Ultraviolet Spectra of Hydroxide, Alkoxide, and Hydrogen Sulfide Anions Petr Zuman" and Wayne Szafranskl' Department of Chemistry, Clarkson College of Technology, Potsdam, N.Y. 13676
Spectra of solutions Containing to M alkall hydroxides or alkoxides show a gradual Increase of the absorbance below 230 nm with decreasing wavelength, corresponding to an absorption band with a maximum between 180 and 200 nm. Absorbance at 205 nm for hydroxide, 215 nm for methoxide, 220 nm for ethoxlde, and 230 nm for fed-butoxlde ion Is a linear function of anion concentration independent of the nature of cation. DifferencesIn molar absorptlvlty of H2S, HS- (A,, = 230 nm) and S2- enables determinationof both acid dissociation constants of hydrogen sulfide. The absorption probably corresponds to photolysls.
Information on electronic absorption of simple anions of the type RX-, which do not contain r electrons, is very limited. Thus, for hydroxide ions, vacuum spectrography (1) indicated the presence of an absorption band at 186 nm with log E about 3.7. The same process probably yields photochemically generated hydrated electrons, reported to be formed by irradiation of alkali and alkaline earth hydroxide solutions by uv light at 185 nm (2-4) similarly as in pulse radiolysis (5, 6). No spectral data were found for alkoxides, but formation of solvated electrons on pulse radiolysis of alkaline methanolic solutions has been reported (7, 8). Information on spectral data for the hydrogen peroxide anion is best documented and the change of the absorbance has also been used for determination of the acid dissociation constant of H202 (9-11). Even when differential absorption of thiols and their anions has been employed widely (12) for spectrophotometric determination of acid dissociation constants of numerous sulfhydryl compounds, details on actual spectra used appear to be lacking in most instances. Alkaline solution of n-butylmercaptan shows (13) an absorption band a t 237 nm (log e 3.75) and sodium sulfide was reported (13)to absorb at 232 nm (log e 3.87). Wavelength of the absorption band depends on the presence of other ionizable groups like COO- or NH3+ and changes with dissociation of this group (14). Similarly, absorption bands at 243-250 nm (log e 3.85) were reported (12)for organic selenide ions containing COO- or NH3+ groups. In this contribution, the absorption processes due to a number of simple anions should be compared, possibility of analytical applications-practical and for determination of pK values-pointed out and possible interpretation indicated.
EXPERIMENTAL Spectrophotometric measurements were carried out on a Unicam SP800A recording spectrophotometer (Pye Unicam Ltd., Cambridge, England) which enables recording of spectra from 190 nm using 1-cm matched quartz cells. The cell compartment was maintained at 25.0 f 0.1 "C by a Haake Mark FJ (Germany) circulating thermostated bath. Sodium, lithium, and tetramethylammonium hydroxides of reagent grade purity were prepared from saturated solutions in carbonate-free
1 Present address, Distillation Products Industries, Eastman Kodak Company, Rochester, N.Y.
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distilled water and kept under nitrogen. Sodium alkoxide solutions were prepared by dissolving a weighed amount of sodium metal in dry methanol, ethanol, and 2-methyl-2-propanol (tert-butanol). The reaction between the alcohols and sodium metal was assumed to be stoichiometric and complete. Gaseous hydrogen sulfide was dissolved in distilled water and concentration of this stock solution was determined iodometrically using a standard thiosulfate solution and a starch indication. Buffers were prepared from reagent grade chemicals supplied by J. T. Baker Chemical Company (Phillipsburg, N.J.). Constant ionic strength was maintained by addition of sodium chloride or perchlorate. The pH values were measured by a Sargent-Welch pH meter, Model NX, in conjunction with a Sargent combination electrode S30072-15. Standardization was accomplished by using National Bureau of Standards buffers with pH values bracketing the solution being tested.
RESULTS AND DISCUSSION Spectra of lithium, sodium, and tetramethylammonium hydroxides in aqueous solution show an increase of the absorbance in the region of 200-220 nm (Figure 1). The shape of the available portion of the absorption band (assuming symmetrical shape of the absorption peak) indicates a maximum in the 180-190 nm region with molar absorptivity of the order of lo3 to lo4 1. mol-1 cm-l. When absorbance is measured at a constant wavelength (e.g., 205 nm), it is found to be a linear function of the hydroxide ion concentration, as proved over the range from 1 X to 14 X M. As the absorbance is independent of the nature of the cation present, one calibration curve is sufficient for lithium, sodium, and tetramethylammonium hydroxide and probably also for other hydroxides. Similar absorption spectra were observed also for methoxide, ethoxide, and tert-butoxide ions. When an appropriate wavelength is chosen for each alkoxide ion, the absorbance is a linear function of concentration in the range between 1 X loe3 and 10 X M (Figure 2-with possible extension of the concentration range to lower concentrations, when different wavelengths were chosen). The shape of the absorption bands indicates existence of an absorption maxima in the 180-200 nm region with molar absorptivity between lo3 and lo4 1. mol-l cm-l. At a chosen alkoxide concentration, the absorbance at a given wavelength increases in the sequence: CH30- < CH3CH20- < (CH&CO-. This may be due either to an increase in molar absorptivity or to the shift of the absorption maximum to a longer wavelength with increasing number of methyl groups on the a carbon. The shape of the spectra seems to indicate the latter alternative. When hydrogen sulfide was added to aqueous buffer solutions of pH greater than about 7, a well developed absorption band with a maximum a t 230 nm was observed. In buffers of ionic strength p = 1.0, the absorbance a t 230 nm increases in a shape of a dissociation curve with an inflexion point a t pH 7.2 (Figure 3), followed by a pH-independent region between pH 8.5 and 11.0. This region is followed by a decrease of absorbance to a lower value with an inflexion point a t pH 12.0. Hence a t 230 nm the molar absorptivity of species H2S predominating at pH < 5 is negligibly small, that of HS- (at pH 9.5) corresponds to 8 X lo3 1. mol-' cm-l and that of S2- (at pH > 13) to 4.6 X lo3 1. molt1 cm-l. Hence the value 7.4 X lo3
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
A 0.4
.I
7 i' 3 0
lnml
Figure 1. Ultraviolet spectra of
aqueous sodium hydroxideat 25.0 OC. Concentration range: 1.0-14.0 X lov4 M
OO
4.0
8.0
12.0
PH Figure 3. Dependence of absorbance on pH for hydrogen sulfide at 230 nm and 25 O C .
