Indian Journal of Chemistry Vol. 28A, April 1989, pp. 298-301

Behaviour of some seleniumtl'V ) and (VI) compounds in 100% sulfuric acid

R J Gillespie! & R Kapoor* Department of Chemistry, Panjab University, Chandigarh 160014 Received 3 October 1988; accepted 28 December 1988

Cryoscopic and conductometric measurements and 77SeNMR data have shown that dioxide,

selenous acid and selenite dissolve in 100% sulfuric acid to yield Se02H ", Se204H +, Se)06H + and possibly other higher polymeric species.Similarstudies on selenicacid, sodium selenate and seleniumtriox- ide show that behaves as a weak base and that selenium trioxide forms disulfoselenic acid,

H2SSe07 which ionizes in 100% sulfuric acid as a weak acid.

The behaviour of in 100% sulfuric urn trioxide inside the apparatus and it was resu- acid and dilute oleum has been studied previously by blimed at 90-100° at 1mm Hg9. Selenium oxychloride Flowers et al) who showed that it behaves as weak (Alfa) was used as such. base, i.e. it is partially ionized to give Se02H + and The preparation of 100% sulfuric acid has been de- possibly other higher polymeric species such as scribed previously 10,11.The freezing point depression

Se204H +.Paetzold and Garsofke/ have explained the and electrical conductivities of solutions were mea- Raman spectrum of selenous acid in perchloric acid sured by methods described earlier!"!". Selenium-77 in terms of protonated selenous acidium ion, NMR spectra were obtained at 95.38 MHz on a Bru- Se(OH){. In view of the well known tendency of sele- ker AM 500. nous acid to undergo dehydration, it seems improb- able that selenous acid exists in protonated form in Results and Discussion perchloric acid. We have, therefore, reinvestigated SeleniumUV) compounds, viz: selenium dioxide, the behaviour of selenous acid, sodium selenite and selenous acid and sodium selenite selenium dioxide in 100% sulfuric acid. We have also The conductometric and cryoscopic results along studied the behaviour of some selenium(VI) com- with the interpolated v and vvalues' are given in Table pounds. 1. The formation of the selenous acidium ion (see Eq. 1) Materials and Methods H Se0 + H S0 =Se(OH){ + HSOi ...(1) Selenium dioxide (BDH) was purified by evapora- 2 3 2 4 tion with fuming nitric acid and then sublimed in a dry (v=2, y= 1) atmosphere at 200°C. The white needle-like is not in accord with v and yvalues which are greater crystals were stored over P 0 in a desiccator. Sele- 4 lO than 2 and 1, respectively. Since selenous acid is nous acid (BDH) was recrystallized from water, and known to dehydrate readily (see Eq. 2) it seems rea- dried for several days in a desiccator over KOH. sonable to postulate the formation of Se0 and its Commercially available selenic acid was dehydrated 2 subsequent protonation as suggested by Flowers et by bubbling dry air through the liquid at 150°C. The li- aU quid was allowed to partially crystallize severaltimes and each time the mother liquor was discarded. The H2Se03 + 2H2S04 pure selenic acid thus obtained had a of =Se02H++H30++2HSOi ... (2) 56- 58° (see ref. 7). Selenic acid corresponding to min- (v=4, y=2, v-y=2.0) imum conductivity (5.6 x 10 - 2 ohm - I em - I at 25°C) However, reaction (2) alone cannot account for the was prepared in the conductivity cell by addition of observed v and y values (Table 1). In order to obtain small amounts of selenium trioxide". Selenium triox .•. further information on this system we have recorded ide was prepared by heating equal weights of 96% sel- the 77SeNMR spectrum of solution of selenium diox- enicacid and P4010at 150° under reduced pressure (1 ide in 100% sulfuric acid. The spectrum displays mm Hg). A cold finger was used to collect the seleni-

t Yis the number of moles of HSO 4 formed by one mole of the so- t Department of Chemistry, McMaster University, Hamilton, On- lute and v is the total number of moles of ions and molecules pro- tario, Canada, 1..8S4M 1. duced in solution by one mole of the solute

