Dissolving Rhodium sponge at a large scale

[sazaidi]

I need to dissolve rhodium sponge to use it further in catalyst preparation. Let me know a step by step procedure to dissolve e.g. 100 g of rhodium sponge. Any process that does not use chlorine gas or HCl gas will be my safe bet.
I have tried aqua regia, H2O2 in conc. HCl, NaOCl and H2O2 in conc. HCl. None of them seems to work with rhodium sponge. Aqua regia was a total fail, whereas the last two did some dissolution but only a fraction amount was dissolved.
It will be really a great help for me if somebody knows how to dissolve rhodium sponge completely and quickly. Please note I am aware of using Cl2 gas or HCl gas in conc. HCl but that will raise personal safety issues and I want to avoid that.

 

[butcher]

Try a fuseion in sodium bisulfate, the salt heated to syrup, and then Rh becomes water-soluble.

Sorry for step-by-step process learning you need to do homework.

Hot concentrated H2SO4 can also put it in solution, as a water soluble compound but this process is terribly dangerous, and not as good as fusion process above.

 

[Lou]

100 g is small scale and there is no "safe bet" (what do you consider safe? that's relative and pretty subjective) when you're working with rhodium and trying to get it soluble.


Make it into sodium hexachlororhodate. Mix with twice its weight in fine sodium chloride and heat with chlorine at 400-450*C for 8 h. This will make a water soluble salt with about 90% efficiency of conversion. Or just chlorinate with chlorine in quartz tube and dissolve in THF or Et2O and put onto catalyst that way.

Alternatively, put it into microwave pressure digester and put it into solution with sodium chlorate and 12 M HCl at about 165*C and 8 bar.

I have soluble rhodium salts available if you just want them already prepared. You can trade your fine rhodium metal sponge for whichever common rhodium salt you want. Send me a message if you are interested.

 

[Palladium]

100 g is small scale and there is no "safe bet" (what do you consider safe? that's relative and pretty subjective) when you're working with rhodium and trying to get it soluble.


Make it into sodium hexachlororhodate. Mix with twice its weight in fine sodium chloride and heat with chlorine at 400-450*C for 8 h. This will make a water soluble salt with about 90% efficiency of conversion. Or just chlorinate with chlorine in quartz tube and dissolve in THF or Et2O and put onto catalyst that way.

Alternatively, put it into microwave pressure digester and put it into solution with sodium chlorate and 12 M HCl at about 165*C and 8 bar.

I have soluble rhodium salts available if you just want them already prepared. You can trade your fine rhodium metal sponge for whichever common rhodium salt you want. Send me a message if you are interested.

Even though i only understand half, i love it when Lou makes a post.
Have you done a post on the nucleation topic from the other day yet Lou?

 

[Lou]

No. Am swamped with the day job (which I guess is the same thing?).

 

[freechemist]

Try a fusion in potassium pyrosulfate/sodium chloride salt-mixture.
Potassium pyrosulfate, K₂S₂O₇, can be regarded as the addition product of potassium sulfate, K₂SO₄ with sulfur trioxide, SO₃, which is the active oxidant to oxidize Rh-metal, Rh(0) to Rh(III), formally Rh³⁺ in the final product RhCl₆^3-, the complex hexachlororhodate(III)-anion, and is itself reduced to sulfur dioxide, SO₂.

K₂SO₄ + SO₃ ==> K₂S₂O₇

2 Rh(0) + 6 S₂O₇^2- ==> 2 Rh³⁺ + 9 SO₄^2- + 3 SO₂

2 Rh³⁺ + 12 Cl^- ==> 2 RhCl₆^3-

Example given: 5.0 g Rh-metal (999.5%o from Métalor; gray powder) were intimately ground together with 34.0 g NaCl and 74.0 g K₂S₂O₇ to a very fine powder. The mixture was placed in a pyrex-beaker (fill no more, than half it's volume), covered with a watch glass (also pyrex) and heated up slowly to ca. 550^oC in a muffle-oven. This temperature was maintained for 1 hour. After cooling down to room temperature the solid, red salt-cake was dissolved in a mixture of 500 ml water and 100 ml conc. HCl to give a red solution and a grey to black residue of unreacted metal. The dissolution rate of Rh in one melting-procedure was about 80%.

 

[Lou]

Concentrated HBr will also dissolve rhodium sponge pretty well too at its reflux point.

 

[rcarbon]

what happens to Potassium and Sodium in the water soluble Rh mixture

 

[butcher]

They would form potassium or sodium sulfate (dissolved in water) with the Rhodium sulfate, and in the equation above I suppose you would have a mixture of potassium or sodium sulfates and chloride salts in solution when the rhodium formed chlorides in the later reactions, free chemist did not put these cations in his reaction above as they are not important in the reaction, the chemical equations can be written with or without them, I like the equations that include these in the equations, as sometimes the by products of reactions can come in handy around the lab (instead of disposing the chemicals as waste.

