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60Co y-radiolysis of iron(III)nitrolotriacetate in aqueous solutions
Radiat. Phys. Chem. Vol. 20, No. 5-6, pp. 341-346, 1982 Printed in Greal Britain.
0146-5724/82/110341--06503.00/0 Pergamon Press Ltd.
60Co 3,-RADIOLYSIS OF IRON(III)NITRILOTRIACETATE IN AQUEOUS SOLUTIONS B. K. SHARMA Department of Chemistry, University of Delhi, Delhi 110007, India and K. SAHUL Department of Chemistry, Jamal Mohamed College, Tiruchirapalli 620020, India
(Received 21 May 1982) Abstract--Aqueous solutions of Ferric nitrilotriacetate(F) at p H - 2 were radiolysed in deaerated medium as well as in the presence of scavengers---oxygen, t-butyl alcohol and sodium formate. In deaerated and oxygenated conditions, G(Fe2+) was 6.2-+0.2 and G(iminodiacetic acid) = G(glyoxalic acid) was 3.2-+0.2. A reaction mechanism for the radiolytic degradation of F in aqueous solutions has been proposed; H or HO2 directly reduces Fe(III) of the chelate and OH does so by abstraction of alpha hydrogen from the ligand followed by intramolecular electron transfer. Competition kinetic studies using t-BuOH gave k(OH÷F3= 1.1 X l0s M-~ S-~.
INTRODUCTION RECENTLY some attention has been paid to the radiation chemistry of Fe(III)EDTA."-3) However, little is known regarding the radiolytic behaviour of Fe(III)NTA, (NTA = Nitrilotriacetic acid, N(CH2COOH)3 which is also a strong chelating agent like EDTA). In view of the stability of aminopolycarboxylates of Fe(IIl) over a wide range of pH, these constitute ideal Fe(II)/Fe(III) systems for radiolytic studies. Among the aminopolycarboxylate chelates, Fe(III)NTA system appears to be the most suitable since its radiolytically degraded ligand product, namely IDA (iminodiacetic acid, HN(CHzCOOH)2), can be conveniently monitored which could not be done in the case of Fe(III)EDTA. Hence to throw some more light on the radiolytic behavior of Fe(II)/Fe(III) systems of aminopolycarboxylic acids, a study of Fe(III)NTA has been undertaken.
Fig. 1, revealed the absence of any free carboxylate stretching frequency, indicating the coordination of all the three carboxylate groups of the ligand with Fe(III). The metal to ligand ratio 1:1 was verified by estimation of iron by 1,10-phenanthroline method, and NTA, after removal of iron as hydrated oxide, by titration against copper sulphate solution at pH 9.6 to a murexide end point. Since Fe(III)NTA is photosensitive, it was stored under dark. Triply distilled water was used for preparing solutions for irradiations. Deaeration of solutions was carried out by N2-saturation.
Irradiation Irradiations were carried out by ~/-rays obtained from 6°Co Gammacell-220, Atomic Energy of Canada Ltd. The average dose rate, as determined by Fricke's dosimetry was 0.6 x 10t7 eV ml-~ min-~.
o~
EXPERIMENTAL
Materials Ferric alum (M&B England), NTA (Lobo-chemie Germany), 1,10-phenanthroline, ninhydrin, sodium formate, alcohols and all other chamicals used were of analytical grade. Fe(III)NTA was prepared ~4)by mixing equimolar solutions of Fe(III) and NTA at 95°C followed by vigorous stirring. The yellow complex formed was filtered, washed several times with triply distilled water till the filtrate gave negative test for free Fe 3+ and sulphate, and finally dried at 105"C. The IR spectra of the complex shown in
E c t--
1 1800
~600
Wove r~L~mber,
1400 c m -I
FIG. 1. IR spectra. L, Nitrilotriacetic acid; C, Fe(Ill)Nitrilotriacetate.
