Communicationsto the Editor
1315
vent in the vicinity of the moving ion is considered to be the same as in the vicinity of a moving uncharged sphere; thus it is assumed not to be affected by the electrostatic interaction of the moving ion with the solvent dipoles. This electrostatic interaction would increase the residence time of oriented solvent dipoles near the ion and, consequently, the friction due to the dielectric relaxation of the solvent would be reduced. The neglect of this electrostatic effect would be particularly important when the electrostatic force is large, i.e., for small and/or polyvalent ions. Departamento de Fisicoquhica Facultad de Farniacia y Bioquimica Universidad de Buenos Aires Buenos Aires, Argentina
L
R. Fernander-Prini
b
Received November 3, 1972
Raman Spectroscopic Evidence for Contact ion Pairing In Aqueous Magnesium Sulfate Solutions
Figure 1. Raman spectrum of (a) 2.0 M MgS04 cm-l) and (b) 1.72 M NaaS04 (900-1200 cm-'1.
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TABLE I: Comparison between the Concentration of Magnesium
Sir: A number of physiochemical methods1-5 have been used to study ion association in aqueous MgS04 solutions. Based on u1trac;onic absorption studies, Atkinson and Petruccie have proposed that the following three-step process occurs in aqueous MgS04 solutions Mg2+(aq)
I + i?042-(aq) e
[Mg(WW)SOJ
n
#
EMg(W)SO,] # [MgSOJ (1) Recently Raman spectroscopy has been used to study aqueous MgSOr. Daly, Brown, and Kester7 have proposed that the only type of ion pairing in MgS04 solutions consistent with their Raman data is solvent separated ion pairing. Their evidence is based on a Raman study of the effect of Na+ and Mg2+ on the dissociation of the HS04ion. In the course of Raman work in our laboratory it was observed that the Raman spectrum of aqueous MgS04 (Figure l a ) is different from that of aqueous N a ~ S 0 4(Figure l b ) . As Figure l b shows, the v l ( A 1 ) mode of the sulfate ion, for a Ma2S04 solution, is a symmetric band centered around 982 cm-l. For a MgS04 solution (Figure l a ) , the envelope is slightly asymmetric to the high-frequency side and the envelope may be resolved into two bands a t 982 (half-width, 14 cm-l) and 995 cm-I (half-width, 25 cm-1). The relative intensity of the band a t 995 cm-l increases when MgC12 is added to the solution. No differences between Na2S04 and MgS04 solutions were observed in the other frequency regions of the Raman spectrum (vz(E) = 450 cm-l; v3(F2) = -1115 cm-1; v4(F2) = 616 cm-1). Further spectroscopic evidence for a n interaction between Mg2+ and SO*- in aqueous solution is provided by the infrared spectrum, where the normally infrared forbidden vl(A1) band occurs as a weak band centered around 985 cm-l. In nitrate systems, perturbations in the vl(A1) region of the Raman spectrum have been shown to occur only when a complex or contact ion pair is formed;8-1° therefore, it appears that contact ion pairs are formed in aqueous MgS04 solutions. The previous IRaman study of dilute magnesium sulfate solutions7 (
