F. P. DALY,C. W. BROWN, AXD 33. R. KESTER
3864
Sodium and Magnesium Sulfate Ion Pairing: Evitlence from Raman Spectroscopy
J
Francis P. Daly, Chris W. Brown,* and Dana R. Kester
Department of Chemistry and Graduate School o j Oceanography, University of Rhode Island, Kingston, Rhade Island 08881 (Received April 20,197.9) Publication costs assisted by the Ofice of Naval Research
Raman spectra of aqueous solutions of sodium and magnesium sulfates were measured but failed to reveal any evidence of ion pairing. However, sulfate solutions with HCl added did exhibit relatively strong bands due to HX04-. Evidence for MgSOd and NaS04- ion pairs has been obtained by the addition of sodium and magnesiuir salts to the latter solution; both cations compete with hydrogen ions for the SO4?-. This is clearly demoiistrated in the Raman spectra by a decrease in intensity of the band assigned to 13301-at 1053 cm-l and an increase in intensity of the band assigned to Sod2- at 982 cm-’ in the solutions with sodium or magnesium added.
Introduction
qualitative measure of the extent of dissociation of HS04-. Considerable attention has been given to the use of In the present investigation Raman spectra of multiion association models to interpret the physical-chemcomponent aqueous solutions of HCI, NaC1, SazSQa, ical character-stica of electrolytic solutions. l - This arid MgClz in the range 0.3-1.3 ionic strcngth have been work considera the ion pairing of sulfate ions with soof an ion-pairing model. We have examined in terms dium and magnesium ions. The tendency of magassumed that the chlorides of H + , S a + , and Mg2+are nesium and sulfat,e ions to form ion pairs has been excompletely dissociated. Even though this assumption amined extensively using condu~tometric,~ potentiosorubility,* and ultrasonic r e l a x a t i ~ n ~ - ’ ~is contrary to some interpretation^.^^-^^ it is consistent techniques. The consistency of these studies supports (1) C. W. Davies, “Ion Association,” Butterworths, Washington, the ion association model for magnesium and sulfate. D. c . , 1962. The case for sodium ion pairing with sulfate is not as (2) G. H. Nancollas, “Interactions in Electrolyte Solutions,” clearly established as that for magnesium. In some Elsevier, Amsterdam, 1966. studies Na2504has been assumed to be a completely (3) R. A. Robinson and R. H. Stokes, “Electrolyte Solutions,” Butterworths, London, 2nd revised ed, 1965. dissociated e l c c t r ~ l y t e , ~ wliercas J ~ , ~ ~ others have con(4) H. W. Jones and C. B. Monk, Trans. FaradaQ Soc., 48, 929 sidered the formation of NaSQ4- ion pairs.13-15 It (1952). has been subsequently sbown that Harned’s rule be(5) V. S. K . Nair and G. H . Nancollas, J . Chem. Soc., 3706 (1958). havior of activitj coefficients in multicomponent solu(6) J. M . T. M . Gieskes, 2. Phys. Chem. (Frankfurt a m M a i n ) , 50, 78 (1966). tions does not, preclude an ion association model for (7) D. R . Kester and R. M . Pytkowica, Limnol. Oeeanogr., 13, 670 the ionic interactwnH.’5-17 (1968). The effects 3f various cations on the Raman spectra (8) W. L. Marshall, J . Phys. Chem., 73, 3584 (1967). of nitrates, sulfates, and perchlorates have been in(9) M . Eigen and K. Tamm, 2. Elektrochem., 66, ‘101(1962). vestigated by Sester and Ylane.ls Only Ind+produced (10) G. Atkinson and S. Petrucci, J . Phys. Chem., 70, 3122 (1966). new bands in the spcctrum of the s04’- ion; all other (11) R. D. Lanier, ibid., 69,3992 (1965). cations studied, including N a + and Mg2+, gave ap(12) J. N. Butler, P. T. Hsu, and J. C. Synnott, ibid., 71, 910 (1967). proximately tae same spectra. I n this work w e ex(13) E. C. Rigliellato and C. W. Davies, Trans. Faraday Soc., 26, 592 (1930). tend the observations of Nester and Plane to lower (14) J. M. Austin and A. D. Mair, J . P h y s . Chem., 66, 519 (1962). ionic strengths a n d to multicomponent sulfate solu(15) R . M. Pytkowicz and D. R. Kester, ilmer. J . Sci., 267, 217 d ions. (1969). Recently. Irish and Chen examined the Raman spec(16) J. N. Butler and R. Huston, J . Phys. Chem., 74, 2976 (1970). tra of various solutions containing HSO4- and s042-.19--21 (17) J. N. Butler and R. Huston, A n a l . Chem., 42, 1308 (1970). They found that the integrated intensity of the Sod2- (18) R. E. Hester, R. A . Plane, and G. E. Walrafen, J . Chem. Phys., 38, 249 (1963); R. E. Nester and R. A. Plane, Inorg. Chem., 3, 769 band at 981 em-’ is proportional t o the S042- molarity (1964). and that the integrated intensity of thc HSO4- band a t (19) D. E. Irish and H. Chen, J . P h y s . Chem., 74, 3796 (1970). 1053 cm-‘ is proportional to the HS04- molarity. (20) H. Chen and D. E. Irish, ibid., 75, 2612 (1971). ‘I‘hey furtbcr s,howed that the ratio of these bands is a (21) H. Chen and D. E. Irish, ibid., 75, 2681 (1971). The JOuTnal oj‘ Physical Chemistry, Vol. 76, N o . .9& 1972
SODIUM ANI) MAGNESIUM SULFATE ION PAIRING
3665
Table I : ObserTed Frequencies (em-') and Assignments of the Bands in the Raman Spectra of the Sulfate Solutions Assignments
0 . 1 M NazS01
0.1 M NazSOr 1 . 0 M NaCl
0 . 1 M NazSOr 0 . 1 M MgClz
SO*+, u3 HS04-, SO3sym atretch SO*2-, P'1 HSQe-, S-OH ~ t r e t c h SO4'-, 5'4 HSOa-, SGa syni bend
-1110 w, b
-1110 w, b
-1110 w, b
0 . 1 M NalSOa0 . 1 iM FIG1
10.53 m 982 vs
982 vs
982 v s
982 v s
-620 w, b
-615 w, b
-610 w, b
448 w
448 w
446 w
4 3 9 5 w, b
sod2--, V?
-595 w, b 445 w 410 w
HSQa-, S-'3H ~ 7 a g
with many of the present concepts of electrolytic solut i o n ~-2s. ~The ~ success ~ ~ ~ of~ the ~ following ~ analysis supports the adequacy of this assumption.
Experimental ~
~
~
~
t
~
~
All so1ution:Jwere prepared from J. T. Baker reagent grade chemicals. The NaCl and Na2SO4were dried a t 125" and 0.5 d m pressure for 2 hr prior to weighing. The RC1 was added by weight from a 37.5% aqueous solution. Magnesium was added as MgClz.6Hz0. Raman spectra were recorded a t 24 f 1" using a Spex lndustri es h4odel 1401 double monochromator with photon counting detection and a C.R.L. Model 52A argon ion laser emitting a t 4880 8 as the excitation source. Specxa w t w measured of samples contained in Pyrex capillary tubes with an inner diameter of 1.2 man, and duplicate spectra were measured of samples contained in B verlirally mounted, quartz liquid cell with an inner diameter of 8 mm. During the experiments in which relative integrated intensities of bands of different samples were measured, the integrated in~ of water (vz) was meatensity of the 1 6 2 5 - ~ m -band sured for each sample. It was found that the intensity of this band rtmseined the same from one sample to the next, if care was talien in exchanging samples. The spectra of -I he samples to be compared were measured consecutively, arid the spectrum of each sample was measured a minimum of five times. The Raman speclmm of a 0.1 N NazSOdsolution in the 800-1200-cm-' region is shown in Figure 1. There i s a very sirorg band a t 982 cm-' assigned as vl, the symmetric SO? stretching mode, and a weak, broad band at - j l l l i O cni assigned as v3, the antisymmetric SO, stretching rnode.2g Two other very weak, broad bands are (&nerved at lower frequencies. All of the observed f Fecuenciw and assignments are listed in 'Fable 1. To test the feasik~lityof using Raman spectroscopy to detect sodium and magnesium ion pairs with sulfate, spectra of t b ree additional solutions were recorded. In the first solution excess sodium was added to the 0.1 fWNazS04solution by the addition of KaCl (making the solution 1 0 M ir NaC1). It was assumed that the
i
~ II
n
0.1M Na,SO,
1200
1100
I
1000
FREQUENCY,
I
I
goo
I
800
CM-'
Figure 1. Top, Raman spectrum of 0.1 M NasS04 solution between 800 and 1200 cm-l (a, sensitivity increased by 3 X ). NaeSOh---O.lM HCl Bottom, Raman spectrum of 0.1 solution. Spectral slit widths