341
342
B. K. SHARMAand K. SAHUL
Analytical Methods Fe(II), IDA and glyoxalic acid were identified as radiolytic decomposition products by means of standard methods. Fe(II)swas determined as its 1,10-phenanthroline complex.() IDA in the radiolysed solution was determined by ninhydrin reagent(6) as such and also after decomposing the complex with NaOH, followed by filtration of the hydrated Fe203. The yields were identical in both cases. Glyoxalic acid was determined by using 2,4-dinitrophenylhydrazine (7) reagent. Test for formaldehyde was carried out by chromotropic acid,(s~but it was found to be absent. The presence of H202,if any, in the irradiated samples was tested with titanium sulphate reagent. (9) The ferrous production in N2-saturated solution was, however, measured without opening the radiation cell to air to avoid any aeration in the solution. The optical density measurements of Fe(II) 1,10-phenanthroline in the radiolysed solution were done after one hour. The reagents like buffer solution, 1,10-phenanthroline and triply distilled water for analysis, were saturated with N2. The optical density measurements were carried out in Beckman Model DU-2 spectrophotometer and pH measurements, in Beckman Expandomatic SS-2 pH meter. The hydrated Fe203 was centrifuged in Servall High Speed (USA) centrifuge. R E S U L T S AND DISCUSSION The effect of chelate initial concentration on G-values was studied between the range (0.55.0)x 10-3M of Fe(III)NTA solution both in deaerated and aerated c o n d i t i o n s ( F i g . 2). The G(products) reached the limiting value at chelate concentration >1 2.0 x 10-3 M. When acidic (pH 2) 2.5 × 10-3 M Fe(III)NTA solution was radiolysed, accumulation of Fe(II), IDA and glyoxalic acid with dose occurred and is shown in Figs. 3 and 4. G(Fe2+), G(IDA) and G ( C H O COOH) were calculated from the initial slopes of the respective curves and found to be G(Fe 2+) =
/
30
/
25
_
/ /
o_ 2 0 x ÷ e~
J.5
0.5
0
0.5
Jo
~5
2.0
25
O, deaerated conditions; O, aerated conditions. 6.2, G(IDA) = G(CHO-COOH) = 3.2. No H202 was detected both in deaerated and oxygen saturated solutions. The G-values obtained under various conditions are given in Table 1. When v-radiation passes through water, the radiolytic reaction is generally expressed as, <1°) (1)
H20 --'~ e ~-q,H, OH, H2, H20:, H30 +.
The radical and molecular yields of water would, then, attack the chelate. It can be seen from Table 1 that in deaerated medium in the absence of any scavenger, the ferrous yield is G(Fe 2+) = 6.2 - 0.2, which is also equal to G ( H ) + G(OH); since at /
12' o
3.0
[ D o s e ] x lO -m, eVrnL -t min -t FIG: 3. Radiolysis of 2.5 x 10-3 M Fe(III)NTA at pH - 2.
I
•
//
°-Jo
Y
•
A
(Fe 2+) 5
--
o
J
8
i,m
6
4
< C3 L-j
•
eO
2
I
o 0
o/
_
(IDA)
of ° 2
O O L) i O T U
1
4
G
~8
o
t
[
~
l
I
L
~
2
3
4
5
6
[Fe(Ill)NTA]
x 10 3 ,
M
FIG. 2. Effect of initial concentration of Fe(III)NTA on G(Fe2÷) and G(IDA): 0 , deaerated; O, aerated.
J~'l
os
i
rc
i
15
i
20
[
2.s
i
3o
[ D o s e ] x IO-Je, eV mE-I m i n - '
FIG. 4. Formation of products in the radiolysis of 2.5 x 10-3M Fe(III)NTA at p H - 2 . IDA: O, deaerated condition; O, aerated condition. CHO-COOH: it, deaerated condition; A, aerated condition.
343
~°Co y-radiolysis of Iron(III)Nitrilotriacetate in aqueous solutions
TABLE 1. G-VALuESIN THERADIOLYSISOF Fe(IIDNTA AT pH ~ 2 [Fe(III)NTA]
Scavenger (S)
[S]
G(Fe 2+)
G(IDA) = G(CHO-COOH)
(2-5) × 10-3 M (2-5) x 10-3 M 2.3 x 10 -3 M 2.3x 10-3M
Deaerated Oxygen t-BuOH HCOO-
-2.6 x 10-4 M 2 × 10-t M" 4 x 10-2 M
6.2 +-.0.2 6.2 -+0.2 -6.4
3.2 +-0.2 3.2 -+0.2 0 0
The other possibilities are those in which the radical disappears by intermolecular electron transfer with Fe(III)NTA (reaction 8) and/or, by disproportionation (reaction 9).
pH 2, all e2q is converted into H atoms, °°) it suggests that both H and OH radicals reduce Fe(III) of the chelate. The reduction of Fe(III) of the chelate by H may, however, take place in either of the two ways; (i) it may attack the central metal ion directly (reaction 2) with no concomitant degradation of the ligand:
(2)
(8)
Fe3+RN-CH2--COO- + H ) Fe2+RN-CH2--COO- + H + (Fe3+RN-CHz--COO- = Fe(III)nitrilotriacetate molecule)
Fe3+RN-CH2--COO- + H > Fe3+RN-CH-COO - + H2.
Likewise OH radical may also abstract H of the ligand, reaction (4), as has been assumed in the radiolysis of many aminopolycarboxylate chelates, ~2-~4)
(4)
Fe3+RN-CH2-COO - + OH ) Fe3+RN-CH-COO - + H20.
