How insoluble is rhodium in nitric acid ?

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orvi

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Hi
this is maybe dumb question, but did anyone noticed any kind of dissolution of rhodium in nitric acid (cold,boiling, 65%..., 100%...) ? Just nitric, no added HCl or H2SO4 or anything, straight nitric. Pretty much every textbook or paper says it is completely insoluble in nitric. With platinum, many pages said that it is insoluble in nitric and hence i was unhappy to see it goes into the solution (HNO3) in not the negligable quantities.
I know and i performed the dissolution of Rh material in AR and it gone to some extent, but the Rh was reluctant to go into the solution. This give me some confidence that nitric does not anything to Rh. But after being previously skunked by textbooks, i rather ask here :)
I just want to be sure that i dont gonna make another mess. Rh is in very finely divided state in the material, structurally it is close to Ranney Ni, with very high surface area. Active.
With relatively low content of Rh in the material (few % at most) im bit paranoid about accidentaly dissolving part of it, which will then be painful to recover from base metals and other PGM present (Pd mainly).
Practical observations would be greatly appreciated.
Thanks
 

Lino1406

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On the contrary, nitric acid will passivate rhodium surface, so preventing its dissolution, especially if alone
 

orvi

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On the contrary, nitric acid will passivate rhodium surface, so preventing its dissolution, especially if alone
that makes sense, thank you for the point. in these terms i am worried that platinum, which is also present in the material in few %, will dissolve into nitric solution (it will hapen) and MAYBE possibly reprecipitate on the undissolved less noble metals. but if Rh is passivated, then this will not likely to happen.
 

4metals

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That is pretty hot and the pressure is up there. If that is what is required to break the passivation Lino mentioned, I dare say a heavy stainless pressure vessel wouldn’t stand a chance.
What equipment is used for this digestion? And is it do able in anything but lab sample quantities? Anything like a microwave bomb used in analytics?
 

orvi

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very interesting numbers, Lou :) im just wondering, like 4metals, how this could be done except sealed glass ampoules. not attempting to even try this on my own.
my aim is to reduce bulk of the PGM, except rhodium. after some thinking, i come up with idea to alloy PGM residue with copper or aluminium, forming uniform (hopefully not granular - with intermetallic grains of PGMs) alloy, which could be easily digested in nitric acid or NaOH (for Al), forming very fine PGM precipitate - this could be very prone to dissolution in nitric acid, i expect even a good part of Pt would go into the solution after prolonged boiling (65% nitric), but this is OK. Im only curious about that finely divided rhodium... Would be great if this pathway work. That is why i posted here, because i know there are experts in the field, which know better than textbooks.
I found online this "paper" (link printscreened on the picture) on rhodium properties, which say, that after alloying Rh with Cu, it will dissolve in HNO3... That is exactly what i wanted to do.rh characteristics.jpg
 

Lou

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4metals, it is very hot and very nasty. We have a 2 gallon C276 autoclave that goes to about 5000 psi. That's what's needed to dissolve Ir/Ru sponge via what's called a Carius Tube digestion. We would use pure quartz tubes, 37% w/v HCl and fuming nitric acid. You put that into the container, evacuate it, and then add a known quantity of CO2 from the tank and heat it until you're at pressure. They're very long and skinny.

Last year I worked with these people to get a method to dissolve Ru/Ir but we didn't have great success at the 500+ mg scale we need to do ICP-OES on 4N+ Ir and Ru sponge and get the LOD we need. Rh would dissolve however, so I can say it's Ir>~Ru>>Rh insofar as chemical resistance is concerned with oxidizing/complexing aqueous media. Download Methods - UltraWAVE - Milestone
They have a great system but they're near 6 figures and only do about 70% of the type of work I need. I usually rely on Na2O2 as the sledge hammer.

As to inorganic ventures (good people by the way), the problem is they collate a bunch of theory. For instance, if alloyed in Pt, it is definitively NOT attacked to any great degree by nitric acid but the aqua regia treatment they mention will get it done. Then of course the Rh must be removed via hydrolysis. That Rh(OH)4 is then cationic when it is redissolved. Rh chemistry is odd.

One way to get bulk rhodium soluble is crush it and then to alloy it with Cu, Zn, Al (one of them), digest away those metals, then fuse it with NaOH/Na2O2. That product--which may be pyrophoric or at least smoke on standing-- is then soluble in HCl and gives the characteristic anionic wine red chlororhodate species. Nitre can be substituted but it decomposes into nitrite at temperature and that may cause problems for aqueous work up depending on where the refining is going and what else is in it.

Generally many analytical labs will do the Cl2+NaCl method which I've already shown on this forum, years ago.
 

Yggdrasil

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Thanks for yet another informative post Lou.
I guess the Carius tube digestion are impractical for other than analytic work?
Is Oxone (Potassium peroxymonosulfate) usable as fusing agent in these scenarios?

