J . Phys. Chem. 1986, 90, 5460-5464
5460
Selenium(V). A Pulse Radiolysis Study U. K. Klaning* Chemistry Department, University of Aarhus, DK-8000 Aarhus C, Denmark
and K. Sehested Accelerator Department, R i m National Laboratory, DK-4000 Roskilde, Denmark (Received: February 19, 1986) Two Se(V) species, assumed to be SeO< and HSeO?-, in equilibrium with each other were observed in pulse radiolysis of aqueous selenite and selenate solutions. The following reactions and equilibria were observed: Se03- + OH- 9 HSe042-, K = 0.78 dm3 mol-'; Se03'- + OH + HSe042-,kf = (3.5 f 0.2) X lo9 dm3 mol-' s-l, k, = (7.3 & 0.5) X lo5 s-'; 0- + H 2 0 SeO?- FZ HSeO2- + OH-, kf = (1.1 f 0.1) X lo7 dm3mol-' s-I, k, (3 & 1) X lo5 dm3mol-' s-I; HSeOY + OH (H2Se04-) Se03- + H20, k = (1.6 f 0.1) X 10' dm3 mol-I s-'; H2Se03+ OH (H3Se04) Se03- + H30+,k = (1.0 f 0.1) X lo9 dm3 mol-I s-'; 2Se03Se(1V) + Se(VI), k = ( 5 . 2 f 0.5) X lo8 dm3 mol-l s-I; 2HSe042- Se032-+ Se042-+ H20, k 5 X 10' dm3 mol-' s-I; HSe04'- + Se03Se03'- + Se042-+ H20, k lo9 dm3 mol-' 8;C 0 3 2 -+ Se03C 0 3 - + Se032-,k = (6 f 1) X lo6 dm3 in&' s-'; Se042-+ e a i 2 HSe042-(SeOY + OH-), k = 1.1 X lo9 dm3 mol-' s-I; HSe04- + H (H2Se04-) Se03-+ H20, k lo6 dm3 mol-' 8. The standard Gibbs energy of formation of Se03and HSeO?-, i&CaO0(SeO3-)= -201.6 kJ mol-' and A1C,oo(HSe042-)= -358.2 kJ mol-', and the standard reduction potentials = 1.68 f 0.01 V and Eaoo(HSe042-/Se032-) = 1.69 f 0.01 V were determined from the rate constants Eaoo(Se03-/Se0~-) and the standard Gibbs energy of formation of OH, OH-, H20, and Se032-. The relatively small values for AfiaOo(SeO3-) and Afcaco(HSe042-)agree with the observation that one-equivalent oxidation of Se(1V) or reduction of Se(V1) oxoacids and oxoanions generally are fast processes and indicate that direct electron transfer to selenate or from selenite and transfer of O-/OH to selenite or from selenate are feasible processes.
-+
-
-
-
-
-
-
-
-
-
-
N
Introduction Se(V) produced by pulse radiolysis of aqueous selenate solutions has tacitly been assumed to be the three-coordinated species Se03and protonated forms thereof.'*2 However, in X-ray-irradiated crystals containing Se042-, both Se03- and SeO4'- have been ~bserved.~ The object of the present work is a description of Se(V) in aqueous solution including reactions, structure and standard electrode potentials. By kinetic spectrophotometry we have studied the Se(V) species which arise in pulse radiolysis of aqueous selenate and selenite solutions at 0.3 < pH < 14 by the reaction of H and eaq- with selenate and by 0- and O H with selenite.
TABLE I: Summary of Reactions
Experimental Section The H R C linac at Riser delivered 10-MeV electrons in a single pulse of maximum 1.1 A and with a pulse length 0.2-1 p s . The dose varied from 0.5 to 4 krd/pulse and was measured with the hexacyanoferrate(I1) dosimeter using c420 = 1000 dm3 mol-' cm-' and G = 5.9. Absorbance changes were measured with an optical system which consists of a 150-W Varian high-pressure xenon lamp, a Perkin-Elmer double quartz prism monochromator, a 1P28 ph~tomultiplier,~ and a Nicolet Explorer I11 digital storage oscilloscope. The data were treated on an on-line PDP8 computer. Experiments at high pressure were performed as de~cribed.~ The temperature was ambient (21 f 1 "C). Na2Se03,NaOH, and HC104 were Merck p.a. or Suprapur. Na2Se04was BDH Analar. Gasses were "Dansk Ilt og Brint", N 2 0 (99.5%), Ar (99.99%), and O2 (99.7%). Solutions containing < Se(IV) < Se(V1) mol dmw3< 0.2 and/or 5 X 2X mol dm-3 < were prepared from triply distilled water. The pHs of the solutions were adjusted with N a O H and HC104. In dilute N a O H and HCIO4 solutions pH was measured with a Radiometer P H M 5 2 fitted with a G H 2301C electrode set.