Then, the radical, Fe3+RN-CH-COO -, may disappear by means of intramolecular electron transfer followed by hydrolysis through reactions (5)(7). (5)
F e 3 + R N _ C H _ C O O_
H20
2Fe3+RN-CH-COO -
(9)
Fe3+RN-CH(OH)-COO - + Fe3+RN-CHz-COO -. On the basis of the observed values, G(Fe 2+) = 6.2 -+ 0.2 and G(CHO-COOH) = G(IDA) = 3.2 -+0.2, reactions (8) and (9) can be ruled out since the observed G(Fe 2+) would have been much less than 6.2. Reaction (7) can also be ruled out because, though, it accounts quantitatively for the ferrous production, it does not explain the formation of IDA and giyoxalic acid during radiolysis. Therefore, the fate of the radical is given by reactions (5) and (6). Of the two sources for the production of this radical, reaction (3) can be ruled out since had it been so, G(IDA) would have been equal to G(Fe 2+) = 6.2. But the observed value is G(IDA) = 3.2 - 0.2. Though the postulation of reactions (2), (4)-(6) appears to offer a ready explanation for the observed radiolytic yields, the mode of disappearance of H202 remains unanswered. The disappearance of H202 formed during radiolysis is explained by reaction (10) in which each mole of
i . . . . . le~ut.~ ) Fe2+RN_~H_CO Oelectron transfer
+
(6)
Fe2+RN_CH_COO_
H2o ) Fe2÷RNH + CHO--COOH
(Fe2+RNH = Ferrous iminodiaeetate) +
(7)
Fe2÷RN-CH-COO -
)
Fe3+RN-CH(OH)-COO - + Fe2÷RN-CHz-COO-
or; (ii) it may attack the ligand by the way of abstracting alpha hydrogen of the ligand as in the case of radiolysis of free N T A <") (reaction 3) and, subsequently the metal ion may be reduced by electron transfer with consequent degradation of the ligand: reactions (5)-(7).
(3)
H20
Fe3+RN-CH-COO - + Fe~+RN-CH2--COO-
H20
) Fe2+RN-CH(OH)-COO - + H +.
B. K. SHARMAand K. SAHUL
344
H 2 0 2 oxidises one mole of Fe(II) and also one mole of OH radical is produced, which in turn reduces one mole of Fe(III) (reactions (4)-(6)), thus, eliminating the effect of H202 on G(Fe2+).
(10)
Fe(II) + H202
other possibility of dimerisation of S O 2 (18) by reaction (14), is ruled out since G(Fe 2+) would then only be equal to G(OH), while the experimental value of G(Fe2+) = 6.2---0.2 is equal to G(OH) + G(HO2).
* Fe(III) + OH + OH-. (14)
In accordance with the above reaction, G ( I D A ) = G(OH) + G(H202) = 2.8 + 0.6 = 3.4; the observed value of G(IDA) is close to the theoretical value and clearly supports the reaction (10). It can be seen from Figs. 3 and 4 that G(Fe 2+) and G(IDA) gradually decrease with increase of dose. Obviously OH radicals, which are directly responsible for the production of IDA, are being utilised somewhere else also. One could expect, then, that Fe(II) formed during radiolysis might react with OH radicals, thus ensueing a competition between reactions (11) and (4) for OH radicals. (11)
Fe(II) + OH
> Fe(III) + OH-.
The rate constant ktoH+~,tm) is of the order of 3.2 x l0 s M -1 S - l ( t 6 ) which is higher than the rate constants k ( O H + F e ( I I I ) N T A ) = 1.1 x 10s M -1 s -I, as reported by us in the latter part of this investigation and hence, it appears that the reaction (11) favourably competes with reaction (4), resulting in the fall of G(Fe 2÷) and G(IDA) with increase of dose. To sum up, then, the ligand part of the chelate is degraded by OH radicals to yield IDA and glyoxalic acid and Fe(III) of the chelate is reduced both by H and OH.
HO2+ H02
Moreover, had reactions (14) and (10) been taking place, rather than (13), in O2 saturated conditions, G(IDA) would have been equal to G ( O H ) + G(H202)+½G(HO2)=5.2, which is not in agreement with the experimentally observed value of G ( I D A ) = 3.2---0.2 equal to G ( O H ) + G(H202). Similar role of HO: has also been observed in the radiolysis of Fe(III)EDTA (3) and Fe(CN)0-. (19)
Effect of alcohol t-Butanol (2°) is a well-known scavenger for OH radical and hence its effect on G(IDA) was studied. It can be seen from Table 1 that at 2 × 10 -t M of t-BuOH, no IDA formation was observed. When the alcohol concentration was varied between 0.1 x 10 -3 M and 1.0 x 10 -3 M, a competition ensued between reactions (4) and (15).
(15)
OH + CH3-C(CH3)2-OH , • CHz-C(CH3h-OH + H20.
