When spectroscopy fails: The measurement of ion pairing

Abstract

Spectroscopic techniques such as UV/vis, NMR, and Raman are powerful tools for the investigation of chemical speciation in solution. However, it is not widely recognized that such techniques do not always provide reliable information about ion association equilibria. Specifically, spectroscopic measurements do not in general produce thermodynamically meaningful association constants for non-contact ion pairs, where the ions are separated by one or more solvent molecules. Such systems can only be properly quantified by techniques such as dielectric or ultrasonic relaxation, which can detect all ion-pair types (or equilibria), or by traditional thermodynamic methods, which detect the overall level of association. Various types of quantitative data are presented for metal ion/sulfate systems in aqueous solution that demonstrate the inadequacy of the major spectroscopic techniques for the investigation of systems that involve solvent-separated ion pairs. The implications for ion association equilibria in general are briefly discussed.

Temperature Effects on Ion Association and Hydration in MgSO4 by Dielectric Spectroscopy

“…The thermodynamics of Equilibrium (1) have been studied in detail. Numerous high-quality conductometric and other studies indicate log K A 8 = 2.24 AE 0.05 at 25 8C, [12,23] and direct calorimetric determinations [42][43][44][45] of DH A 8 and DS A 8 are in good agreement. Unusually, DC pA 8 has also been determined from direct calorimetry.…”

“…[9] Typical of many higher-valent electrolytes, magnesium sulfate can be described as an almost strong electrolyte in aqueous solution. The supporting evidence of moderate levels of ion association in MgSO 4 (aq) from conductometric, thermodynamic, Raman spectroscopic and other studies, is overwhelming, [10][11][12] despite occasional suggestions to the contrary. [13,14] The presence of associated species significantly affects the properties of magnesium sulfate solutions and creates difficulties for their theoretical description.…”

Ion association and hydration in 3:2 electrolyte solutions by dielectric spectroscopy: Aluminum sulfate

“…It is particularly noteworthy that the normally powerful spectroscopic techniques, such as NMR, IR, Raman and UVvis, are of limited use for studying such equilibria because they generally detect only CIPs (Hefter, 2006). On the other hand, thermodynamic methods such as potentiometry detect only the overall association, Scheme (I) (Hefter, 2006).…”

“…Spectroscopic techniques such as NMR (Akitt et al, 1969(Akitt et al, , 1972(Akitt et al, , 1985Jin and Ichikawa, 1988;Akitt and Howarth, 1989), Raman Schö nherr, 1989, 1991a,b;Rudolph and Mason, 2001) and UV-vis (Kryzhanovskii et al, 1971;McIntyre et al, 1982) have also been employed. However, as discussed in detail elsewhere (Rudolph et al, 2003;Hefter, 2006), such techniques usually detect only CIPs and their interpretation is often problematic. In particular, values of K A obtained from such measurements may be lower (even by many orders of magnitude) than those determined by thermodynamic methods (Hefter, 2006).…”

“…However, as discussed in detail elsewhere (Rudolph et al, 2003;Hefter, 2006), such techniques usually detect only CIPs and their interpretation is often problematic. In particular, values of K A obtained from such measurements may be lower (even by many orders of magnitude) than those determined by thermodynamic methods (Hefter, 2006). Furthermore, the constants obtained are not always thermodynamically well-defined, because they may contain unknown contributions from the species not detected (Rudolph et al, 2003;Hefter, 2006).…”