 

[rcarbon]

Thank you Butcher for such useful information.

Please do let me know how to separate out unwanted Potassium or Sodium sulfate.

I need to have only water soluble Rh, be it Rhodium sulfate or Rhodium Chloride.

 

[butcher]

Unless you precipitated out the Rhodium, I do not believe you could separate these other salts, (sodium and potassium are so reactive), I also do not see why you would need to separate them normally.

It would be like trying to separate the salt (NaCl sodium chloride), from a solution of dissolved gold chloride (AuCl3), unless you precipitated the gold, these both would be in solution.

 

[HAuCl4]

Alternatively, put it into microwave pressure digester and put it into solution with sodium chlorate and 12 M HCl at about 165*C and 8 bar.

I'd choose this method as the simplest, cleanest, and safest, with proper equipment. The heater needs not be a microwave, but the pressure digester is usually Teflon (expensive).
With proper equipment everything can be made "safe". With improper equipment/procedures the simplest thing can be lethal.

Cheers Lou. Nice post.

 

[rcarbon]

While precipitating, lots of Rhodiun Hydroxide remains suspended in solution, I tried Alum coagulant, but it being acidic, simply dissolve the suspended particles of Hydroxide

 

[freechemist]

Hi rcarbon,

What do you want to know? You don't even ask a precise, specific question, nor do you give us information about what you did, what you want to precipitate from which solution, the composition of the solution etc., etc. - Without much more and precise information nobody will be able to help you, based on speculative guesses only.

Regards, freechemist

 

[rcarbon]

Hi FreeChemist,

Example given: 5.0 g Rh-metal (999.5%o from Métalor; gray powder) were intimately ground together with 34.0 g NaCl and 74.0 g K2S2O7 to a very fine powder. The mixture was placed in a pyrex-beaker (fill no more, than half it's volume), covered with a watch glass (also pyrex) and heated up slowly to ca. 550oC in a muffle-oven. This temperature was maintained for 1 hour. After cooling down to room temperature the solid, red salt-cake was dissolved in a mixture of 500 ml water and 100 ml conc. HCl to give a red solution and a grey to black residue of unreacted metal. The dissolution rate of Rh in one melting-procedure was about 80%.

Unless you precipitated out the Rhodium, I do not believe you could separate these other salts, (sodium and potassium are so reactive), I also do not see why you would need to separate them normally.

It would be like trying to separate the salt (NaCl sodium chloride), from a solution of dissolved gold chloride (AuCl3), unless you precipitated the gold, these both would be in solution.



I AM TRYING TO PRECIPITATE OUT RH TO GET RID OF SODIUM AND POTASSIUM IN ABOVE PROCESS

 

[Lou]

Precipitate the hydroxide with KOH until a pH of 6.5. Have excess peroxide present to help in agglomeration. Then you can wash this or centrifuge and wash until it tests relatively free of K+ or Na+. You may be able to remove those with a cation exchange resin.

If you want it absolutely free of spectator cations, you will want to microwave pressure digest it in HCl/Cl2.

 

[freechemist]

Hi rcarbon,

I AM TRYING TO PRECIPITATE OUT RH TO GET RID OF SODIUM AND POTASSIUM IN ABOVE PROCESS

That is the only information of possible value to help you, you give to me on my request for more information. The rest is a combined copy-paste action with my receipe, now more than a year old and an also more than a year old comment from butcher.

http://goldrefiningforum.com/phpBB3/viewtopic.php?f=37&t=14894#p154068
http://goldrefiningforum.com/phpBB3/viewtopic.php?f=37&t=14894#p156437

There are two ways, to get rid of alkali-metal-ions by precipitation of rhodium, one by forming insoluble rhodium hydroxide, Rh(OH)₃, the other by cementation with zinc, to obtain raw metallic rhodium.

Rhodium hydroxide is formed by first neutralizing excess acid (e.g. 100 ml conc. HCl; ca 1 mole HCl) with NaOH (e.g. as a 30% solution) until a pH of about 3, followed by adding slowly 3 moles of NaHCO₃ per mole/gram-atom of rhodium to be precipitated to the constantly well stirred solution. Be careful, there may be considerable foaming by evolution of CO₂.

Rh³⁺ + 3 HCO₃^- ==> Rh(OH)₃ + 3 CO₂(g)

After complete addition of NaHCO₃ (e.g. 10-12 g), the reaction mixture is heated to boiling, under constant stirring and held at the boiling temperature for about 1-2 hours. Usually a well settling yellow solid is obtained, which can be purified further by filtration and repeated washings with deionized water.

Regards, freechemist

 

[Bawa]

Whats the best method of dissolving Rh in H2So4