Sample Preparation - Digesting Pt oxide

I'm trying to dissolve Pt isotopic spikes. I've dissolved Pt metal before with no problem by first dissolving the metal in concentrated aqua regia in a teflon beaker, then evaporating the aqua regia by placing the beaker on a hotplate with the lid off, redissolving the residue in concentrated HCl, evaporating that, and ultimately dissolving this residue in 6M HCl. The Pt isotopic spikes are in the form of a very fine powder. In fact the powder was so fine that I could not remove all of it from the vials without washing them out with something. I chose to wash the vials out with methanol because I could evaporate it quickly afterwards both to begin the aqua regia dissolution and to reweigh the shipping vial after removal of the spike so that I knew how much spike I had dissolved. I also thought methanol would not react with the Pt at low temperatures. After washing the Pt+methanol into teflon beakers, I put the beakers (unlidded) on a hotplate set to 100 C to evaporate. After they were totally dry, I added the concentrated aqua regia and allowed the spikes to dissolve for two days. After two days, the spikes appeared to be dissolved, although it was hard to tell since the powder is so fine. I have dissolved a greater mass of Pt wire in the same amount of time, so I suspect the spikes were dissolved, or would have dissolved if they were still Pt metal. I evaporated the aqua regia and instead of the normal dark red residue (which dissolves in conc. HCl in seconds), I had a black residue. I added the concentrated HCl anyway, and a small amount of residue dissolved but most did not, even after sitting in lidded beakers on a hotplate for 24 hours. This has happened with two Pt metal spikes that were on a hotplate at 100 C with methanol and never with Pt metal that did not contact methanol, so it seems that Pt and methanol do react at temperatures as low as 100 C.I've tried re-doing the aqua regia step and have gotten to the point where I now have both an easily-dissolved reddish residue and an insoluble black residue in each beaker. I need to dissolve everything because I don't want to fractionate my isotopic spikes. But the problem is I have no idea what I made and so no clue how to ultimately get it to dissolve into 6M HCl. Do you have any suggestions?

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admin Total posts: 529	Thank you for the detailed information regarding the Pt digestion method. The residues you are observing are consistent with chloroplatinic acid (reddish), which is easily soluble in HCl, and platinum oxide (black), which can be difficult to solubilize. The direct approach to getting the black residue in solution is to reduce the platinum to the metal, which should easily dissolve in aqua regia. One approach we’ve used to clean platinum crucibles that have developed a black oxide coating is to heat ascorbic acid in the crucible using the reducing part of a flame until the ascorbic acid is burned off. Another possible reducing agent could be an aldehyde (e.g. formaldehyde), though we have not explored this option. A third approach could be to perform the aqua regia digestion again, but increase the HCl/HNO3 ratio significantly (e.g., 1 part HNO3 to 5-10 parts HCl); we’re concerned, however, that this may not fully complete the dissolution. As for the methanol-Pt interaction, we believe that if a Pt-oxide was formed, it was more likely to have resulted from the gentle (100 C) heating, and not the addition of methanol, as the fine powder form of the material you received would enhance any potential oxidation reactions that could occur, particularly as temperature increases. Please let us know if you have any further questions or comments. Posted: 06/14/19 10:14:34

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admin Total posts: 529

Thank you for the detailed information regarding the Pt digestion method. The residues you are observing are consistent with chloroplatinic acid (reddish), which is easily soluble in HCl, and platinum oxide (black), which can be difficult to solubilize. The direct approach to getting the black residue in solution is to reduce the platinum to the metal, which should easily dissolve in aqua regia. One approach we’ve used to clean platinum crucibles that have developed a black oxide coating is to heat ascorbic acid in the crucible using the reducing part of a flame until the ascorbic acid is burned off. Another possible reducing agent could be an aldehyde (e.g. formaldehyde), though we have not explored this option. A third approach could be to perform the aqua regia digestion again, but increase the HCl/HNO3 ratio significantly (e.g., 1 part HNO3 to 5-10 parts HCl); we’re concerned, however, that this may not fully complete the dissolution. As for the methanol-Pt interaction, we believe that if a Pt-oxide was formed, it was more likely to have resulted from the gentle (100 C) heating, and not the addition of methanol, as the fine powder form of the material you received would enhance any potential oxidation reactions that could occur, particularly as temperature increases. Please let us know if you have any further questions or comments.

Posted: 06/14/19 10:14:34

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Sample Preparation - Digesting Pt oxide

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