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Kinetics of electron transfer reactions between dioxoactinide(VI) ions and a viologen radical anion
Journal of the Less-Common
Metals, 149 (1989)
193
- 196
193
KINETICS OF ELECTRON TRANSFER REACTIONS BETWEEN DIOXOACTINIDE(V1) IONS AND A VIOLOGEN RADICAL ANION* C. G. PIPPIN,
G. R. CHOPPIN
Department
of Chemistry, Florida State University, Tallahassee, FL 32306
D. MEISEL
and J. C. SULLIVAN
Chemistry Division, Argonne National Laboratory, IL 60439 (U.S.A.) (Received
June
27, 1988;
in revised form
(U.S.A.)
9700 South Cass Avenue, Argonne,
September
19, 1988)
Summary
The rates of electron transfer between the dioxoactinide ions (UOz2+, NPO~~+and Pu02”) and the 4,4’dipropylsulfonate-2,2’-bipyridium radical anion (ZV-) were determined by pulse radiolysis. The measured rate parameters are compared with those predicted by the Marcus relation using the known reduction potentials and self exchange rate parameters for U022+ and NPO~~+.
1. Introduction
The reactions of the dioxocations U(VI), Np(V1) and Pu(V1) with a common reductant provide an opportunity to delineate the effects of variation in thermodynamic driving force on the rates of electron transfer reactions. The An(V1) cations present a unique series in that the co-linear O-An-O configuration is maintained throughout the triad in both oxidation states. The present study was motivated, in part, to test the applicability of the Marcus formalism [l] to calculate the values for the cross-reactions using previously determined values for the self-exchange reactions of the actinides. The reductant of choice in the present investigation, 4,4dipropylsulfonate-2,2’-bipyridinium radical anion, ZV-, is a well-characterized outersphere reductant [2], i.e. the potential and self-exchange reaction rate have been determined.
*Paper presented at the September 12 - 16, 1988. 0022-5088/89/$3.50
18th
Rare
Earth
Research
0 Elsevier
Conference,
Sequoia/Printed
Lake
Geneva,
WI,
in The Netherlands
194
2. Experimental details Details of preparation and standardization of the U(VI), Np(V1) and Pu( VI) stock solutions have been given in a previous publication [ 31, as have the characteristics and operation of the Argonne National Laboratory linear accelerator. 3. Results and discussion The reactions that occur when an argon-saturated solution 0.1 M 2propanol, 2 X lo4 M in ZV is irradiated with a beam of electrons are Hz0 -+
eaq-, 6H, H, H,, HzO,
(CH,),HCOH + 6H(H) = (CHs)&H (CHs)&H
(1) + H,O(H,)
+ ZV = (CH,),CO + ZV’ + H+
eaq- + zv = zv’
(2) (3) (4)
The above sequence demonstrates that the resulting system is predominantly a one radical reducing system. When the solution also contains An(V1) ions the additional reactions An(V1) + ZV’ = An(V) + ZV
(5)
will occur. In all experiments the concentration of An(V1) was 50 - 100 times greater than that of the ZV’ radical. The values of the first-order rate parameters as a function of An(V1) concentration are summarized in Table 1. The complete data set for each system was adequately correlated by the linear function k ,+ = a + b An(V1)
(6)
where b is identified as Fz,. In all cases a was not statistically defined as different from zero. The values calculated for the rate are presented in Table 2. It is apparent from the data presented in this table that there is a marked increase in the rate parameters when the oxidant is varied from U(V1) to either Np(V1) or Pu(V1). The values of the rate parameters for the last two systems approach the values typical of diffusion-controlled reactions. Therefore the following analysis of the data, based on the Marcus formalism for an outer-sphere electron transfer reaction, was modified to correct for the effects of diffusion. The initial calculation of the rate parameter k, uses the simplified form of the Marcus equation k,, = (k,,k,,Q=)“2
(7)
where k,, and k,, are the rate parameters for the respective self-exchange reactions of the An(VI)/(V) and ZV/ZV’, K, is the value calculated for the equilibrium constant of reaction (5) and
195 TABLE 1 Observed rates for the reduction of An0z2+ by ZV’ AnOz2+
uo*2+
radicals *
(x
[An022+] lo4 M)
k obs (x 10-4 s-l)
1.00 2.00 4.00 10.0
1.17 2.63 4.78 10.1
f + + +
0.15 0.24 0.01 0.4
(x10-S
s-r)
Np0a2+
0.40 0.70 1.9 3.2
0.805 f 0.006 1.62 + 0.02 3.78 f 0.12 7.32 f 0.09
puo22+
0.250 0.624 1.24 1.86 2.48
0.379 k 0.062 1.36 5 0.04 1.82 * 0.07 4.43 + 0.01 5.27 * 0.05
* AI1 experiments were performed in argon-saturated solution with [ ZV] = 2.0 [ 2-propanol] = 0.13 M, pH 4.0, and [NaClOe] = 0.10 M.
