ELECTRONIC SPECTRA OF THE OXYANIONS OF SELENIUM IN SOLUTION
4131
various dcgrees of neutralization of polymrylic acid can be obtained as shown in Figure 9. Using the results of Figure 9, the values of AGe1 corresponding to the present experiment are obtained. Figure 10 shows a AG+ vs. AGE1plot, where AG* is obtained for Zn(I1) and Co(I1) ions, using the Na-PA systems containing 10-90 M NaC104. Figure 11shows a AG vs. AGE1plot when the degree of neutralization of polyacrylic acid is varied (a = 0.9-0.4). I n Figure 12, the result is shown when 40 mM of sodium perchlorate is added to the systems at various degree of neutralization. I n Figures 10-12, it is clearly exhibited that AG* is proportional to AGel with a slope of about unity. These results indicate that a decrease in AG* comes from that in AGe1. I n other words, the present results are consistent with the results obtained from the potentiometric titration. I n conclusion, it appears that the present model for the diffusion is reasonable for the system under investigation.
*
5 t
L
0
3
2
AGd-'
Kwl/~
Figure 12. Relationship between AG* for Zn(I1) and Co(I1) ions and AGel at various degrees of neutralization (a= 0.4-0.9). Sodium perchlorate (40 mM) is added to the systems at various degrees of neutralization.
ometric titration. Nagasawa and his c o t ~ o r k e r are~~ ported the titration curves for polyacrylic acid at various ionic strengths. From these data, the relationship between AGE, and the ionic strength of the solvent at
Acknowledgments. We are greatly indebted to Dr. Y. Arata for valuable discussions. (22) 11. Nagasawa, T. Murase, and K. Kondo, J . Phys. Chem., 69, 4003 (1965).
Electronic Spectra of the Oxyanions of Selenium in Solution by A. Treinin and J. Wilf Department of Physkal Chemistry, The Hebrew University, Jerusalem, Israel
(Received May 19, 1870)
The electronic spectra of SeOs2-,HSeOa-, Se042-,and HSe04- were analyzed by studying their response to environmental effects. The first absorption band in each spectrum is assigned to an allowed sub-Rydberg transition involving the nonbonding electrons on the oxygen atoms. In the spectra of SeOs-, HSeOa-, and Se02- this band is followed by a charge-transfer-to-solvent (ctts) band. The ctts bands of the dinegative ions appear to be twice as sensitive as that of HSeOs- towards solvent effects, whereas their temperature sensitivities are nearly the same. pH and temperature effects yield some thermodynamic data on the equiH* SeO,+. The vertical ionization potential and the effective crystallographic radius libria HSe0,of HSeOa- are derived from the analysis of solvent effects on its ctts band. Under the conditions employed the uv spectra do not reveal the formation of new species, such as dimers or esters.
+
The aim of this research is to examine the effect of electrical charge on electronic spectra of closely related anions. Solutions of mono- and dinegative oxyanions of selenium, readily converted into each other by pH adjustment, display considerable absorptions in readily accessible spectral regions. The solutions are rather stable and pure tetraalkylammonium salts can be prepared that dissolve in alcohols and acetonitrile. Thus environmental effects on the spectra could be studied in detail. Some studies of this type were
already conducted with oxyanions of phosphorus,' but their scope was somewhat limited owing to relatively low absorptions and insolubility of the salts employed in pure organic solvents. Previous information on the spectra of oxyselenides2 is meagre and in some discord with our results. Here (1) M. Halmann and I. Platzner, J. Chem. SOC.,1440 (1965); H. Renderly and M.Halmann, J. Phys. Chem., 71, 1053 (1967). (2) H.Ley and E. Konig, 2.Phys. Chem. Abt. B , 41,365 (1938).
The Journal
of
Physical Chemistry, Vol. 74, No. $8,1970
A. TREININ AND J. WILF
4132 we present an analysis of electronic transitions in SeOa2-, HSe03-, Se04z-, and HSe04-. The first three display charge transfer-to-solvent (ctts) transitions; this gives us an opportunity to examine environmental effects on ctts bands of polyvalent anions and compare them with theorya3 Some thermodynamic data are derived from spectroscopic results.
