Separation and quantification of [RhCl n(H 2O) 6− n] 3− n ( n = 0–6) complexes, including stereoisomers, by means of ion-pair HPLC–ICP-MS

Abstract

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 2O) 3], cis-[RhCl 4(H 2O) 2] −, trans-[RhCl 4(H 2O) 2] − and [RhCl n (H 2O) 6− n ] 3− n ( n = 5, 6) species. The [RhCl n (H 2O) 6− n ] 3− n ( n = 5, 6) complex anions eluted as a single band due to the relatively fast aquation of [RhCl 6] 3− 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] 3− 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 2O) 6− n ] 3− n ( n = 5, 6) species can be chromatographically separated. In addition, aquation/anation rate constants were determined for [RhCl n (H 2O) 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-[RhCl 4(H 2O) 2] − stereoisomers have been obtained. Furthermore, it was found that relatively large amounts of ‘highly’ aquated [RhCl n (H 2O) 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 [RhCl 5(H 2O)] 2− species is only 8–10% of the total, far from the 70–80% as previously proposed. A 95% abundance of the [RhCl 6] 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.