# rhodium dissolution



## arthur kierski (Apr 12, 2011)

when cleaning the rhodium powder with ar to eliminate pd and other metals ,i often dissolve together with the pd some rhodium,and sometimes all the rh goes to solution-----is this normal?
why this happens?most times the rh remains intact and there is no loss of weight-------
Arthur


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## Alquimi (Apr 12, 2011)

First you must be sure that it is Rh.

When there was the dissolution of all black powder, it is best to test whether Pd is hypochlorite and NH4Cl, if there is precipitation and still remain in doubt the mother solution can be reduced with zinc or other reducing agent and repeat the procedure with HR and then with hypochlorite and NH4Cl.
Check also if it is not platinum.
Another hypothesis might be that the powder metals are somehow grouped together and occur in consequence "drag"of others. But I wonder if this happens because in fact it happens in certain metal alloys specific proportions.

Alquimi :?: :?:


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## Lou (Apr 12, 2011)

Arthur,

Fine rhodium does dissolve in aqua regia. I've dissolved Rh powder (and bulk Rh) in a FEP bottle with 95% HNO3/37% HCl 1:4. 
I would've leached out the Pd with hot nitric acid several times, and used the nitric to go dissolve more Pd in another vessel.

Rh is best dissolved by boiling 85% sulfuric acid (flask with reflux condenser). Rhodium black is usually all dissolved in about an hour, rhodium sponge in 2-3 h, and Rh bulk from 3 h- 2 days.


Lou

This needs corrected. Even at fumes of sulfuric I am unable to get the more coarse, crystalline pure Rh sponges dissolved in reflux Inc sulfuric without ten times as much time invested. The preferred practice now is chlorination with sodium chloride.

I now realize that Rh blacks cover a wide variety of morphologies and surface areas. What claims I was making were most applicable to materials with surface areas exceeding 5 m2/gram. Apologies for the incorrect use of terms, I did not know better. Rh is much easier to get into solution when in alloyed or adulterated form.


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## arthur kierski (Apr 12, 2011)

thanks Lou for all these informations---it helped me to understand better what is happening-------
regards 
Arthur


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## skippy (Apr 21, 2011)

Lou, that 37% HCl will boil at 50C or so - it that how hot you were running it? Always interesting to hear another way to do something! 
Of course it's a lot easier and cheaper to get sulfuric acid for most people than 95% HNO3, a bit of a toss up which is safer though, no?
The fuming nitric acid is probably a bigger handling and storage hazard, and the sulfuric more dangerous during the reaction.


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## Lou (Apr 22, 2011)

I was using it under pressure, but the digester is rated for that. 

Also, 90% nitric is every bit as bad as concentrated sulfuric. I have a cigarette burn looking scar left underneath my right wrist from where a drop of it fell from the bottle cap and landed. I remember I felt instant pain.

I was dealing with a rhodium boride alloy, that's why I was using the higher concentration chemicals.


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## Yggdrasil (Apr 1, 2016)

Hi!
I've seen recommendatios to dissolve PGMs in Cats with the HCl/Cl method.
Why is this reco[stt]o[/stt]mmended?
Do anyone have numbers for how much PGMs are left after this?
I ask, because I found a document from "The Mining and Materials Processing Institute of Japan"
I can't actually remember where I found it, but it is highly likely it was in here somewhere 



> Quote


5. Conclusion
Various chloride based leaching solutions had been
examined to dissolve PGM from automotive catalyst residue.
The results are summarized below:
– Maximum dissolution of PGM in HCl–H2O2 system
could be achieved by using HCl (11.6 mol/L)–H2O2
(1 vol%). The dissolution of PGM was 95.5, 100 and
85.6% for Pt, Pd and Rh, respectively, at 65C for 3 h.
– The chloride in HCl can be substituted by NaClO.
However, the substitution of Cl ions supplier from HCl
to NaClO gives only limited capability to form chlorocomplexes
of PGM. The optimum PGM dissolution by
using NaClO (3 vol%)–HCl (5 mol/L)–H2O2 (1 vol%)
was 87.7, 98.7 and 76.9%, for Pt, Pd and Rh,
respectively. In addition, NaClO might also take a role
as an oxidizing agent.


> Quote



[stt]If I remember right from some other post, Pd oxides are hard to reclaim and they form at approximately 350 centigrade.
Since the Pd is completely gone one could roast the cat at high temperature and oxygen to form Rh and Pt oxides which should be easier to dissolve.[/stt]

Are my deduction from this in the vicinity of the playground?

Comments are welcome 

I guess Lou and Lazersteve might be the authorities on this.

Regards Per-Ove

Edit:
PS!
No response in this thread will try to find a more recent thread.


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## Harshrao (Aug 10, 2020)

what do you mean by (1 vol %) ?


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## Yggdrasil (Aug 11, 2020)

I guess that it means 1% by volume of the liquid, but I can't say for sure.
It was a quote from somewhere in the forum and it was 4 years ago :wink: 
Try searching for some of the terms in the quote.


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## Xxxmen (Sep 9, 2020)

Hı gays. ..What is precipitation and temperature respectively guys?specifically how to precipitate rhodium


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