10e4 M,
X
TABLE 2 Summary of measured and calculated rate parameters for the reaction between An(V1) and ZV’ radicals An0a2+
AR” (v)s
K12
Radius (8)
k5 (M-l s-~)~
uo22+ Np02’+ puoz2+
0.408 1.48 1.26
7.89 x lo6 1.13 x 102s 2.24 X 102’
3.3 3.7 4.0
9.80 ?r:0.04 x 10’ 2.27 + 0.09 x 10’ 2.26 + 0.18 x 10’
AnOz2+
h
kc
(M-' s-l)
klz (M-
8.55 x 10’ 8.20 x 10’ 7.96 x 10’
8.15 x 10’ 8.40 x 10” 5.3 x 1010
(M-' s-l )
uo22+
NpOz2+ puoz2+
15f 105s 1OOh
*E” for ZV = 0.345 V us. SHE; kll b Error corresponds to *u. ‘Radius of ZV- = 5.4 8. d Equation (7 ). ek = knkcltklz + kc). f IGfT4. g Ref. 5. h Estimated.
= 5.4
x
lo8 M-’
s-l.
d
ke 1 s-1
)
(M-
1 s-1
I
8.08 x 10’ 7.47 x 109 6.9 x 10’
and 2 = 10” M-’ s-l. The values for diffusion-controlled k = (7.4X d (eb -
reaction were then calculated
109)b 1)
(9)
where b = 14ZJ&z, a is the sum of the ionic radii and k is corrected from zero ionic strength to that of the reaction medium, 0.1, by the simplified Debye-Htickel relation h, = lzd1ozizzc11’2/(1 + /..F)
(10)
The corrected value of the rate parameter is then k=
k,,k, @I2
(11)
+kJ
The results of these calculations are presented in Table 2. The agreement between the experimentally determined rate parameters and those calculated is excellent. This agreement provides strong evidence for the validity of the estimated value of the Pu(VI)/(V) exchange reaction a quantity that is not subject to direct measurement but is of importance in the characterization of plutonium oxidation state speciation.
Acknowledgment This investigation was supported by the Office of Basic Energy Sciences, Division of Chemical Science, U.S. Department of Energy, under Contract W31-109-ENG-38.
References R. A. Marcus, J. Chem. Phys., 43 (1965) 679; 2654. A. L. Rieger and P. H. Rieger, J. Phys. Chem., 88 (1984) 5845. W. A. Mulac, S. Gordon, K. H. Schmidt, D. Wester and J. C. Sullivan, Znorg. Chem., 23 (1984) 1639. K. R. Howes, G. Pippin, J. C. Sullivan, D. Meisel, J. H. Espenson and A. Bakac, Znorg. Chem., 27 (1988) 2932. Calculated from values listed by E. Pelizzetti, M. Woods and J. C. Sullivan, Znorg. Chem., 20 (1981) 3973.
