# rhodium sulfate to Rhodium



## samuel-a (Apr 22, 2010)

so... i have dissolved some Rh in boiling sulfuric acid...
the acid is now deep brownish red almost black which fit the photo uploaded by steve of a dissolved Rh.

- now, what should i do? how do i retrieve it back to metal state?
- this Rh is from an auto-cat leech and was boild first in AR (twice) to to remove any Pt/Pd. does it really need more processing or can i consider it as relatively pure Rh?

thanks, SAMUEL


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## lazersteve (Apr 22, 2010)

You can electroplate it out onto a good clean copper surface then dissolve the copper in dilute nitric leaving a piece of metallic Rh.

Here's a site with a good Rh cell setup:

Rhodium Plating Process

Be sure the back side of your copper item is protected with some acid proof tape, if you need some let me know. You'll need to get a good clean surface to produce the best possible plating. Here's a quick video I found on you tube of the process:

Rhodium plating Video

Alternately you can use any one of several reducing agents ( e.g.: hydrazine ) or you can cement the rhodium out as a powder with zinc or copper. Cemented Rh will need to be cleaned with an appropriate acid to remove the base metal contamination.

Another course of action is to add ammonium hydroxide to produce yellow Rhodium hydroxide. The hydroxide can then be redissolved in H2SO4 for the greatest purity before using the resulting solution for plating.

There are other ways as well.

The important thing to remember is what form does your buyer want the Rh in and will it be redissolved. If it will be redissolved the hydroxide may be the best form for you to produce. 

Steve


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## goldsilverpro (Apr 22, 2010)

Steve's idea has merit, but there could be some problems.

The following are only educated guesses. I apologize to Steve, but I have learned over the years to first look at every possible problem I can think of. However, I really don't know exactly what will happen. All you can do is try it.

If the sulfuric concentration is too high, it might either plate very slowly or not at all. Excess acid could throw the cathode reaction more towards the generation of hydrogen gas rather than the deposition of rhodium. 

The Rh plating solution in the video was most probably done with a standard sulfate plating bath of:

Rhodium metal, as rhodium sulfate - 2 to 10 g/l
Sulfuric acid, conc. - 20 to 50 ml/l
Temp. - 104 to 113 F
Cathode Current density - .06 to .24 amps/sq.in.

If it does plate, it is unlikely that all of it will plate out. In most solutions, as the metal concentration decreases beyond a certain level, the cathode efficiency decreases rapidly. This means the metal plates out at a slower rate. Also, when the metal in the solution gets low, the quality of the deposit gets worse. It could get so bad that it falls off the cathode as a powder. If this becomes a problem, you might remedy it by lowering the current. That last bit might be plating at such a slow rate that it will take forever to plate out.

When we recovered Rh from standard sulfate plating baths, the standard way was to precipitate the Rh with powdered magnesium. However, Mg is soluble in sulfuric. If there is a great excess of sulfuric, the Rh might not start dropping until the excess sulfuric is used up. This might take a lot of magnesium. Also, this might produce a very vigorous reaction. If I were to use Mg, for safety's sake, I would add it in small amounts.

If you do this again, I would try to keep the sulfuric usage at a bare minimum.


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## lazersteve (Apr 22, 2010)

Gsp,

I totally agree with all the points you have made.

All of the Rh in solution would definitely not fully plate out in a perfect sheet or ingot, but you could run the cell until the quality of the deposit degraded, then precipitate the remaining Rh with ammonium hydroxide and make another solution of less volume with fresh sulfuric. 

The up side to this process is that you don't need to melt Rh to get solid piece of metallic Rh.

Steve


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## samuel-a (Apr 23, 2010)

that is some good solid advise from you guys, many thanks.

i'll try it both ways according to your suggestions.

i have here about 0.6g of Rh that was boiled in 6 ml of acid, it took about an 2 hours until the Rh stopped reacting to the acid, and i belive it to be saturated and without any water. it looks like i have dissolved only 0.1-0.2g of Rh into solution (plausible?).

steve thank you for your suggestion, but i just pulled out of a CRT some laminated copper strip, it looks to be resistant to dilute acid (i hope so anyway).

if plating go's well, when done, maybe i can drop the traces of Rh from the plating solution with zinc...


BTW, seems like i need to use Pt or Pt coated Anode... well.. i don't have that.... can i use somthing else? gold?
thanks again. all the best


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## goldsilverpro (Apr 23, 2010)

> BTW, seems like i need to use Pt or Pt coated Anode... well.. i don't have that.... can i use somthing else? gold?


That's a very good question. Unless the sulfuric is fairly low, it might "dissolve" the gold. Art Corbit, on another forum, was playing around with using battery acid (about 35% sulfuric by weight or 20% by volume) to strip gold from pins. He found that the gold reacted similarly as it did in the standard concentrated sulfuric stripper and ended up as a powder on the bottom. However, the sulfuric also attacked the copper with a vengeance and, therefore, it wasn't a very usable process.