IDA and glyoxalic acid were determined from the radiolysed solution containing t-BuOH. For this study one can write, 1
(16)
1
G(IDA)
(12)
H + 02
~' H02
(13)
Fe3+RN-CH2--COO- + HO2
•
Fe2*RN-CH2-COO - + H + + 02. Since in aerated conditions G(Fe 2+) = 6.2-+ 0.2, it is evident that HO2 radical, like H in the deaerated conditions, behaves as a reducing agent. ¢ln The
1
G ( C H O - C O O H ) = G(OH)'
Effect of oxygen From Fig. 2, it can be seen that in presence of O2, the limiting values of G(Fe2÷)=6.2 and G(IDA) = 3.2 are reached at the same chelate concentration as in deaerated conditions• Again, from Figs. 3 and 4, it is clear that there is no change in G(Fe2+), G(IDA) and G(CHO-COOH) values due to oxygen saturation in the reaction medium. This observation is explained as follows,
:' H202+ O2
1 k~5[t-BuOH] G(OH)' k,[Fe(IIDNTA]
(G(OH)' = G(OH) + G(H202)). A
plot
of
I/G(IDA)
against
[t-
BuOH]I[Fe(III)NTA] gave a straight line (Fig. 5) with intercept equal to 0.32 {=I/[G(OH)']}. From the slope of the straight line, assuming t~' the value of kt5 = 4.2 × l0 s M -1 s -t, k4 = k(OH+Fe(nl)NTA) w a s calculated from equation (16) and was found to be equal to 1.1 × l0 s M-~s -t. On comparison of this value with the rate constant k
6°Co T-radiolysis of Iron(III)Nitrolotriacetate in aqueous solutions
.,//
0.9
0,6
/"
I
2.0 16
ief -
-
-
~2
Z-
G
, , . J
08
~''~°/e
OS
04
/ • •
(Fe2+) l
0
J 0
0.15
030
045
[ t - BuOH] / [ F e ( l l l ) N T A ]
FIG. 5. Kinetic plot for the competition reaction for OH radicals between t-Butyl alcohol and Fe(III)NTA at pH 2 under aerated conditions.
Effect of [ormate The effect of formate as a scavenger for OH radicals in aerated condition is interesting because, unlike t-BuOH, the CO2- radical brings about reduction of Fe(III) of the chelate through reactions (17)-(19) followed by reaction (13).
(17)
OH+ HCO0-
I__
COF + O2
Hence, in presence of formate in the solution during radiolysis, there should be a decrease in G(IDA) but no change in G(Fe:+). At 4 x 10-2M formate concentration, G(Fe 2÷) = 6.4 and G(IDA) = 0, indicating the complete scavenging of OH radicals by formate ions. In the presence of intermediate concentrations of the formate ion, a competition will ensue between reactions (4) and (17). Then, one can write the equation (20),
1 (20)
3, 4. 5. 6. 7. 8. 9. 10.
1
G(IDA) = G ( C H O - C O O H )
1
1
k15[HCOO-] - G(OH)~' + G(OH)~' k4[Fe(HI)NTA]"
11. 12. 13.
A plot of IlG(IDA) or I/G(CHO--COOH) against H C O O - / F e ( I I I ) N T A yielded a straight line (Fig. 6) with intercept equal to 0.32 {= I/[G(OH)']}. Taking k4 = 1.1 x 108M -1 s -l, kl7 was calculated to be
IlO
I
o,8
AcknowledgementsmOne of the authors (K. Sahul) acknowledges with thanks the teacher fellowship offered by Centre for Advanced Studies, Department of Chemistry, University of Delhi, Delhi-ll0007. He is grateful to the Management and Professor E. P. Mohamed Ismail, Principal of Jamal Mohamed College, Tiruchirapalli, India, for grant of study leave.
> CO2 + O~~ HO2.
~
equal to 2.6x 109 M -~ s -1 which very well agrees with the value cited in Ref. (24).
>H20+CO2-
02- + H +
O }
FIG. 6. Kinetic plot for the competition reaction of OH between HCOONa and Fe(III) NTA at p H - 2 under aerated conditions.
2. (19)
g !
003 006 009 o~z O~S [HCO0-]/[Fe (111)NTA]
REFERENCES and E. KocsIs, Acta. Chim. Acad. Sci. (Hung) 1978, 97, 255. N. G. RASSOKHIN,S. A. TEVLIN and A. I. MILAEV, Teploenergetica 1975, 22, 74. S. N. BHATACHARYYAand K. P. KUNDU, Int. J. Radiat. Phys. Chem. 1971, 3, I. M. KRISHNAMOORTHYand K. B. MORRIS, J. Inorg. Nucl. Chem. 1972, 34, 719. C. A. PARKERand C. G. HATCHARD,Proc. Roy. Soc. (London) 1956, 518. S. MOOREand W. H. STEIN,.t. Biol. Chem. 1948, 176, 367. T. F. FRmDMANNand G. E. HAUGEN,J. Biol. Chem. 1943, 147, 415. C. E. BRICKERand H. R. JOrtNSON, Ind. Engng Chem. Anal. Ed. 1945, 17, 400. R. M. SELLERS,Analyst 1980, 105, 950. I. G. DRAGANICand Z. D. DP,AGANIC, The Radiation Chemistry of Water, p. 41. Academic Press, New York, 1971. S. N. BHATACHARYYAand E. V. SRISHANKAR,Int, J. Radiat. Phys. Chem. 1976, 8, 667. N. MATSUURA,N. SHINOrlARA,M. NISI-IIKAWAand M. TAKIZAWA,Bull. Chem. Soc. Jpn. 1967, 410,2042. S. N. BnATACHARYYAand K. P. KUNDU, Int. J. Radiat. Phys. Chem. 1973, 5, 183. B. K. SHARMAand R. Gur'rA, Radiat. Eft. Lett. 1980, 57, 149. A. O. ALLEN and W. G. ROa~-ISCmLD,Radiat. Res. 1957, 7, 591.
