I think,  Spectrophotometric determination is the simplest one. Please read the papers mentioned below:

1. Orville et al. (1971). Spectrophotometric determination of rhodium with nitriso R. Formula for the Rh (III)- nitroso R complex. Analytical Chemistry 43, 146-148.

2. Shelar et al. (2015). Extraction spectrophotometric determination of rhodium (III) with o-methylphenyl urea. Journal of Saudi Chemical Society 19, 616-627.

Hi Tarafdar

thank you for the response and articles. However I am more interested in solution chemistry of rhodium. what type of complexes are formed ( cationic, acionic, ion pairing etc) depending on the pH, temperature etc. those articles seem to related to detection and extraction of trivalent rhodium rather than analyzing the formed complexes.

Mr Kas,

These are UV-VIS, IR, MS, XRD, NMR, etc.