Experimental Section Materials. The KazSe03(B.D.H.) was recrystallized from concentrated aqueous solution by adding acetonitrile. A second recrystallization proved to have no effect on the spectrum. NaHSe03 (B.D.H.) was recrystallized from CH3CN; both materials exhibited the same spectrum a t the same pH. N(C2H&HSeO3 and (N(C2H5)&Se03 were prepared by neutralization of HzSeOI (Riedel-De Haen) with a solution of N(CzH6)40H (Fluka, Purum; some yellow impurities were removed from this solution by several extractions with CH3CN); pH 6 and 10.5 were chosen as end points for their preparation, respectively. Water was expelled under reduced pressure and the dry materials recrystallized from CH3CN. The biselenite was then dried under vacuum at 40". The selenite was redissolved in a solution of N(CzH5)40H,water evaporated, and the material was thoroughly dried. I n this way an excess of N(CzH&OH was added to the selenite; the pH of M in water was 10.5. This was done so as to avoid conversion of SeOS2- to HSeOa- in the organic solvents employed. Na2SeO4(Analar) was used without further purification. (N(C2H&)2Se04was prepared by neutralization of HzSe04(Riedel De Haen) with N(CzH&OH (pH 6 as end point), the material crystallized from concentrated solution by adding CH3CN, rinsed with CHaCN, and dried under vacuum a t 40". The purity of tetraethylammonium salts was checked by measuring their spectra in aqueous solutions; the spectra were identical with that of the sodium salts (see, e.g., Figure 5, which also indicates that N(CzHs)40H present in excess did not contribute to the absorption). The tetraethylammonium salts were used to introduce the oxyselenide ions into organic solvents. The following experiment was performed to check the identity of ionic species in CHaCN: further addition of N(CZH5)kOHto selenite had no effect on its spectrum while a little concentrated HC104 changed the spectrum to that of biselenite in the same solvent. Concentrated HC104 was also used to convert Se0d2- to HSe04- while adding only little HzO to the organic solvent. (The pH of acidic solutions was below 1, as measured with indicator paper.) The organic solvents were of spectroscopic grade. Special care was taken to dry CH&N because traces of water exerted a considerable effect on the spectra of SeOa2- and Se042-. For this purpose CH3CX (Matheson Coleman and Bell) was shaken with anhydrous The Journal of Physical Chemistry, VoL 74, N o . d d , 1970
Cas04 for 70 hr. (Similar results were obtained with CuSOe.) Water was triply distilled, DzO (99.7%, Fluka, Puriss) was used without further purification, and all other materials were of Analar grade. Measurements. Spectra were measured with a 450 Perkin-Elmer spectrophotometer equipped with a thermostated cell compartment to keep temperature constant within 0.1". Absorption cells with 10 mm, 1 mm, and variable path length (down to 0.1 mm) were employed. Absorbance up t o 1.8 could be read without distinct deviations from Lambert's law. The instrument was flushed with NZfor measurements below 190 nm. pH was measured with a Metrohm pH meter (ApH f 0.01). Perchloric acid, phosphate, and borate buffers were employed to vary the pH in the range 0-11.5.
*
Results All the oxyselenide ions were found to obey Beer's law over all the wavelength region studied. Range of ooncentrations studied was 10-2-10-4 M in water and in organic solvents. With HSeOa- in water the law was checked from loV5M to 1 M and no deviation was detected; this is in contrast to the behavior of HSOa-, which dimerizes to 5202- under such condition^.^ Another difference lies in the stability of HSeOs- towards oxidation by 0 2 : no change in spectra was displayed by air containing solutions after 48 hr, while HSOa- was considerably oxidized under the same treatment. From the pH effect on the spectra of aqueous systems the thermodynamics of
+
HSe0,H+ SeOB2(1) could be studied. A sharp isosbestic point is exhibited by the selenite system (Figure 1). The selenate system appears to
I
I
1
I
9.10 9.2 -10.6
X.nm
Figure 1. Isosbestic point displayed by the HSeOa--SeOa*- system. (3) A. Treinin, J. Phys. Chem., 68, 898 (1964). (4) R. M. Golding, J. Chem. Soc., 3711 (1960).
ELECTRONIC SPECTRA OF THE OXYANIONS OF SELENIUM IN SOLUTION Table I: Thermodynamic Data for the Equilibrium HSe0,-
* H+ + SeOn2-
7 H S e O s - P Ht
PK
AGO (kcal/mol)
A H o (kcal/mol)
A S o (eu)
4133
+ SeOs’---
Ht
-HSeOd-
T,‘ C
Present work
Previous data
10 20 26
8.50 & 0.02 8.43 f 0.02 8.39 =k 0.03“
8.0b 8.32b 6.600
30 50 25, 25 25
8.37 & 0.03 8.23 & 0.03 11.4 =k 0.4 2.8 & 0.2 -29 f 2
+ SeOG------
Present work
Previous data
1.80 =k0.02 1.84 & 0.04
1.82b 1.86b 1.88b 1.70 A 0.01d
2.5 -2.20 -16.3C
1.10
-26.5’
From the plot of pK against 1/T. b “Stability Constants,” Special Publication No. 17, The Chemical Society, London; see this reference for more data. 0 W. M. Latimer, ‘‘Oxidation Potentials,” 2nd ed, Prentice-Hall, Englewood Cliffs, N. J., 1952, p 83. d A. K. Covington and J. V. Dobson, J . Inorg. Nucl. Chem., 27,1435 (1965).