1. M. WEBER, G . FOLDIAK
(18)
345
14. 15.
346
B. K. SH~d~:a~ and K. S~quL
16. M. ANBAR and P. NETA, Int. J. Appl. Radiat. Isotopes 1967, 18, 493. 17. A. O. ALLEN, V. D. HOGAN and W. G. ROTHSCHILD, Radiat. Res. 1957, 7, 603. i8. G. CZAPSKI and B. H. J. BmLSKI, Ji Phy. Chem~ 1963, 67, 2180. 19. G. HUGHES and C. WILLIS, Disc. Faraday Soc. 1963; 36, 223. 20. G. SCHOLES, P. SHAW, R. L. WILSON and M. EBERT, Pulse Radiolysis (Edited by J. H. Baxendale, M.
21. 22. 23. 24.
Ebert, J. P. Keene and A. J. Swallow), p. 151. Academic Press, New York, 1965. J. W. T. SPINKS and R. J. WooDs, Wiley, New York, 1963. M. Z. HOFFMAN and M. SIMIC, Inorg. Chem. 1973, 12, 2471. M. Z. HOFFMAN and M. SIMIC, J. Am. Chem. Soc. 1972, 94, 1757. J. RABANI and G. STEIN, Trans. Faraday $oc. 1962, 58, 2150.
0146-5724/82/110341--06503.00/0 Pergamon Press Ltd.
60Co 3,-RADIOLYSIS OF IRON(III)NITRILOTRIACETATE IN AQUEOUS SOLUTIONS B. K. SHARMA Department of Chemistry, University of Delhi, Delhi 110007, India and K. SAHUL Department of Chemistry, Jamal Mohamed College, Tiruchirapalli 620020, India
(Received 21 May 1982) Abstract--Aqueous solutions of Ferric nitrilotriacetate(F) at p H - 2 were radiolysed in deaerated medium as well as in the presence of scavengers---oxygen, t-butyl alcohol and sodium formate. In deaerated and oxygenated conditions, G(Fe2+) was 6.2-+0.2 and G(iminodiacetic acid) = G(glyoxalic acid) was 3.2-+0.2. A reaction mechanism for the radiolytic degradation of F in aqueous solutions has been proposed; H or HO2 directly reduces Fe(III) of the chelate and OH does so by abstraction of alpha hydrogen from the ligand followed by intramolecular electron transfer. Competition kinetic studies using t-BuOH gave k(OH÷F3= 1.1 X l0s M-~ S-~.
INTRODUCTION RECENTLY some attention has been paid to the radiation chemistry of Fe(III)EDTA."-3) However, little is known regarding the radiolytic behaviour of Fe(III)NTA, (NTA = Nitrilotriacetic acid, N(CH2COOH)3 which is also a strong chelating agent like EDTA). In view of the stability of aminopolycarboxylates of Fe(IIl) over a wide range of pH, these constitute ideal Fe(II)/Fe(III) systems for radiolytic studies. Among the aminopolycarboxylate chelates, Fe(III)NTA system appears to be the most suitable since its radiolytically degraded ligand product, namely IDA (iminodiacetic acid, HN(CHzCOOH)2), can be conveniently monitored which could not be done in the case of Fe(III)EDTA. Hence to throw some more light on the radiolytic behavior of Fe(II)/Fe(III) systems of aminopolycarboxylic acids, a study of Fe(III)NTA has been undertaken.
Fig. 1, revealed the absence of any free carboxylate stretching frequency, indicating the coordination of all the three carboxylate groups of the ligand with Fe(III). The metal to ligand ratio 1:1 was verified by estimation of iron by 1,10-phenanthroline method, and NTA, after removal of iron as hydrated oxide, by titration against copper sulphate solution at pH 9.6 to a murexide end point. Since Fe(III)NTA is photosensitive, it was stored under dark. Triply distilled water was used for preparing solutions for irradiations. Deaeration of solutions was carried out by N2-saturation.
Irradiation Irradiations were carried out by ~/-rays obtained from 6°Co Gammacell-220, Atomic Energy of Canada Ltd. The average dose rate, as determined by Fricke's dosimetry was 0.6 x 10t7 eV ml-~ min-~.
o~
EXPERIMENTAL
Materials Ferric alum (M&B England), NTA (Lobo-chemie Germany), 1,10-phenanthroline, ninhydrin, sodium formate, alcohols and all other chamicals used were of analytical grade. Fe(III)NTA was prepared ~4)by mixing equimolar solutions of Fe(III) and NTA at 95°C followed by vigorous stirring. The yellow complex formed was filtered, washed several times with triply distilled water till the filtrate gave negative test for free Fe 3+ and sulphate, and finally dried at 105"C. The IR spectra of the complex shown in
E c t--
1 1800
~600
Wove r~L~mber,
1400 c m -I
FIG. 1. IR spectra. L, Nitrilotriacetic acid; C, Fe(Ill)Nitrilotriacetate.
