How do you assay for platinum and rhodium?

Answer 1

The answer to your question depends in part on the form of the sample requiring analysis. There are two general forms in which refined Rhodium and / or Platinum may be found - as an alloy of Pt/Rh or in the form of a mixture of the ions of Platinum and / or Rhodium. The former is how one may encounter Platinum Rhodium mixtures in thermocouple wires, or in furnace elements intended for specific high temperature applications. For simple metals and their alloys sequential digestion into acids may be the preferred method of generating an initial sample for further analysis. The information obtained should be sufficient to determine the relative proportions of Pt : Rh within a percent. For plating solutions, it is presumed that the Platinum and Rhodium values are simultaneously present in solution. In either of the cases cited a reasonable approach would be to subject the dissolved metals to Inductively Coupled Plasma analysis. A benefit of this approach is that the two metals may be simultaneously analyzed and their relative proportions quantified. Moreoever, as ICP is a very sensitive method, the quantity of sample sacrificed to this procedure is small. To conduct sequential digestion into acid the intrinsic properties of the metals can form a guide. http://www.britannica.com/eb/article-9063458/rhodium

Platinum is readily dissolved by Aqua Regia (a specific mixture of concentrated Nitric and Hydrochloric acids) but Rhodium remains unaffected by this mixture.

Meanwhile Rhodium is susceptible to dissolution in Sulfuric Acid - in which Platinum would remain undissolved. Thus, one might contemplate taking a Pt/Rh alloy, dissolving the Platinum metal with Aqua Regia, then removing the Platinum rich solution. The remaining solids could then be treated with Sulfuric Acid to dissolve the remaining Rhodium. Dilution of the respective solutions could permit sequential assay for the two metals by ICP. Because each solution uses concentrated acids it is imperative that they be diluted by addition of the solution to water, rather than water to the acid extracts. For plating solutions, the Pt/Rh mixtures may be directly and simultaneously assayed after dilution to the range of detection, by ICP. For the most accurate analyses of Pt and Rh in more complex matrices classical approaches are generally prefered, often beginning with a fire assay. This method is also the preferred approach to assaying ore bodies or minerals that contain traces of the Platinum group metals.

http://www.britannica.com/eb/article-81568/platinum-group A description of Fire Assay methods is found here:

http://www.mine-engineer.com/mining/assay2.htm