Metals, 149 (1989)
193
- 196
193
KINETICS OF ELECTRON TRANSFER REACTIONS BETWEEN DIOXOACTINIDE(V1) IONS AND A VIOLOGEN RADICAL ANION* C. G. PIPPIN,
G. R. CHOPPIN
Department
of Chemistry, Florida State University, Tallahassee, FL 32306
D. MEISEL
and J. C. SULLIVAN
Chemistry Division, Argonne National Laboratory, IL 60439 (U.S.A.) (Received
June
27, 1988;
in revised form
(U.S.A.)
9700 South Cass Avenue, Argonne,
September
19, 1988)
Summary
The rates of electron transfer between the dioxoactinide ions (UOz2+, NPO~~+and Pu02”) and the 4,4’dipropylsulfonate-2,2’-bipyridium radical anion (ZV-) were determined by pulse radiolysis. The measured rate parameters are compared with those predicted by the Marcus relation using the known reduction potentials and self exchange rate parameters for U022+ and NPO~~+.
1. Introduction
The reactions of the dioxocations U(VI), Np(V1) and Pu(V1) with a common reductant provide an opportunity to delineate the effects of variation in thermodynamic driving force on the rates of electron transfer reactions. The An(V1) cations present a unique series in that the co-linear O-An-O configuration is maintained throughout the triad in both oxidation states. The present study was motivated, in part, to test the applicability of the Marcus formalism [l] to calculate the values for the cross-reactions using previously determined values for the self-exchange reactions of the actinides. The reductant of choice in the present investigation, 4,4dipropylsulfonate-2,2’-bipyridinium radical anion, ZV-, is a well-characterized outersphere reductant [2], i.e. the potential and self-exchange reaction rate have been determined.
*Paper presented at the September 12 - 16, 1988. 0022-5088/89/$3.50
18th
Rare
Earth
Research
0 Elsevier
Conference,
Sequoia/Printed
Lake
Geneva,
WI,
in The Netherlands
194
2. Experimental details Details of preparation and standardization of the U(VI), Np(V1) and Pu( VI) stock solutions have been given in a previous publication [ 31, as have the characteristics and operation of the Argonne National Laboratory linear accelerator. 3. Results and discussion The reactions that occur when an argon-saturated solution 0.1 M 2propanol, 2 X lo4 M in ZV is irradiated with a beam of electrons are Hz0 -+
eaq-, 6H, H, H,, HzO,
(CH,),HCOH + 6H(H) = (CHs)&H (CHs)&H
(1) + H,O(H,)
+ ZV = (CH,),CO + ZV’ + H+
eaq- + zv = zv’
(2) (3) (4)
The above sequence demonstrates that the resulting system is predominantly a one radical reducing system. When the solution also contains An(V1) ions the additional reactions An(V1) + ZV’ = An(V) + ZV
(5)
will occur. In all experiments the concentration of An(V1) was 50 - 100 times greater than that of the ZV’ radical. The values of the first-order rate parameters as a function of An(V1) concentration are summarized in Table 1. The complete data set for each system was adequately correlated by the linear function k ,+ = a + b An(V1)
(6)
where b is identified as Fz,. In all cases a was not statistically defined as different from zero. The values calculated for the rate are presented in Table 2. It is apparent from the data presented in this table that there is a marked increase in the rate parameters when the oxidant is varied from U(V1) to either Np(V1) or Pu(V1). The values of the rate parameters for the last two systems approach the values typical of diffusion-controlled reactions. Therefore the following analysis of the data, based on the Marcus formalism for an outer-sphere electron transfer reaction, was modified to correct for the effects of diffusion. The initial calculation of the rate parameter k, uses the simplified form of the Marcus equation k,, = (k,,k,,Q=)“2
(7)
where k,, and k,, are the rate parameters for the respective self-exchange reactions of the An(VI)/(V) and ZV/ZV’, K, is the value calculated for the equilibrium constant of reaction (5) and
195 TABLE 1 Observed rates for the reduction of An0z2+ by ZV’ AnOz2+
uo*2+
radicals *
(x
[An022+] lo4 M)
k obs (x 10-4 s-l)
1.00 2.00 4.00 10.0
1.17 2.63 4.78 10.1
f + + +
0.15 0.24 0.01 0.4
(x10-S
s-r)
Np0a2+
0.40 0.70 1.9 3.2
0.805 f 0.006 1.62 + 0.02 3.78 f 0.12 7.32 f 0.09
puo22+
0.250 0.624 1.24 1.86 2.48
0.379 k 0.062 1.36 5 0.04 1.82 * 0.07 4.43 + 0.01 5.27 * 0.05
* AI1 experiments were performed in argon-saturated solution with [ ZV] = 2.0 [ 2-propanol] = 0.13 M, pH 4.0, and [NaClOe] = 0.10 M.