341
342
B. K. SHARMAand K. SAHUL
Analytical Methods Fe(II), IDA and glyoxalic acid were identified as radiolytic decomposition products by means of standard methods. Fe(II)swas determined as its 1,10-phenanthroline complex.() IDA in the radiolysed solution was determined by ninhydrin reagent(6) as such and also after decomposing the complex with NaOH, followed by filtration of the hydrated Fe203. The yields were identical in both cases. Glyoxalic acid was determined by using 2,4-dinitrophenylhydrazine (7) reagent. Test for formaldehyde was carried out by chromotropic acid,(s~but it was found to be absent. The presence of H202,if any, in the irradiated samples was tested with titanium sulphate reagent. (9) The ferrous production in N2-saturated solution was, however, measured without opening the radiation cell to air to avoid any aeration in the solution. The optical density measurements of Fe(II) 1,10-phenanthroline in the radiolysed solution were done after one hour. The reagents like buffer solution, 1,10-phenanthroline and triply distilled water for analysis, were saturated with N2. The optical density measurements were carried out in Beckman Model DU-2 spectrophotometer and pH measurements, in Beckman Expandomatic SS-2 pH meter. The hydrated Fe203 was centrifuged in Servall High Speed (USA) centrifuge. R E S U L T S AND DISCUSSION The effect of chelate initial concentration on G-values was studied between the range (0.55.0)x 10-3M of Fe(III)NTA solution both in deaerated and aerated c o n d i t i o n s ( F i g . 2). The G(products) reached the limiting value at chelate concentration >1 2.0 x 10-3 M. When acidic (pH 2) 2.5 × 10-3 M Fe(III)NTA solution was radiolysed, accumulation of Fe(II), IDA and glyoxalic acid with dose occurred and is shown in Figs. 3 and 4. G(Fe2+), G(IDA) and G ( C H O COOH) were calculated from the initial slopes of the respective curves and found to be G(Fe 2+) =
/
30
/
25
_
/ /
o_ 2 0 x ÷ e~
J.5
0.5
0
0.5
Jo
~5
2.0
25
O, deaerated conditions; O, aerated conditions. 6.2, G(IDA) = G(CHO-COOH) = 3.2. No H202 was detected both in deaerated and oxygen saturated solutions. The G-values obtained under various conditions are given in Table 1. When v-radiation passes through water, the radiolytic reaction is generally expressed as, <1°) (1)
H20 --'~ e ~-q,H, OH, H2, H20:, H30 +.
The radical and molecular yields of water would, then, attack the chelate. It can be seen from Table 1 that in deaerated medium in the absence of any scavenger, the ferrous yield is G(Fe 2+) = 6.2 - 0.2, which is also equal to G ( H ) + G(OH); since at /
12' o
3.0
[ D o s e ] x lO -m, eVrnL -t min -t FIG: 3. Radiolysis of 2.5 x 10-3 M Fe(III)NTA at pH - 2.
I
•
//
°-Jo
Y
•
A
(Fe 2+) 5
--
o
J
8
i,m
6
4
< C3 L-j
•
eO
2
I
o 0
o/
_
(IDA)
of ° 2
O O L) i O T U
1
4
G
~8
o
t
[
~
l
I
L
~
2
3
4
5
6
[Fe(Ill)NTA]
x 10 3 ,
M
FIG. 2. Effect of initial concentration of Fe(III)NTA on G(Fe2÷) and G(IDA): 0 , deaerated; O, aerated.
J~'l
os
i
rc
i
15
i
20
[
2.s
i
3o
[ D o s e ] x IO-Je, eV mE-I m i n - '
FIG. 4. Formation of products in the radiolysis of 2.5 x 10-3M Fe(III)NTA at p H - 2 . IDA: O, deaerated condition; O, aerated condition. CHO-COOH: it, deaerated condition; A, aerated condition.
343
~°Co y-radiolysis of Iron(III)Nitrilotriacetate in aqueous solutions
TABLE 1. G-VALuESIN THERADIOLYSISOF Fe(IIDNTA AT pH ~ 2 [Fe(III)NTA]
Scavenger (S)
[S]
G(Fe 2+)
G(IDA) = G(CHO-COOH)
(2-5) × 10-3 M (2-5) x 10-3 M 2.3 x 10 -3 M 2.3x 10-3M
Deaerated Oxygen t-BuOH HCOO-
-2.6 x 10-4 M 2 × 10-t M" 4 x 10-2 M
6.2 +-.0.2 6.2 -+0.2 -6.4
3.2 +-0.2 3.2 -+0.2 0 0
The other possibilities are those in which the radical disappears by intermolecular electron transfer with Fe(III)NTA (reaction 8) and/or, by disproportionation (reaction 9).
pH 2, all e2q is converted into H atoms, °°) it suggests that both H and OH radicals reduce Fe(III) of the chelate. The reduction of Fe(III) of the chelate by H may, however, take place in either of the two ways; (i) it may attack the central metal ion directly (reaction 2) with no concomitant degradation of the ligand:
(2)
(8)
Fe3+RN-CH2--COO- + H ) Fe2+RN-CH2--COO- + H + (Fe3+RN-CHz--COO- = Fe(III)nitrilotriacetate molecule)
Fe3+RN-CH2--COO- + H > Fe3+RN-CH-COO - + H2.