What function has the NaOH with the Na2O2? Forming Rhodium Hydroxide?

Regards Per-Ove
 

Lou

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Oxone gives up its oxygen readily and would probably end up being just like melting it with bisulfate, where the active oxidizing agent is mostly SO3. As for Carius...guess it depends on the size of the autoclave?? Usually the big issue with Ir is getting it fine enough to dissolve under those conditions without contaminating it with the media used to comminute it. This is why many people press pellets and do spark-OES or melt with UHP indium as a binder and do GD-MS.

I think it's sodium rhodite/rhodate that forms. Palladates, rhodates, iridates...they all do that. They all seem soluble in aqua regia or very concentrated hydrochloric acid.
 

goldandsilver123

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That autoclave method is so interesting, I've searched for it, but didn't find much about it.

If the quartz ampoule sealed? or the CO2 is poured inside the autoclave with the quartz tubes open?

Does Rh(OH)4 turns to Rh3+ on boiling the HCl solution? First time seeing Rh4+

The method that Freechemist posted with bisulfate and NaCl does work, but it's too much expensive and time consuming...
How long it would take for total dissolution of rhodium?
 

Lou

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The tubes can be open or sealed. If open, enough gas (in this case, nitrogen) is introduced to keep sufficient over pressure to prevent boiling of the acids. If sealed, CO2 is added to reactor.

Rh in the tube is done in 30-60 minutes.

Check out the ultraclave. For Rh for plating solution quantities (say 20 g per hour) it may be all you need.
 

orvi

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4metals, it is very hot and very nasty. We have a 2 gallon C276 autoclave that goes to about 5000 psi. That's what's needed to dissolve Ir/Ru sponge via what's called a Carius Tube digestion. We would use pure quartz tubes, 37% w/v HCl and fuming nitric acid. You put that into the container, evacuate it, and then add a known quantity of CO2 from the tank and heat it until you're at pressure. They're very long and skinny.

Last year I worked with these people to get a method to dissolve Ru/Ir but we didn't have great success at the 500+ mg scale we need to do ICP-OES on 4N+ Ir and Ru sponge and get the LOD we need. Rh would dissolve however, so I can say it's Ir>~Ru>>Rh insofar as chemical resistance is concerned with oxidizing/complexing aqueous media. Download Methods - UltraWAVE - Milestone
They have a great system but they're near 6 figures and only do about 70% of the type of work I need. I usually rely on Na2O2 as the sledge hammer.

As to inorganic ventures (good people by the way), the problem is they collate a bunch of theory. For instance, if alloyed in Pt, it is definitively NOT attacked to any great degree by nitric acid but the aqua regia treatment they mention will get it done. Then of course the Rh must be removed via hydrolysis. That Rh(OH)4 is then cationic when it is redissolved. Rh chemistry is odd.
Thanks for very interesting information. I appreciate the expertise always given in your posts. When I started at the university as organic chemist, i thought that inorganic chemistry is something what starts to be boring like year after doing so, but i was horribly wrong :) in next few years i worked with transition metal catalysts (with funding that we "have", all made on my own from basic, mostly PGM salts) and practically i was more like inorganic chemist :D
As you start to add more and more elements to the mixtures, much more is going on, and it started to fascinate me everyday i stumble across some unexpected results from XRF or how you cannot separate 6 elements in 21. century in some easy and elegant fashion :)
Started to doing some refining, as a side "job", picked up the most difficult category of metals, definitely sure that with level of my knowledge it would be easy and straightforward... haha :D wrong

But in the next few days, we are planning to conduct few experiments, in "analytical manner-XRF backed", so if some at least somewhat interesting firures could be made out of it, ill post it here :)
 

Yggdrasil

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Oxone gives up its oxygen readily and would probably end up being just like melting it with bisulfate, where the active oxidizing agent is mostly SO3. As for Carius...guess it depends on the size of the autoclave?? Usually the big issue with Ir is getting it fine enough to dissolve under those conditions without contaminating it with the media used to comminute it. This is why many people press pellets and do spark-OES or melt with UHP indium as a binder and do GD-MS.

I think it's sodium rhodite/rhodate that forms. Palladates, rhodates, iridates...they all do that. They all seem soluble in aqua regia or very concentrated hydrochloric acid.
Thanks again Lou.

In your initial post you implied that it was not viable to use bisulfate, is that due to price/time or speed and efficiency?
Bisulfate is cheap and readily available, Sodium Peroxide is hard to get and expensive.

Are the sealed Carius tubes flexible enough to not crack if the differential pressure get too high?
And why CO2?
Will N2 or Ar be too expensive or do CO2 participate in the reaction?

What about Oxone in the Carius tubes?