1.6
Buffers used: (0)HCI; ( 0 )CH&OOH-CH&OONa; ( 0 )NaH2PO4-NaZHPO4;( 0 ) borate: ( 0 )Na2HP04-Na3P04;( 0 )NaOH, p = 1.0M
1.2
in those cases where information on hydroxide ion concentration (as opposed to the activity) is of interest. It can be assumed that in all anions of the type RX-, which do not contain ir electrons, a similar type of excitation process is involved. The large value of the molar absorptivity, of the order of 103-104 1. mol-l cm-I seems to exclude the possibility that forbidden transitions are involved. This leaves u LT* transition and photolysis as alternative interpretations. Experiments to prove the latter, which seems to be indicated by the reports of literature (2-8), are in progress.
A
0.8
-
0.4
LITERATURE CITED 0 [RO~ x103 Figure 2. Dependence of absorbance on concentration for (0)CH30at 215 nm; ((>) CH3CH20- at 223 nm; and (0) (CH3)&O- at 230 nm and 25 O C
(1)H. Leyand B. Arends, 2.Pbys. Cbem. B, 6, 240 (1929). (2)M. S. Matheson, W. A. Mulac, and J. Rambani, J. Pbys. Cbem., 67, 2613 (1963). ( 3 ) J. Jortner, M. Ottolenghi, and G. Stein, J. Phys. Cbern., 66, 2029,2037, 2042 (1962). (4)C. Gopinathan, E. J. Hart, and K. H. Schmidt, J. Pbys. Cbem., 74, 4169 (1970). ( 5 ) V. N. Shubin, S. A. Kabakchi, L. P. Beruchashviii, and P. i. Dolin, int. J. Radiat. Pbys. Cbem., 2, l(1970). (6)S.A. Kabakchi and V. N. Shubin, Radiat. Eff., 15, 23 (1972). (7)A. K. Pikaav, T. P. Zhestkova, and G. K. Sibirskaya, J. Pbys. Cbem., 76, 3765 (1972). (8) E. P. Kalyazln, V. M. Byakov, and A. V. Pestov, Vestfl. Moskov. Univ., Khim.,
1. mol-l cm-' attributed in the literature (13)to sulfide ion, is probably due to hydrogen sulfide ion. The approximate practical value of acid dissociation constant K1' = 0.63 X a t y = 1.0, is in good agreement with reported (15)for K1 in literature. values 1.08 - 0.44 X Practical value for the second dissociation constant K2' = 1.0 x a t y = 1.0 seems to be more reliable than data in the literature (16). Selenide and telluride anions show similar absorption spectra at longer wavelengths. Similar uv absorption has been observed also for geminal diol anions of formaldehyde ( I 7) and chloral (17,18). The linear relationship between measured absorbance and concentration can be used for analysis of samples containing hydroxide, alkoxide, sulfide, or similar anions provided that they do not contain other components absorbing a t shorter wavelengths than about 250 nm. In particular, such measurements are suitable for the study of kinetics of reactions involving such anions, formation of oxo complexes (19)and
27, 600 (1972). (9)J. Jortner and G. Stein, Bull. Res Counc. isr., Sect. A, 6, 239 (1957);Cbern. Abstr., 50, 17447 (1957). (10)S.S.Mohammad and T. Navaneeth Rao, J. Chem. SOC., 1957, 1077. (1 1) A. J. Everett and G. J. Minkoff, Trans. Faraday SOC.,49, 410 (1953). (12)W. H. H. Gunther. J. Org. Cbem., 32, 3931 (1967)andreferences therein.
(13)L. H. Noda, S. A. Kuby, and H. A. Lardy, J. Am. Cbem. SOC., 75, 913 (1953). (14)R. E. Benesch and R. Benesch, J. Am. Cbem. SOC.,77, 5877 (1955). (15)"Gmellins Handbuch der Anorganischen Chemie E", Aufi. Syst. Nr. 9, Schwefel, Teil B, Lieferung 1, Verlag Chemie, 1953,p 86. (16)D. D. Perrin, Pure Appl. Chem., 20, 133 (1969). (17)D. Barnes, Ph.D. Thesis, Birmingham University, England, 1968. (18)W. Szafranski, M.Sc. Thesis, Ciarkson College of Technology, Potsdam, N.Y., 1975. (19)P.Zuman and E. Maytin, unpublished results.
RECEIVEDfor review June 17, 1976. Accepted August 19, 1976. W.S. thanks Distillation Product Industries, a division of Eastman Kodak Company, for educational leave of absence and financial support of this research. The paper was reported in part a t the 22d Spectroscopy Symposium, Montreal, Que., Canada, 1975.
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