Likewise OH radical may also abstract H of the ligand, reaction (4), as has been assumed in the radiolysis of many aminopolycarboxylate chelates, ~2-~4)
(4)
Fe3+RN-CH2-COO - + OH ) Fe3+RN-CH-COO - + H20.
Then, the radical, Fe3+RN-CH-COO -, may disappear by means of intramolecular electron transfer followed by hydrolysis through reactions (5)(7). (5)
F e 3 + R N _ C H _ C O O_
H20
2Fe3+RN-CH-COO -
(9)
Fe3+RN-CH(OH)-COO - + Fe3+RN-CHz-COO -. On the basis of the observed values, G(Fe 2+) = 6.2 -+ 0.2 and G(CHO-COOH) = G(IDA) = 3.2 -+0.2, reactions (8) and (9) can be ruled out since the observed G(Fe 2+) would have been much less than 6.2. Reaction (7) can also be ruled out because, though, it accounts quantitatively for the ferrous production, it does not explain the formation of IDA and giyoxalic acid during radiolysis. Therefore, the fate of the radical is given by reactions (5) and (6). Of the two sources for the production of this radical, reaction (3) can be ruled out since had it been so, G(IDA) would have been equal to G(Fe 2+) = 6.2. But the observed value is G(IDA) = 3.2 - 0.2. Though the postulation of reactions (2), (4)-(6) appears to offer a ready explanation for the observed radiolytic yields, the mode of disappearance of H202 remains unanswered. The disappearance of H202 formed during radiolysis is explained by reaction (10) in which each mole of
i . . . . . le~ut.~ ) Fe2+RN_~H_CO Oelectron transfer
+
(6)
Fe2+RN_CH_COO_
H2o ) Fe2÷RNH + CHO--COOH
(Fe2+RNH = Ferrous iminodiaeetate) +
(7)
Fe2÷RN-CH-COO -
)
Fe3+RN-CH(OH)-COO - + Fe2÷RN-CHz-COO-
or; (ii) it may attack the ligand by the way of abstracting alpha hydrogen of the ligand as in the case of radiolysis of free N T A <") (reaction 3) and, subsequently the metal ion may be reduced by electron transfer with consequent degradation of the ligand: reactions (5)-(7).
(3)
H20
Fe3+RN-CH-COO - + Fe~+RN-CH2--COO-
H20
) Fe2+RN-CH(OH)-COO - + H +.
B. K. SHARMAand K. SAHUL
344
H 2 0 2 oxidises one mole of Fe(II) and also one mole of OH radical is produced, which in turn reduces one mole of Fe(III) (reactions (4)-(6)), thus, eliminating the effect of H202 on G(Fe2+).
(10)
Fe(II) + H202
other possibility of dimerisation of S O 2 (18) by reaction (14), is ruled out since G(Fe 2+) would then only be equal to G(OH), while the experimental value of G(Fe2+) = 6.2---0.2 is equal to G(OH) + G(HO2).
* Fe(III) + OH + OH-. (14)
In accordance with the above reaction, G ( I D A ) = G(OH) + G(H202) = 2.8 + 0.6 = 3.4; the observed value of G(IDA) is close to the theoretical value and clearly supports the reaction (10). It can be seen from Figs. 3 and 4 that G(Fe 2+) and G(IDA) gradually decrease with increase of dose. Obviously OH radicals, which are directly responsible for the production of IDA, are being utilised somewhere else also. One could expect, then, that Fe(II) formed during radiolysis might react with OH radicals, thus ensueing a competition between reactions (11) and (4) for OH radicals. (11)
Fe(II) + OH
> Fe(III) + OH-.
The rate constant ktoH+~,tm) is of the order of 3.2 x l0 s M -1 S - l ( t 6 ) which is higher than the rate constants k ( O H + F e ( I I I ) N T A ) = 1.1 x 10s M -1 s -I, as reported by us in the latter part of this investigation and hence, it appears that the reaction (11) favourably competes with reaction (4), resulting in the fall of G(Fe 2÷) and G(IDA) with increase of dose. To sum up, then, the ligand part of the chelate is degraded by OH radicals to yield IDA and glyoxalic acid and Fe(III) of the chelate is reduced both by H and OH.