H2Se03 OH SeO< H 2 0 HC H HSeO, Se0,H20 (Se042-,H2Se03,HSeO 12. The HSe0,2- Radical. At pH > 1 2 the spectrum assigned to Se(V) is changed. The band which at pH < 11 is situated at 420 nm shifts at pH > 12 to shorter wavelengths with increasing pH. In N*O-saturated selenite solutions the kinetics of disappearance remains a second-order process. However, in Ar-saturated solutions containing selenate and tert-butyl alcohol, the kinetics of disappearance of the absorbance at pH > 12 is a first-order process with a rate constant increasing with pH. In 0.2 mol am-3 selenate solution, pH 13.5 and saturated with 02,the eaq-reacts preferentially with Se0,'and 0- with oxygen forming 03-.The first-order rate constant for Se(V) disappearance is the same in Ar-saturated tert-butyl alcohol and in 0,-saturated solutions. An additional O< formation at a rate matching the rate of Se(V) disappearance is observed. Similar processes have been detected in the photochemistry and radiation chemistry of p e r i ~ d a t e ' ~and , ' ~ p e r ~ e n a t e , and ' ~ consequently the present observations suggest that the Se(V) species formed in solutions at pH > 12 decomposes with formation of OH/O- radicals, which then subsequently reacts with oxygen. 0 - + 0, 2 0,-
k,, = 3
X
IO9 dm3 mol-' s-I (ref 19)
k - , , = 3.3
X
(14)
lo3 s-' (ref 18)
On the basis of these observations we assign the formula HSeOd2to the additional Se(V) species observed at pH > 12 and suggest the reactions and equilibria (Table I) shown in Scheme I . SCHEME I eaq- + Se04*-
H2 0
HSeOd2-
+ OH-
+ Se03*- z HSeO,*+ Se0,- HSeO,*H 0- + Se032HSeO,*- + OHSe0,- + Se03- Se(1V) + Se(V1) HSe0,2- + HSeO.,Se(IV) + Se(V1) Se(1V) + Se(V1) Se03- + HSe0,2OH
OH-
in solutions at [NaOH] = 0.5, 0.6, 0.8,and 1.Omol dm-). G values were corrected for reactions in spurs.I3 At pH > 13, Gse(") was taken equal to (Ge + GOH+ GH) since at pH > 13 H atoms are converted into ea; by reaction 197before they can enter into other reactions.
+ OH-
H
-
(6,-6) (15)
-
(l6,-16) (13) (17)
---* (18) Spectrum of HSe042-. Equilibrium Constant K15.The composite spectrum at [NaOH] = 0.1 mol dm-3 was determined from measured absorbances, dose, and G values at pH 13.20 G values at pH 1420were used in the determination of the composite spectra
(14) Robinson, R. A,; Stokes, R. H. Elecrrolyte Solutions, 2nd ed. (revised); Butterworths: London, 1970. (15) Klaning, U. K.; Sehested, K. J . Chem. Soc., Farada~vTrans. I 1978. 7 4 . 2819. (16) Klaning, U.K.: Sehested, K.: Wolff, T. J . Chem. Soc., Faraday Tram 1 1981, 77, 1707. (17) Klaning, U. K.: Sehested, K.; Wolff, T.; Appelman, E. H. J . Chem. Soc., Faraday Trans. 1 1982, 7 8 , 1539. (18) Gall, B. L.; Dorfman, L. M. J . Am. Chem. Soc. 1969, 91, 2199. (19) Farhataziz; Ross, Alberta B. Narl. Stand. Ref Data Ser. ( L ' S . Narl. Bur. Srand.) 1977, No. 59. (20) Bjergbakke, E.: Sehested, K.; Lang Rasmussen, 0.;Christensen, H. Input Files for Computer Simulation of Water Radiolysis. Riso-M-2430. Riso National Laboratory. Denmark. 1984
eaq-
+ H,O
k = 1.5 X lo7 dm3 mol-' s-' (ref 7) (19)
In accordance with our assumption of only two Se(V) species, Se03- and HSe042-, the corrected spectra at pH < 1 1 and at [NaOH] = 0.1, 0.5, 0.6, 0.8, and 1.0 mol dm-3 display an isobestic point at 402 nm (Figure 1). The composite spectra thus measured were used for the determination of the equilibrium constant K 1 5and the spectrum of HSeO,?-. Due to the fact that the ionic strength varied between 0.3 and 1.3 the method of determining K,, and the spectrum of HSeO,*are not straightforward. Since it was not convenient to use solutions prepared to constant high ionic strength by salt addition, the following method, which proved to give fairly consistent results, was adopted Kl5
= xfHSeOd2-/((1 - .~)fSeO,~[OH-lfOH~)
(20)
where
+ [SeO,-1)
(21)
/ (6HSe042- - %eo3-)
(22)
x = [HSe0,2-]/([HSe0,2-] x may also be expressed by
x=
(cobsd
-
€SeO;)
where t&d is the extinction coefficient of Se(V) in solution containing Se03- and HSe042- in equilibrium concentrations and tHSeO 2- and em,of HSe0,2- and SeO0.99.The value used in the thermodynamic calculations below is K , , = 0.78 dm3 mol-'. The spectra calculated from eq 23 and 24 and formulas 1-111 display a band maximum at 375 f 10 nm with an extinction coefficient ranging from 1500 to 1800 dm3 mol-' cm-' (Figure 1).