10e4 M,
X
TABLE 2 Summary of measured and calculated rate parameters for the reaction between An(V1) and ZV’ radicals An0a2+
AR” (v)s
K12
Radius (8)
k5 (M-l s-~)~
uo22+ Np02’+ puoz2+
0.408 1.48 1.26
7.89 x lo6 1.13 x 102s 2.24 X 102’
3.3 3.7 4.0
9.80 ?r:0.04 x 10’ 2.27 + 0.09 x 10’ 2.26 + 0.18 x 10’
AnOz2+
h
kc
(M-' s-l)
klz (M-
8.55 x 10’ 8.20 x 10’ 7.96 x 10’
8.15 x 10’ 8.40 x 10” 5.3 x 1010
(M-' s-l )
uo22+
NpOz2+ puoz2+
15f 105s 1OOh
*E” for ZV = 0.345 V us. SHE; kll b Error corresponds to *u. ‘Radius of ZV- = 5.4 8. d Equation (7 ). ek = knkcltklz + kc). f IGfT4. g Ref. 5. h Estimated.
= 5.4
x
lo8 M-’
s-l.
d
ke 1 s-1
)
(M-
1 s-1
I
8.08 x 10’ 7.47 x 109 6.9 x 10’
and 2 = 10” M-’ s-l. The values for diffusion-controlled k = (7.4X d (eb -
reaction were then calculated
109)b 1)
(9)
where b = 14ZJ&z, a is the sum of the ionic radii and k is corrected from zero ionic strength to that of the reaction medium, 0.1, by the simplified Debye-Htickel relation h, = lzd1ozizzc11’2/(1 + /..F)
(10)
The corrected value of the rate parameter is then k=
k,,k, @I2
(11)
+kJ
The results of these calculations are presented in Table 2. The agreement between the experimentally determined rate parameters and those calculated is excellent. This agreement provides strong evidence for the validity of the estimated value of the Pu(VI)/(V) exchange reaction a quantity that is not subject to direct measurement but is of importance in the characterization of plutonium oxidation state speciation.
Acknowledgment This investigation was supported by the Office of Basic Energy Sciences, Division of Chemical Science, U.S. Department of Energy, under Contract W31-109-ENG-38.
References R. A. Marcus, J. Chem. Phys., 43 (1965) 679; 2654. A. L. Rieger and P. H. Rieger, J. Phys. Chem., 88 (1984) 5845. W. A. Mulac, S. Gordon, K. H. Schmidt, D. Wester and J. C. Sullivan, Znorg. Chem., 23 (1984) 1639. K. R. Howes, G. Pippin, J. C. Sullivan, D. Meisel, J. H. Espenson and A. Bakac, Znorg. Chem., 27 (1988) 2932. Calculated from values listed by E. Pelizzetti, M. Woods and J. C. Sullivan, Znorg. Chem., 20 (1981) 3973.