HO2+ H02
Moreover, had reactions (14) and (10) been taking place, rather than (13), in O2 saturated conditions, G(IDA) would have been equal to G ( O H ) + G(H202)+½G(HO2)=5.2, which is not in agreement with the experimentally observed value of G ( I D A ) = 3.2---0.2 equal to G ( O H ) + G(H202). Similar role of HO: has also been observed in the radiolysis of Fe(III)EDTA (3) and Fe(CN)0-. (19)
Effect of alcohol t-Butanol (2°) is a well-known scavenger for OH radical and hence its effect on G(IDA) was studied. It can be seen from Table 1 that at 2 × 10 -t M of t-BuOH, no IDA formation was observed. When the alcohol concentration was varied between 0.1 x 10 -3 M and 1.0 x 10 -3 M, a competition ensued between reactions (4) and (15).
(15)
OH + CH3-C(CH3)2-OH , • CHz-C(CH3h-OH + H20.
IDA and glyoxalic acid were determined from the radiolysed solution containing t-BuOH. For this study one can write, 1
(16)
1
G(IDA)
(12)
H + 02
~' H02
(13)
Fe3+RN-CH2--COO- + HO2
•
Fe2*RN-CH2-COO - + H + + 02. Since in aerated conditions G(Fe 2+) = 6.2-+ 0.2, it is evident that HO2 radical, like H in the deaerated conditions, behaves as a reducing agent. ¢ln The
1
G ( C H O - C O O H ) = G(OH)'
Effect of oxygen From Fig. 2, it can be seen that in presence of O2, the limiting values of G(Fe2÷)=6.2 and G(IDA) = 3.2 are reached at the same chelate concentration as in deaerated conditions• Again, from Figs. 3 and 4, it is clear that there is no change in G(Fe2+), G(IDA) and G(CHO-COOH) values due to oxygen saturation in the reaction medium. This observation is explained as follows,
:' H202+ O2
1 k~5[t-BuOH] G(OH)' k,[Fe(IIDNTA]
(G(OH)' = G(OH) + G(H202)). A
plot
of
I/G(IDA)
against
[t-
BuOH]I[Fe(III)NTA] gave a straight line (Fig. 5) with intercept equal to 0.32 {=I/[G(OH)']}. From the slope of the straight line, assuming t~' the value of kt5 = 4.2 × l0 s M -1 s -t, k4 = k(OH+Fe(nl)NTA) w a s calculated from equation (16) and was found to be equal to 1.1 × l0 s M-~s -t. On comparison of this value with the rate constant k
6°Co T-radiolysis of Iron(III)Nitrolotriacetate in aqueous solutions
.,//
0.9
0,6
/"
I
2.0 16
ief -
-
-
~2
Z-
G
, , . J
08
~''~°/e
OS
04
/ • •
(Fe2+) l
0
J 0
0.15
030
045
[ t - BuOH] / [ F e ( l l l ) N T A ]
FIG. 5. Kinetic plot for the competition reaction for OH radicals between t-Butyl alcohol and Fe(III)NTA at pH 2 under aerated conditions.
Effect of [ormate The effect of formate as a scavenger for OH radicals in aerated condition is interesting because, unlike t-BuOH, the CO2- radical brings about reduction of Fe(III) of the chelate through reactions (17)-(19) followed by reaction (13).
(17)
OH+ HCO0-
I__
COF + O2
Hence, in presence of formate in the solution during radiolysis, there should be a decrease in G(IDA) but no change in G(Fe:+). At 4 x 10-2M formate concentration, G(Fe 2÷) = 6.4 and G(IDA) = 0, indicating the complete scavenging of OH radicals by formate ions. In the presence of intermediate concentrations of the formate ion, a competition will ensue between reactions (4) and (17). Then, one can write the equation (20),
1 (20)
3, 4. 5. 6. 7. 8. 9. 10.
1
G(IDA) = G ( C H O - C O O H )
1
1
k15[HCOO-] - G(OH)~' + G(OH)~' k4[Fe(HI)NTA]"
11. 12. 13.
A plot of IlG(IDA) or I/G(CHO--COOH) against H C O O - / F e ( I I I ) N T A yielded a straight line (Fig. 6) with intercept equal to 0.32 {= I/[G(OH)']}. Taking k4 = 1.1 x 108M -1 s -l, kl7 was calculated to be
IlO
I
o,8
AcknowledgementsmOne of the authors (K. Sahul) acknowledges with thanks the teacher fellowship offered by Centre for Advanced Studies, Department of Chemistry, University of Delhi, Delhi-ll0007. He is grateful to the Management and Professor E. P. Mohamed Ismail, Principal of Jamal Mohamed College, Tiruchirapalli, India, for grant of study leave.
> CO2 + O~~ HO2.
~
equal to 2.6x 109 M -~ s -1 which very well agrees with the value cited in Ref. (24).
>H20+CO2-
02- + H +
O }
FIG. 6. Kinetic plot for the competition reaction of OH between HCOONa and Fe(III) NTA at p H - 2 under aerated conditions.
2. (19)
g !
003 006 009 o~z O~S [HCO0-]/[Fe (111)NTA]
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