Many why's here but I got curious
😃🤔

Thanks again for your excellent posts Lou.

Regards Per-Ove

Edited for clarity.
 
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goldandsilver123

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The tubes can be open or sealed. If open, enough gas (in this case, nitrogen) is introduced to keep sufficient over pressure to prevent boiling of the acids. If sealed, CO2 is added to reactor.

Rh in the tube is done in 30-60 minutes.

Check out the ultraclave. For Rh for plating solution quantities (say 20 g per hour) it may be all you need.

I did find the ultraCLAVE, it seems great, but 6 figures after customs tax and everything will be in the 7 figures range, sucks to be so far away and have high import taxes.

Now I'm modifying my furnace for a 55 mm diameter 300 mm long tube, for the freechemist fusion.

In the future I will try to buy a Cl2 cylinder and try to dissolve the rhodium powder with HCl on a round bottom flask.

Thanks again Lou.

In your initial post you implied that it was not viable to use bisulfate, is that due to price/time or speed and efficiency?
Bisulfate is cheap and readily available, Sodium Peroxide is hard to get and expensive.

Are the sealed Carius tubes flexible enough to not crack if the differential pressure get too high?
And why CO2?
Will N2 or Ar be too expensive or do CO2 participate in the reaction?

What about Oxone in the Carius tubes?

Many why's here but I got curious
😃🤔

Thanks again for your excellent posts Lou.

Regards Per-Ove

Edited for clarity.

I never saw this procedure, but I imagine like this:

The sample + acids is added in the quartz or borosilicate tube, the tube is closed with a flame and put in the heating vessel, normally the tube will handle the pressure, but the heating vessel is designed to handle the eventual explosion. Perhaps CO2 is added outside (inside the heating vessel) to diminish the pressure difference experienced by the tube.

They are not flexible.

To my knowledge CO2 doesn't participate, much less Ar or N2

In my view sodium peroxide is used for analytic work when you must dissolve 100% of the sample (otherwise there would analytical error), for other purposes having less dissolution rates are acceptable.
 

Lou

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The reason for not using CO2 in open digestion is because it will dissolve under pressure in the liquid media and may cause sample loss as it changes phases/states. For open digestion, nitrogen or argon are much less soluble in the acid and can suppress the boiling. The CO2 on closed tubes or N2/Ar on open tubes serves to counter the vapor pressure of the acid mixture and keep the tubes from exploding.

10 mm quartz tubes we would use are here:

Typically it is 1 to 5 parts red fuming nitric acid to as high a concentration of HCl as possible. 25 mL can be frozen with LN2 and have 1000-500 mg of iridium (or whatever) sponge, evacuate and flame seal the other end.

We like sodium peroxide for analytical and for production fusions because there is no nitrite produced and less waste treatment concerns. In bulk, it's about $15000/ton. Relative to how much RuO2/IrO2/Rh2O3 it can oxide, it's cheap.

For Rh, better off to chlorinate with salt at 700 C to get Na3RhCl6, dissolve that in boiling water, filter unreacted Rh, add KOH, re-filter the hydroxide and either repeat it to lower alkali or dialyze it.

As for bisulfate/pyrosulfate fusions...sure they work, even better when NaCl is added. It's a temperature thing. Very very hot, concentrated sulfuric acid will dissolve Rh black in hours timescale. It's frightening, with lots of fumes but it will dissolve.
 

Yggdrasil

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The reason for not using CO2 in open digestion is because it will dissolve under pressure in the liquid media and may cause sample loss as it changes phases/states. For open digestion, nitrogen or argon are much less soluble in the acid and can suppress the boiling. The CO2 on closed tubes or N2/Ar on open tubes serves to counter the vapor pressure of the acid mixture and keep the tubes from exploding.

10 mm quartz tubes we would use are here:

Typically it is 1 to 5 parts red fuming nitric acid to as high a concentration of HCl as possible. 25 mL can be frozen with LN2 and have 1000-500 mg of iridium (or whatever) sponge, evacuate and flame seal the other end.

We like sodium peroxide for analytical and for production fusions because there is no nitrite produced and less waste treatment concerns. In bulk, it's about $15000/ton. Relative to how much RuO2/IrO2/Rh2O3 it can oxide, it's cheap.

For Rh, better off to chlorinate with salt at 700 C to get Na3RhCl6, dissolve that in boiling water, filter unreacted Rh, add KOH, re-filter the hydroxide and either repeat it to lower alkali or dialyze it.

As for bisulfate/pyrosulfate fusions...sure they work, even better when NaCl is added. It's a temperature thing. Very very hot, concentrated sulfuric acid will dissolve Rh black in hours timescale. It's frightening, with lots of fumes but it will dissolve.
Thanks again Lou, for the clarification.
I'm grateful whenever you share your knowledge with us.

Regards Per-Ove
 

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