous Oxide Bromide ions t-butanol Deoxygenated
9.5 9.5 9.5 9.5 6.5
2.1 3.2 0.6 0.5 1.8
2.0 3.1 0 0 1.9
0.75 0:7 0.6 0.6 0.3
0.5 2.0 0 0 1.1
236
B.K. SHARMAand R. GUPTA
and G(IDA) values do not support this viewpoint. (6)
Ce(III)NTA + O H - , C e ( I V ) N T A + OH -
The absence of reaction (6) could be explained as due to the higher probability of reaction (2), since the rate constant of the reaction of OH radicals with free NTA (pH 9.0) is much higher, i.e. 2.5 x 1 0 9 M - ~s - 1,(2o)than that for OH radicals reacting with Ce(III). Moreover, the rate constant of the reaction of OH radicals with the ligand is expected to be further increased on chelation as has also been observed by Bhattacharya et al. (21,22)in the radiolysis of NTA chelates with Co(II) and Ni(II). Thus, in view of the values of these rate constants of reactions of OH radicals on the two possible, alternate sites of attack of the chelate "C", H-abstraction by OH from the ligand site (reaction 2) is preferred over the oxidation of the metal ion by OH radical (reaction 6). One need not invoke the idea of shielding (or steric hindrance) of the metal ion by the ligand, as has been suggested by some workers in the radiolysis of Ce(III) chelates with EDTA 1,3'4)and TTHA, (2) to account for non-occurrence of reaction (6).
Effect of nitrous oxide When radiolysis was done in the presence of nitrous oxide, an e~ scavenger, °3) an increase of OH radicals took place due to reaction (7) (7)
e~ + N20 H2O N2 + OH + O H - .
This was actually found to be so since no detectable IDA or carbonyl compounds could be found (since G(H) at this pH is only 0.5), when a deoxygenated, aqueous solution of Ce3+-NTA (0.5 x 10 -3 M) was radiolysed in the presence of 1 x 1 0 - 2 M bromide ions. G (Ce 4 +) and G (-Ce 3+-NTA) were almost similar, as expected, to G(H2o2)=0.7. Since all OH radicals are being scavenged by bromide ions, Ce 3+ - N T A complex is degraded only by molecular H202 through attack on the central metal ion (reaction 5). The fact that no carbonyl compounds could be detected in the radiolysed solutions further indicated that the oxidation of Ce(III) by H202 was probably occurring through heterolytic dissociation of the latter into OH ÷ and O H - and it was this OH ÷ which was in fact responsible for Ce(IV) production. The OH radicals produced during this process were subsequently being removed by B r - as was shown by the absence of IDA and carbonyl compounds in the radiolysed solutions.
Effect of t-butanol Another effective OH scavenger, ~25) t-butanol was also used for confirming the results obtained earlier in the presence of B r - . The alcohol concentration was kept at 1 x 10-3M and that of the chelate solution at 0.5 x 10 -3 M. On radiolysis of the mixture under deoxygenated conditions, a competition ensued between reactions (2) and (8); although, as expected, (8)
The observed G (Ce4+) = 0.7 in presence of N20 was identical with that in the deaerated system. Had OH radicals been responsible for the oxidation of Ce 3÷, G(Ce *+) would have increased in the presence of N~O. However, as expected, G(-Ce3+-NTA) increased as also the radiolytic yields of IDA and CHO.COOH (Table 1) due to enhanced degradation of the ligand by OH radicals generated through reaction (7). Hence G (--Ce3+-NTA) --- G (IDA) ~--- I G ( H +OH + e ~ + H202) ----"3.3.
Effect of Bromide ions To further confirm the mechanism proposed, it was decided to use a OH scavenger which can completely remove all these radicals instead of increasing their amount as is done by N20. Bromide ions happen to be very efficient scavenger (24) of OH radicals. Radiolysis of Ce 3+-NTA, therefore, in the presence of B r should not show any appreciable ligand degradation.