298 GILLESPIE et al: BEHAVIOUR OF SELENIUM COMPOUNDS IN 100% SULFURIC ACID three distinct peaks at 1402, 1358 and 1275 ppm. These signals occur at high frequency side compared Table l+-Interpolated values of v and y of solutions of selenous acid and sodium selenite in 100% sulfuric acid to that of (CH3)2Se (Fig. 1).This suggests the presence m Freezing point v lOzk. y v - y of at least three selenium containing species in solu- O(C) (ohm-'cm-') H Se0 tions ofSe02 in 100% sulfuric acid. Selenium dioxide Z 3 is known to exist as a trimer in seleninyl chloride.' and 0.03 10.072 2.62 1.340 1.40 1.22 we found that the 77 Se NMR spectrum of this solu tion 0.04 9.960 2.47 1.488 1.30 1.17 exhibits a single peak at 1445 ppm. 0.05 9.850 2.37 1.636 1.26 1.11 0.06 9.740 2.30 1.782 1.22 1.08 The SeNMR spectra and the decr.ease in thev-vva- 0.07 9.622 2.25 1.930 1.20 1.05 lue with increasing concentration of Se02 can be ac- 0.08 9.510 2.20 2.078 1.18 1.02 counted for if it is assumed that the solutions contain 0.09 9.389 2.18 2.224 1.17 1.01 monoprotonated .polymeric species such as 0.10 9.269 2.16 2.370 1.16 1.00

Se 04H+ ,Se 06H+ in addition toSe0 H+ ,asshown 2 3 2 NaZSe01 in Eqs (3-5). 0.01 10.087 6.85 1.270 3.29 3.56 0.02 9.754 6.70 1.656 3.26 3.44 2H Se0 + 3H S0 2 3 2 4 0.03 9.382 6.60 2.070 3.21 3.39 = Se 0 H+ + 2H 0+ + 3HSO" ... (3) 2 4 3 0.04 9.000 6.52 2.478 3.19 3.33 v = 3, Y= 1.5, v - Y= 1.5 0.05 8.608 6.44 2.860 3.17 3.27 0.06 8.210 6.37 3.190 3.16 3.21

3H2Se03 + 4H2S04 = Se306H+ + 3H30+ + 4HSO" ... (4) v = 2.67, Y= 1.33, v - Y= 1.34 SELENIUM(VI) COMPOUNDS Selenium; VI) compounds, vi: selenic acid and sodi- umse/enate 4H Se0 + 5H S0 2 3 2 4 The interpolated v and y values of solutions of sel- = Se 0 H+ + 4H 0+ + 5HSO" ... (5) 4 8 3 enic acid and sodium selenate in 100% sulfuric acid v = 2.25, Y= 1.25, v - Y= 1.0 are given in Table 2. The results suggest that selenic acid behaves as a weak base in 100% sulfuric acid and With increasing concentration of Se02 v decreases is partially ionized in accordance with Eq. (16) from 2.62 to 2.16, y from 1.40 to 1.16 and v-y from H Se0 H S0 ~H3SeO: HSO" 1.22 to 1.0, in good agreement with the values pre- 2 4 + 2 4 + v<2,y<1 ... (6) dicted by Eqs (3-5). The agreement between experi- mental and theoretical values would be further im- Addition of sulfate to solutions proved if the polymeric Se204, Se306, etc. are incom- of selenic acid even in very small amounts increases pletely protonated as suggested by Flowers et al.', the conductivity, This shows that selenic acid ionizes Our conductometric and cryoscopic results on solu- as a base in 100% sulfuric acid and not as an acid. The tions of selenium dioxide in rOO%sulfuric acid are in greater proton-donating power of sulfuric acid' as agreement with those of Flowers et al:' and are there- compared to that of selenic acid may by attributed to fore not discussed here. greater electronegativity of the atom.

---.,-----.,-----.,-----,------~i -----.,-----.,-----.,-----.,-----.,--- 1~40 1~20 1400 1300 1360 13~0 1320 1300 1280 1260 PPM

Fig.l-Selenium-77 NMRspectrumofSeOz solution in 100% HzSO.

299 INDIAN.J CHEM, SEC A, APRIL 1989

Thevalue of Kj l.Z X 1O-2)hasbeencalculatedus- results on sodium selenate can be explained on the ba- ing the expression (7). sis ofEqs (8 and 9):

K, = y.ffiHSOi 11 - y ... (7) Na2Se04 + 2H2S04 = 2Na + + H2Se04 + 2HSOi where mHSO - is the total concentration of hydrogen .. , (8) sulfate ion in~luding that remaining from the partially H2Se04 + H2S04 unrepressed solvent self-dissociation", Similarly the '=H3SeO: + HSOi ... (9)

