After used in thermal analysis(30-1000℃), the internal surface of platinum crucible become black perhaps due to its reaction with the sulphur or halogen elements of the samples. I boiled the crucible in 1M HCl for several hours, rinse it with ultrapure water, and immerse it in HF acid for several days, but cannot make the crucible clean and light as before. Could anyone give me any suggestions, thanks in advance.
You may try to repeat cleaning of the platinum crucible also with boiling conc. aq. hydrochloric acid, but using the considerably higher conc. of ~18 wt% ― about 6-fold higher than that you have mentioned. The acid must be free from oxidizing agent (e.g. nitrate) contamination. If this attempt still does not improve cleaning sufficiently, try cleaning with fused potassium bisulphate (~1 min). The flux residue can be cleaned with hot (to boiling) water.
Also about platinum cleaning ― from gold nano-particles: https://www.researchgate.net/post/How_do_you_clean_your_platinum_crucible_from_Gold_nano-particles
Next reference (p. 98; in Portuguese) addresses cleaning of platinum crucibles from molten glass: https://www.researchgate.net/publication/231157960_Cristalizacao_de_Biomateriais_Vitroceramicos_e_Mineralizacao_em_Meio_Fisiologico_Simulado
Thesis Cristalização de Biomateriais Vitrocerâmicos e Mineralização...
One method I found useful is room-temperature concentrated HBr, and if the HBr itself is not enough add just a droplet of H2O2 (generating a bit of bromine in situ of course). This will result in some loss of platinum, so be sure to decant and inspect the crucible often to minimize the damage. Explain to your supervisor that your time is worth money too, and in the future if possible do not use the Pt crucible for sulfur-containing compounds.
Thank Carlos and Emanuel for your wonderful answers.
I will try hot HCl with a higher concentration, cool concentrated HBr (with a drop of H2O2 if necessary).
Carlos, can I fill the crucible with potassium bisulphate, fuse it for 1h in the muffle furnace, if not, please give me some specific guidance.
BEST REGARDS
Peixin Du
Two things I would like to add:
1. On HBr + H2O2: I was using 10-20 ml concd. HBr + 0.03-0.05 ml H2O2 30% (cleaning fairly large crucibles) - gives you an idea of the ratio.
2. If you keep having to deal with sulfur-containing samples, you may try not to use a noble metal crucible, but rather try tantalum or niobium (or possibly titanium). Of course it depends on what else you have in the samples - fluorine is not good for these, and strong base is not good either (especially for Ta/Nb).
You can clean your crucible also at high temperature by using standard white glass (e.g. from yoghurt or glass bottles) which are typically lime-soda-silica glasses with a melting point of around 1040°C. So, put some glass material into the crucible and put it at 1100°C or 1200°C. The black stuff will then most likely dissolve into the glass. After holding the crucible with the glass at high temperature for while, quench it on a metal plate. Try to get the glass out of the crucible manually and finalize the cleaning by using an HF solution. This procedure should normally give you a very clean crucible.
i did run in the same problem.
I would like to say, first is better not to use sulfur containing products with platinum crucible (try ceramics or zirconia instead)
second even if your crucible is black, you can run a blank measurement on the pan. if it´s stable along the measuring range, then is non reacting and you can use it just like a clean one. I do understand the bad appearance of it, but eventually if it´s thermally stable is just a matter of aesthetics but it will work just fine.
I just re-read the question and the answers. If the surface is darkened, this may be due to the formation of Pt-oxide, especially at temperatures below 1000°C. However, these oxides are rather volatile. The only thing to do is to put the crucible in a furnace and heat up to 1400°C in air. The oxides are volatile and will disappear and the crucible looks as new.
Thanks for the Answer Alexander, at Rossing Uranium Mine Laboratory , we use Potassium bisulfate fusion for cleaning. we keep the temperature to around 1000 OC
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