HS~O;
w
z
a
HSeO;
1.2
A=
HC+SsOi-
--. n++sro;-
!ern optical patn
220nrn
0
m 1.0
1
1 2
I 4
I
I
6
8
10
PH
Figure 2. Typical “titration curves” displayed by the HSeOa--Se082- and HSeOd--Se02- systems.
reach such a point below 185nm (see Figure 3). Figure 2 shows a typical “titration curve” for each system. The pK of equilibrium 1 was determined by means of the approximation
..-
190
200
A, nm
21 0
220
230
Figure 3. Absorption spectra of the oxyselenide ions a t 20’.
where DRSeO,-, Dseo,S- and D are the absorbance values of the system when present in its acidic form, alkaline form, and a t the given pH, respectively; the last term is the simplified correction for log yHSeO,-/ yseO,a- (y = activity coefficient). The ionic strength I was calculated by successive approximations from the amounts of electrolytes introduced and equilibrium constants. The second term on the right (eq 2) was determined from plots of DHse0,- - D against D Dseon2-at the same pH and different wavelengths.6 Calculations were carried out for three pH values within pK f 1.0. From the dependence of pK on temperature AH1° and AX1O were determined for HSeOs- (Table I). The values of A a t various temperatures were taken
from ref 6. The results with HSe04- were less reliable (low pH and high ionic strengths, up to 0.3 M ) ; pK appeared to increase with temperature but AH and A S could not be determined. The results are summarized in Table I; agreement with previous data is satisfactory. Environmental Egects. Figure 3 records the spectra of oxyselenides in water. HSeOa-, SeOsz-, and Se042each displays two overlapping quite intense bands (designated A and B), which are best resolved in the spectrum of HSeOe-. The two bands respond differently to (5) See e.g., J. Jortner and G . Stein, Bull. Res. Counc. Isr., Sect. A , 6, 239 (1957). (6) R. A. Robinson and R. H. Stokes, “Electrolyte Solutions,” 2nd ed, Butterworths, London, 1959, p 468.
The Journal of Physical Chemistry, Val. 74, No. $3,1070
A. TREININ AND J. WILF
4134 I
I
I
i
r
[
CHICN
,*--.
-
,\,,,
9
2 . 5 X 10-'M HSQO; 20'
( p e r l m m opticl path1 Y
3
.\\
-
1.0
'.
'.
I 190
I 200
I 210
220
230
A. n m Figure 6. Solvent effects on the spectrum of HSeOs-. 3.6n1V3 M HSeO;
( p H 6.1)
1.6
yj z
1.2 (per lmrn optical path)
3 0
190
200
21 0
x, nrn
220
Figure 4. Temperature effects on spectra of the oxyselenide ions.
I
I
1
1
*m
0.9
0.4
I
190
200
210
230
220
A, n m Figure 7. Solvent effects on the spectrum of Se04e-.
m
1.5
I0 n
r-
- - -
c
1.54 x lo-' M HSQO;
:
20. (per lmrn optical path) 0.5
190
200
210
220
230
A , nm Figure 5. Solvent effects on the spectrum of SeOsa-.
changes in environment, the single band of HSeOrexhibiting the band-A-type of behavior. Figure 4 shows effects of temperature on the spectra and Table I1 records -d(hv)/dT at various absorptivities. Band B is somewhat more sensitive with -d(hv)/dT 20 cm-' deg-' regardless of ionic charge. The difference in d(hv)/dT brings about a shift in the isosbestic point for Se032--HSe03- to longer wavelengths with rise in temperature.
-
The Journal of Physical Chernietry, Vol. 74, No. 33,1070
-.190
200
210
,220
230
A. n m Figure 8. Solvent effects on the spectrum of HSeO,-.
A more pronounced distinction between A and B is shown by solvent effects (Figures 5-8). Band A undergoes the regular "blue shift" with hv increasing in the DzO. On order CH3CN < glycol < ethanol < HzO the other hand, the order for B is: CHsCN < HzO
