Sciencemadness Discussion Board

Need some advice/help on recovering Palladium from Aqua Regia...

FireLion3 - 21-10-2015 at 15:31

What I thought would be a two day process of dissolving the Pd and boiling excess off, has become a 20 day process due to all kinds of little problems that come working with acid fumes, not having the materials I need, not being able to vent outside, and so on. I wont go into detail about all my failed attempts, I just need to figure out a way that doesn't involve me working with boiling off excess.

Is it possible for me to just add sodium carbonate to neutralize any of the excess acid while simultaneously salting out the Palladium, due to Sodium Carbonates higher water solubility? I think this would work, but only very limited information is availible about the solubility profile of palladium salts in water. I'm just trying to avoid being stuck with a difficult to separate mixture of palladium salts and other salts.

I'm thinking I could wash the leftover palladium with a small amount of water to remove any carbonate/nitrate/chloride salts, but without knowing the water-solubilities profiles of palladium, I'm unsure how to procede.

Any suggestions?

Upsilon - 21-10-2015 at 15:50

Palladium chloride is "soluble in trace amounts" as per Wikipedia. This means that its solubility is pretty much negligible. It also mentions that solubility is better in cold water, so I would get it as hot as you can without boiling the water. All of the sodium carbonate and chloride and nitrate will just be completely dissolved.

Something else you can do if you have a distillation setup that is even cleaner is boiling the aqua regia off in a closed system and piping the vapors into a sodium carbonate or bicarbonate solution. This will prevent these pesky vapors from making it into your space.

Something else to note is that the action of aqua regia on palladium is usually supplemented with dissolved chlorine gas, but I imagine you probably figured that out the hard way.

[Edited on 21-10-2015 by Upsilon]

FireLion3 - 21-10-2015 at 16:06

I wasn't sure how to interpret the soluble in trace amounts note. So you think if I neutralize all the acid, the palladium will just precipitate out, since the acid is what solubizes it (it being soluble in HCl). I wasn't sure if "trace amounts" meant something like 5g per liter, or 0.1g per liter.

It's usually supplemented with chlorine gas? Dissolving the palladium was pretty easy, I just left it in a bottle with aqua regia for a few days. What do you mean?


I tried a distillation set up boiling off the aqua regia. I don't want to take the time to explain all the things I tried, but this is basically what I was trying to set up for 10 days, but there were all these little pesky technical issues that prevented me from being able to do it. Risk of water suck back, concern about the vapor pressure in my high temperature hosing being too high and instead of boiling it into the receiving solution, breaking the glass releasing dangerous fumes into the air, and so on. What you're describing sounds simple... I thought it was, I could visualize it so perfectly in my head... but I was mistaken. I just didn't have the stuff I needed to fully set it up correctly, and when working with those nasty fumes, it's not exactly something you want to mess up on even once.

[Edited on 22-10-2015 by FireLion3]

[Edited on 22-10-2015 by FireLion3]

Upsilon - 21-10-2015 at 19:05

Quote: Originally posted by FireLion3  

I wasn't sure how to interpret the soluble in trace amounts note. So you think if I neutralize all the acid, the palladium will just precipitate out, since the acid is what solubizes it (it being soluble in HCl). I wasn't sure if "trace amounts" meant something like 5g per liter, or 0.1g per liter.

In general, trace concentrations are barely detectable. Technically everything is soluble in "trace amounts" but these can, for almost all purposes, be ignored. While I cannot find an actual number for the solubility of PdCl2, you can probably infer that it would behave similarly to its neighboring noble metal chlorides, rhodium chloride and silver chloride, which are both virtually insoluble.
Quote: Originally posted by FireLion3  

It's usually supplemented with chlorine gas? Dissolving the palladium was pretty easy, I just left it in a bottle with aqua regia for a few days. What do you mean?


Dissolution in aqua regia alone certainly works, but takes extended periods of time much like dissolving platinum. However the process can be accelerated by piping chlorine gas into the solution as an oxidizing source of chloride ions. Given your apparent aversion to using toxic gases, though, I don't imagine you'd want to try it anyway.

FireLion3 - 21-10-2015 at 19:25

I did some googling and found one source say that it's not exclusive HCl that will cause palladium to dissolve, but the Cl- ion in general.. meaning adding Sodium Carbonate will result in Sodium Chloride, which could induce solubility? I imagine separating that would be quite a pain.

