Separation and quantification of [RhCln(H2O)(6-n)](3-n) (n=0-6) complexes, including stereoisomers, by means of ion-pair HPLC-ICP-MS.

A hyphenated ion-pair (tetrabutylammonium chloride-TBACl) reversed phase (C(18)) HPLC-ICP-MS method (High Performance Liquid Chromatography Inductively Coupled Plasma Mass Spectroscopy) for anionic Rh(III) aqua chlorido-complexes present in an HCl matrix has been developed. Under optimum chromatographic conditions it was possible to separate and quantify cationic Rh(III) complexes (eluted as a single band), [RhCl(3)(H(2)O)(3)], cis-RhCl(4)(H(2)O)(2), trans-RhCl(4)(H(2)O)(2) and RhCl(n)(H(2)O)(6-n) (n=5, 6) species. The RhCl(n)(H(2)O)(6-n) (n=5, 6) complex anions eluted as a single band due to the relatively fast aquation of RhCl(6) in a 0.1 mol L(-1) TBACl ionic strength mobile phase matrix. Moreover, the calculated t(1/2) of 1.3 min for RhCl(6) aquation at 0.1 mol kg(-1) HCl ionic strength is significantly lower than the reported t(1/2) of 6.3 min at 4.0 mol kg(-1) HClO(4) ionic strength. Ionic strength or the activity of water in this context is a key parameter that determines whether RhCl(n)(H(2)O)(6-n) (n=5, 6) species can be chromatographically separated. In addition, aquation/anation rate constants were determined for RhCl(n)(H(2)O)(6-n) (n=3-6) complexes at low ionic strength (0.1 mol kg(-1) HCl) by means of spectrophotometry and independently with the developed ion-pair HPLC-ICP-MS technique for species assignment validation. The Rh(III) samples that was equilibrated in differing HCl concentrations for 2.8 years at 298K was analyzed with the ion-pair HPLC method. This analysis yielded a partial Rh(III) aqua chlorido-complex species distribution diagram as a function of HCl concentration. For the first time the distribution of the cis- and trans-RhCl(4)(H(2)O)(2) stereoisomers have been obtained. Furthermore, it was found that relatively large amounts of 'highly' aquated RhCl(n)(H(2)O)(6-n) (n=0-4) species persist in up to 2.8 mol L(-1) HCl and in 1.0 mol L(-1) HCl the abundance of the RhCl(5)(H(2)O) species is only 8-10% of the total, far from the 70-80% as previously proposed. A 95% abundance of the RhCl(6) complex anion occurs only when the HCl concentration is above 6 mol L(-1). The detection limit for a Rh(III) species eluted from the column is below 0.147 mg L(-1).