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 (C18) 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), [RhCl3(H2O)3], cis-[RhCl4(H2O)2]−, trans-[RhCl4(H2O)2]− and [RhCln(H2O)6−n]3−n (n = 5, 6) species. The [RhCln(H2O)6−n]3−n (n = 5, 6) complex anions eluted as a single band due to the relatively fast aquation of [RhCl6]3− in a 0.1 mol L−1 TBACl ionic strength mobile phase matrix. Moreover, the calculated t1/2 of 1.3 min for [RhCl6]3− aquation at 0.1 mol kg−1 HCl ionic strength is significantly lower than the reported t1/2 of 6.3 min at 4.0 mol kg−1 HClO4 ionic strength. Ionic strength or the activity of water in this context is a key parameter that determines whether [RhCln(H2O)6−n]3−n (n = 5, 6) species can be chromatographically separated. In addition, aquation/anation rate constants were determined for [RhCln(H2O)6−n]3−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 298 K 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-[RhCl4(H2O)2]− stereoisomers have been obtained. Furthermore, it was found that relatively large amounts of ‘highly’ aquated [RhCln(H2O)6−n]3−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 [RhCl5(H2O)]2− species is only 8–10% of the total, far from the 70–80% as previously proposed. A 95% abundance of the [RhCl6]3− 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.