An average value of Kb = (1.5 x 102) isgiven by the ex- Table 2-Interpolated values of v and y of solutions of selenic pression (10) acid and sodium selenate in 100% sulfuric acid t m Freezingpoint rC) v .102K(ohm - 'ern -') y Kb=(y-2).mHSOiI3-y ... (10) H2Se04 The nSe NMR spectrum of a 0.050 10.075 1.28 1.100 0.28 solution in 100% sulfuric acid exhibits a single peak at 0.D75 9.880 1.28 1.142 0.28 1016 ppm. This peak is at high frequency compared to 0.100 9.670 1.26 1.190 0.27 that of (CH3)2Se and can presumably be assigned to 0.125 9.458 1.26 1.249 0.26 the equilibrium mixture of H2SeO 4 and H3SeO: . 0.150 9.248 1.25 1.306 0.25 0.175 9.035 1.25 1.360 0.25 Selenium trioxide

Na2SeO.j The conductivity and freezing point results are 0.D2 9.865 5.18 1.386 2.17 summarized in Table 3 and Fig. 2. The ionization is 0.04 9.258 5.17 1.940 2.16 expressed by Eqs (11 and 12). . 0.05 8.946 5.16 2.225 2.16 Se0 + 2H2SO 4 = H SeO 4 + H2S 0 .=H3SeO: 0.06 8.630 5.14 2.490 2.14 3 2 2 7 + HS 0i ... (11) 0.07 8.312 5.14 2.748 2.12 2 0.08 7.995 5.13 Se03 + 2H2S04 = H2SSe07 + H2S04 .=H:;SO: + HSSeOi ... (12)

10.5

10·0

1·40

•VI.... z a<5, ~ IE C> ,/ Z u

[j 95 ,/ /' I w ,/ E a:: s: I.L 1·20~

N-" S!

9·0'------,,------,,------_...1,·00 0·00 0·05 m

Fig. 2-Freezing points (curves Aand B)and conductivities at 25°C(curves C and Djof solutions of ScO. in IOO°/.,H:SO.j

300 • GILLESPIE et al: BEHAVIOUR OF SELENIUM COMPOUNDS IN 100% SULFURIC ACID

id in 100% sulfuric acid". It seems clear therefore that Table 3-Solutions of selenium trioxide in 100% sulfuric acid: selenium trioxide adds to sulfuric acid to give disulfo- Freezing points and conductivities at 25°C selenic acid, H2SSe07 which ionizes in sulfuric acid as m Freezing point (0C) m 102K(ohm - Iem - I) a weak acid, as expressed by Eq. (12). 0.0000 10.370 0.0000 1.0440 0.0148 10.276 0.0178 1.0922 References 0.0274 10.157 0.0316 1.1419 1 Flowers R H, Gillespie R J & Robinson E A, J inorg nucl 0.0423 9.996 0.0444 1.1892 Chern, 9 (1959) 155. 2 Paetzold R & GarsoffkeM, Z anorgallg Chern, 336 (1965) 52. 0.0623 9.803 0.0616 1.2390 3 Greenwood N N & EarnshawA, in Chemistry ojthe elernents , 0.1221 9.208 0.0849 1.3302 (Pergamon Press, Toronto) 1986 pp 911. 1.3652 0.0961 4 Flowers R H, Gillespie R J & Robbinson E A, Can J Chern, 38 (1960) 1363. 5 Gillespie R J & Wasif S, J chem Soc, (1953) 209." According to Eq. (11) selenium trioxide dehydrates 6 Bass S J, Flowers R H, Gillespie R J, Robinson E A & Solom- 100% sulfuric acid to give selenic acid and disulfuric ons C, J chem Soc, (1960) 4315. acid and selenic acid is protonated by disulfuric acid 7 Gilbertson LI & King G B, lnorg Synth, 3 (1950) 137. 8 NourM M & WasifS,J chernSoc(A),(1968) 3024. to give H3SeO: and HS20i ions. Since neither of these ions takes part in the chain-conduction process 9 Toul F & Dostal K, Coli Czech Chern Cornrnun, 16 (1951) 531. of sulfuric acid? the conductivity of the solvent should 10 Gillespie RJ,OubridgeJ V & Solomons C,Jchern Soc,(1957) not be appreciably increased. In fact the present re- 1804. sults show that solutions of selenium trioxide are only 11 Gillespie R J, Hughes E D & Ingold C K, J chern Soc, (1950) slightly less conducting than solutions of disulfuric ac- 2504.

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