Has anyone ever experienced this with Palladium salts?

Upsilon - 21-10-2015 at 19:40

The Cl- ion is in general a poor oxidizer (despite the fact that Cl2 is a very strong oxidizer). When HCl dissolves common metals, it is a simple single replacement reaction between hydrogen and the reactive metal (it is not a redox reaction). Aqua regia works on noble metals because a redox reaction occurs with the nitric acid (nitric acid is a strong oxidizer which does the dirty work of oxidizing the noble metal to the noble metal cation), while the hydrochloric acid is merely present to provide a constant stream of Cl- ions for the dissolved noble metal ions to bind with (nitrates do not form because they are reduced into nitrogen oxides which exit the solution as a gas).

Adding sodium carbonate to your mixture would form sodium chloride and sodium nitrate, but these will have no effect on the solubility of the PdCl2.

[Edited on 22-10-2015 by Upsilon]

gdflp - 22-10-2015 at 06:42

Quote: Originally posted by Upsilon  
The Cl- ion is in general a poor oxidizer (despite the fact that Cl2 is a very strong oxidizer). When HCl dissolves common metals, it is a simple single replacement reaction between hydrogen and the reactive metal (it is not a redox reaction). Aqua regia works on noble metals because a redox reaction occurs with the nitric acid (nitric acid is a strong oxidizer which does the dirty work of oxidizing the noble metal to the noble metal cation), while the hydrochloric acid is merely present to provide a constant stream of Cl- ions for the dissolved noble metal ions to bind with (nitrates do not form because they are reduced into nitrogen oxides which exit the solution as a gas).

Adding sodium carbonate to your mixture would form sodium chloride and sodium nitrate, but these will have no effect on the solubility of the PdCl2.

That's just plain wrong. Sodium chloride does indeed complex with palladium chloride to form the water soluble sodium tetrachloropalladate(II), which is water soluble. In general, yes chloride ions do improve the solubility of palladium chloride by complexing with the palladium to form tetrachloropalladate(II) ions. And that has nothing to do with the oxidizing potential of chlorine or chloride.

During the dissolution of noble metals such as platinum and gold, not palladium, the nitric acid does indeed oxidize their surface. But the reason that it is necessary for chloride ions to be present is not the the nitrate is being reduced to nitrogen dioxide. It is that the metal ions produced will not dissolve without chloride ions coordinating to them to increase their solubility. Palladium is not one of these metals, and will dissolve in hot nitric acid alone if given enough time.

And dissolution of palladium in aqua regia is normally supplemented with chlorine gas? That's pointless as refluxing with aqua regia dissolves moderate amounts of palladium in several hours. I'm not even sure adding chlorine gas would help the dissolution rate.

My 2003 CRC lists palladium(II) chloride as soluble in water, but unfortunately it doesn't give a number. This means that it has a non-negligible solubility and I would trust that book over Wikipedia any day. Adding chloride ions will markedly improve the solubility.

@FireLion3
What is your goal? Do you want palladium metal again, or do you want to isolate palladium chloride? If the latter, unfortunately you need to reflux the solution with hydrochloric acid to remove any remaining nitric acid, then evaporate to dryness, preferably thermally assisted. Unfortunately, I can't see anyway around this. I wouldn't neutralize the solution at all with sodium carbonate as this will introduce a number of ions. As for your suckback issues during distillation, do they really matter? If the neutralization solution gets sucked back into the receiving flask it will neutralize the acid in there, but it won't contaminate the palladium solution. I would just refill the scrubber if that happened and continue on, emptying the receiving flask if necessary.

FireLion3 - 22-10-2015 at 16:14

@gdflp

Back suction of the neutralizing solution would introduce the carbonate resulting in sodium chloride in my palladium solution, which as you said is something that should be avoided.

Edit:

Oh I see what you mean. I think I'll have to modify my setup a bit, but I think I can get it done.

[Edited on 23-10-2015 by FireLion3]

gdflp - 22-10-2015 at 17:13

Yes, if you use a simple distillation setup, then connect the scrubber to the vacuum take-off adapter; then the palladium solution can be isolated from any suckback issues.