Lots of metals will be soluble, electrolytically, in the sulfuric. Steel or SS won't work. Silver might not work for the reasons I gave for gold above. About the only 2 things I can think of are lead or carbon. However, I have seen carbon or graphite anodes fall apart in acid cells. I think I might try the lead. It shouldn't dissolve in the sulfate system. I would think that, at the worst, it might develop an oxide coating from the anode reaction that would increase the voltage and decrease the current flow. Whatever you do, I would use about twice as much anode area as cathode area and use about 0.1 to 0.15 amps per sq.in., based on the cathode surface area. You might also warm the solution to about 100F.

I still think the sulfuric concentration might be the key to success in Steve's process. Do you have any idea how much concentrated sulfuric, by volume, you have in a liter of solution? If the Rh doesn't plate, you might try diluting a small portion of what you have, with distilled water, and try that.

I might mention that rhodium plating is extremely brittle. I will be surprised if, when dissolving away the copper, the Rh holds together in one piece. Due to the inherent internal stress of the deposit, I would expect it to fall apart into a jillion shiny slivers. In the literature, the use of a little magnesium sulfamate is suggested as a stress reliever - it doesn't say how much to use. I doubt if that is readily available, but I would guess that it could be easily made by reacting a solution of sulfamic acid with a slurry of Mg hydroxide or oxide.

Here again, I'm just thinking out loud. It's an interesting project.


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## Barren Realms 007 (Apr 23, 2010)

goldsilverpro said:


> That's a very good question. Unless the sulfuric is fairly low, it might "dissolve" the gold. Art Corbit, on another forum, was playing around with using battery acid (about 35% sulfuric by weight or 20% by volume) to strip gold from pins. He found that the gold reacted similarly as it did in the standard concentrated sulfuric stripper and ended up as a powder on the bottom. However, the sulfuric also attacked the copper with a vengeance and, therefore, it wasn't a very usable process.
> 
> You might also warm the solution to about 100F.
> 
> I still think the sulfuric concentration might be the key to success in Steve's process. Do you have any idea how much concentrated sulfuric, by volume, you have in a liter of solution? If the Rh doesn't plate, you might try diluting a small portion of what you have, with distilled water, and try that.



The water in the solution is what attacked the copper, if it were concentrated sulfuric it will not attack the copper. Also Steve recomends to keep the temp down because when the temperature on the solution raises it has more of a tendency to attack the copper. I use my cell in an ice bath to keep the temp down. The sulfuric should not be diluted. IMHO. 8)

This is just concerning the gold removal.


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## goldsilverpro (Apr 23, 2010)

> The water in the solution is what attacked the copper, if it were concentrated sulfuric it will not attack the copper. Also Steve recomends to keep the temp down because when the temperature on the solution raises it has more of a tendency to attack the copper. I use my cell in an ice bath to keep the temp down. The sulfuric should not be diluted. IMHO. 8)
> 
> This is just concerning the gold removal.


I certainly agree with everything you say but I don't see how that has anything to do with the subject at hand. We're not talking about stripping gold. I was talking about the possible attack on a gold anode if it was used to plate the Rh out of a solution containing sulfuric acid. I just don't want to confuse the issue.


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## samuel-a (Apr 23, 2010)

goldsilverpro said:


> > The water in the solution is what attacked the copper, if it were concentrated sulfuric it will not attack the copper. Also Steve recomends to keep the temp down because when the temperature on the solution raises it has more of a tendency to attack the copper. I use my cell in an ice bath to keep the temp down. The sulfuric should not be diluted. IMHO. 8)
> >
> > This is just concerning the gold removal.
> 
> ...



thanks chris, i was writing about ten million words of a replay on the subject but, it didn't send out and deleted.. :| 

so, in short.


my Rh solution is 97% or higher, probably higher.

i was thinking of diluting the solution at about 5:1 ratio with distilled water and add more water if it won't work.

lead was my secund guess :mrgreen: 
i'd hate to mess up with the little gold that i have


EDIT: although, he does have a point about a copper anode that will not be attacked by the concentrated acid, on the other hand, it will probably screw up the plating function :mrgreen:


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## Oz (Apr 23, 2010)

Copper (as well as nickel) can and will go into concentrated sulfuric (98%) under a DC current, just not in large quantities. As your sulfuric draws moisture from the air diluting itself, the copper and nickel will go into solution more rapidly.


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## samuel-a (Apr 26, 2010)

well.... the electrolization didn't worked.....
not even a start of a silvery deposite...

working condition of the cell:
diluted the electrolyte 5 to one with dis. water
anode:cathode ratio was 2:1
used lead anode and a coated (one side) copper film cathode
temp. 107F
voltage - 4
the only drawback that the i can think of is to high current dancity, almost one Amp per sq.in. but i didn't had any other transformer .

after a few minutes, the copper (the cathode, the negative lead) film disintegrated with small eruptions....
the lead anode turnd blabk...

i think i'll try to precipitate with zinc or aluminum...

all in all, i would like to know if there is a color test for Rh like we have for gold etc... and not just by looking at the color of the solution...
who knows, maybe i'm doing all of this in vain


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