CH3-C(CH3)2-OH + OH --)(~H2-C(CH3)2-OH + H20
no detectable IDA and/or carbonyl compounds were formed (due to scavenging of OH radicals and G(H) being small). Ce(IV) was formed with G(Ce TM)= 0.6 and the chelate was degraded, G(-chelate) = 0.5. Thus, while radiolysis of Ce(III)NTA in the presence of N20 reveals that OH radicals react with the ligand NTA, experiments conducted in the presence of B r - and t-butanol show that the oxidation of the Ce(III) by H202 occurs, most probably, via heterolytic, and not homolytic dissociation of the oxidant H202. Had dissociation of H202 been homolytic resulting in the formation of OH radicals, no Ce(IV) formation would have taken place in the presence of OH scavengers (Br- and t-butanol). Such mode of oxidations by H202 were also suggested by Edwards (26)in the case of many other inorganic ions. The radiolysis of Ce(III)NTA in oxygenated medium at pH 9.5 has been described elsewhere(4) in a different context and the results again point to the mechanism mentioned before. Since the radiolysis of
On the ~,-radiolysis of aqueous solution of cerium(III)nitrilotriacetate C e ( I I I ) N T A leads to Ce(IV) production which is p H dependent, t27) experiments on radiolysis of C e ( I I I ) N T A were done at p H 6.5 also. The results (Table 2) of radiolysis of C e ( I I I ) N T A in deoxygenated medium at this p H support the proposed mechanism. The reduced G(Ce TM)value of 0.3 at this p H points to only partial consumption of H202 towards oxidation of " C " . Hence both G(--C) and G ( I D A ) would be equal to ½G(H + O H + H202) or to 1.95 and the observed G-values are in good agreement with this mechanism. The comparatively low G ( C H O - C O O H ) = 1.1 as observed by us could not be quantitatively correlated with the primary yields. REFERENCES 1. M. B. HAFEZ, H. ROUSADVand J. HAFEZ,J. Radioanal. Chem. 1978, 43~ 121. 2. M. B. HAFEZ, W. H1GAZland N. HA~z, J. Radioanal. Chem. 1979, 49(1), 45. 3. B. K. SHARMAand R. GUPTA, Radiat. Eft. Lett. 1980, 57(5), 149. 4. B, K, SHARlVtA.and R. GUI'TA, Radiat. Eft. Lett. 1982, 67(5), 147. 5. M. B. HAFEZ and B. M. AI~OELKHAIR,Radiochem. Radioanal. Lett. 1980, 53(6), 375. 6. I. G. DRAGNlC and Z. D. DRAGN1C, The Radiation Chemistry of Water, p. 218. Academic Press, New York 1971. 7. S. MOOREand W. H. STEIN, J. BioL Chem. 1948, 176, 367. 8. G. M. EISENaERG,Ind. Engng Chem., Anal. Ed. 1943, 15, 327. 9. R. M. SELLERS,Analyst 1980, 105, 950. 10. C. E. BRICKERand H. R. JOHNSON,Ind. Engng Chem. Analyt. Ed. 1945, 17, 400.
237
11. G. R. A. JOHNSONand G. SCI-IOLES,Analyst 1954, 79, 217. 12. L. GOROON and A. I. FEIeUSH, AnaL Chem. 1955, 27, 1050. 13. G. L, VARLAMOVA,N. D. MITROFANOVA,L. I. MARa'YNE~rKO, N. I. PECHUROVA and V. G. VARLAMOV, Russian J. Inorg. Chem. 1970, 15(5), 636. 14. S. N. BHATTACHARYAand E. V. SRXSANKAR,Radiat. Phys. Chem. 1976, 8, 667. 15. R. GUPTA, Ph.D. Thesis, p. !10. University of Delhi, Delhi, India, 1981. 16. F. A. CoTtoN and G. WILKINSON,Advanced Inorganic Chemistry, 3rd Edn, p. 1058. Wiley Eastern, New Delhi, 1979. 17. M. ANB^R and P. NETA, Int. J. AppL Radiat. Isotope 1967, 18, 493. 18. J. W. T. SPINKSand R. J. WOODS, An Introduction to Radiation Chemistry, p. 281. Wiley Interscience, New York, 1976. 19. L. F. DORFMANand G. E. ADAMS, Reactivity of the Hydroxyl Radical in Aqueous Solutions, NSRDS-NBS, p. 46. U.S. Department of Commerce, National Bureau Standards, Washington D.C., 1973. 20. J. LATI and D. MEYER.STEIN,J. Chem. Soc. (Dalton Trans.) 1978, 2, i105. 21. S. N. BHATTACHARYAand E. V. SRISANKARA,J. C. S. (Faraday I), 1979, 2089. 22. E. V. SRISANKARand S. N. BHATT^CHARYA,J. Chem. Soc. (Dalton Trans.), 1980, 4, 675. 23. F. S. DAINTONand D. B. P~TERSON,Nature (London) 1960, 186, 878. 24. T. J. SWORSKI,J. Am. Chem. Soc. 1954, 76, 4687. 25. G. SCHOLES,P. SHAW, R. L. WmSON and M. EaERT, Pulse Radiolysis (Edited by J. H. Baxendale, M. Ebert, J. P. Keane and A. J. Swallow), p. 131. Academic Press, New York, 1965. 26. J. D. EDWARDS,Chem. Rev. 1952, 50, 455. 27. R. GUPTA, Ph.D. Thesis, p. 115. University of Delhi, Delhi, India, 1981.