A new approach of microwave stimulation for cloud point extraction of rhodium, palladium and platinum has been developed. The procedure was applied for preconcentration of trace amounts of platinum group elements followed by inductively-coupled plasma mass spectrometry determination. 2-mercaptobenzothiazole and Triton X-100 were selected as hydrophobic ligand and non-ionic surfactant, respectively. The extraction was carried out with model solutions of platinum group metals in hydrochloric media where they present as chloro-complexes. The exchange of chloride ligands with 2-mercaptobenzothiazole was found to be kinetically inert - especially for Rh and Pt. The following special kinetic features are proved: the velocity of extraction of platinum depends on its oxidation degree - for Pt (II) the process proceeds faster than Pt (IV). The presence of Pd in the system accelerates the extraction of Pt in both oxidation states but it has no significant effect on Rh extraction.

One possible way to promote the complex formation processes is to heat the system which fits well with the need of elevated temperature in order to occur the critical phenomenon of cloud point typical for non-ionic surfactants. The parameters for cloud point extraction process carried out in water bath (conventional heating on hot plate) were varied in respect to the recoveries degree. Working under optimal conditions: acidity of 1M HCl; 1000-fold mass excess of ligand; incubation at 95 ⁰C quantitative extraction of Pt (IV) was achieved at least for 120 min. but under the same conditions the degree of extraction for Rh was found to be only 20%. Pd is extracted completely in less than 30 min.

Microwave irradiation increases the efficiency of the preconcentratrion procedure reducing the time needed to obtain quantitative extraction in comparison to the conventional heating. The whole cloud point procedure was successively completed in microwave system for less than 20 min and the extraction of Pd and Pt (IV) was shortened by time factor of 3 and 6, respectively. Unfortunately even with the use of microwave stimulated extraction the recovery of Rh still remained lower than 40%.

The complex formation processes for Pt and Rh was successfully accelerated by adding of reducing agents such as KI and SnCl₂. It was proved that the presence of each reagent leads to equalization of the behavior of both initial oxidation states of platinum. In the case of Rh only SnCl₂ is an efficient promoter of the extraction process. A comparison between conventional heating and microwave irradiation at these conditions was also done. A quantitative microwave-assisted cloud point extraction of platinum and rhodium is obtained in only 10 min (compared to 30 min. for Pt (IV) and 90 min. for Rh, respectively, if conventional heating is used).

Acknowledgements: This work has been undertaken as part of the EU sponsored COST programme (Action D32, working group D32/005/04, "Microwaves and Ultrasound Activation in Chemical Analysis").

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