,,,
Rate Constants k 4 , k 16, k-16rk and k Since the spectrum and the decay kinetics of Se(V) in N20SeOj2- and in 0,-free Se032--Se0,2- solutions are identical we assume that equilibrium 15 is maintained during the decay. With this assumption, kobsdmay be expressed by kobsd/X
= k-6
+k-16[~~-~/f-4
(25)
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5463
Pulse Radiolysis Study of Se(V)
1
I&/ I
I
I
5
I;.,
I
I
10 15 1[OH-]/f! )/mo Id N 3
20
Figure 2. First-order rate constant observed for the decay of Se(V) in alkaline solution, koM divided by x, the fraction of Se(V) which is HSeO?-, plotted against [OH-]/f_4.f-4 calculated by using formula IV (0)and V ( X ) (see text).
where f ? is a function of activity coefficients, which accounts for the primary salt effect on reaction -16. Values of k+ and k-16 were determined graphically from plots of k o M / xagainst [OH-]/f? (Figure 2). With x calculated from eq 20 for KI5 = 0.63 dm3 mol-' and f - 2 given by I and using the 1 p1l2)to calculate corresponding formula, IV, -log f? = 2pL'l2( f?, we find by fitting to a straight line k-6 = 6.7 X lo5 s-' and k-16= 3.9 x IO5 dm3 mol-' s-I (correlation coefficient 0.998). Similarly, using K15 = 0.91 dm3 mol-', f? given by 111, and f-4 given by the corresponding formula, v , f-4 = f&4Na3As04/ C f * Z N a B r ~ f f z ~ we a ~ ~find ) , kd = 7.9 X lo5 s-' and k-16 = 2.1 X lo5 dm3 mol-' s-' (correlation coefficient 0.99). Formula I1 for calculation off? and the corresponding formula -log f-4 = 2 ~ ' / 1~ ( p11/2) - 0.411 were not used since these formulas lead to a strong deviation from linearity. In the calculations below are used k4 = 7.3 X lo5 s-I and k-'6 = 3 X lo5 dm3 mol-' s-l (Table I). An accurate determinaticrn of k16 was not possible, in strongly alkaline solution owing to interference from the reaction of C03zpresent as impurity (see below) and in more dilute alkaline solution owing to the interference from reaction 6. At pH 13 we find, after correcting for reaction 6, values for k I 6ranging from 9 X lo6 to 6 X lo7 dm3 mol-' s-I. Determination of k6, km6,and k-16,however, enables us to calculate a more accurate value of k16. The base constant of 0-
+
+
0-
+ H 2 0 F? OH + OH-
(26)
may be expressed as KB = (k4 k 1 6 ) / ( k k-16). 6 With KB = (8 f 1) X we find k16 = (1.1 f 0.1) X lo7 dm3 mol-' s-'. We estimate k15 k I 7 0.5k18since the rate of decay of absorbance measured in solutions at a constant ionic strength of 0.33 at the wavelength of the isobestic point did not vary with PH.
- -
Thermodynamic Calculations. One-Equivalent Redox Reactions of Selenite, Selenate, Sulfite, and Sulfate Values for the standard Gibbs energy of formation of HSe02and Se03-, A,Gaoo(HSe042-)and A,Gaoo(SeO;), were calculated from A,Gaoo(HSe04Z-)= R T In (k-6/k6)
+ A,Gaoo(Se03z-)+ A@,,"(OH)
(27)
and A,Gaoo@eo3-) = A,Gaoo(HSe04z-)- A,Gaoo(OH-)
+ R T In K I 5 .(28)
We find A,Gaoo(HSe0,2-) = -358.2 kJ mol-' and A,Ga,"(Se03-) = -201.6 kJ mol-' using A,GaOo(OH) = 26.8 kJ mol-Iz1 and (21) Klaning, U. K.; Sehested, K.; Holcman, J. J.,Phys. Chem. 1985, 89,
760.
,
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P
m
-700
El
OxKktnn Nmber
Figure 3. Standard Gibbs energies A@,,' of formation, A of SOj2-,22 SO
