Precipitating palladium help needed

[kurtak]

Here I added Zn powder to a pH = 1 solution with Pd2+ present. As you can see not a lot is happening. Any clue what I'm doing wrong?

You did not add enough zinc

In order for the cementation process to work the "free acid" that is capable of dissolving metal needs to be used up before the metal higher on the reactive series of metals can do it's job of cementing out the metal(s) lower on the reactive series of metals

Cementation is a redox reaction wherein metals higher on the reactive series of metals are oxidized (dissolved) thereby allowing the metals lower on the reactive series of metals to be reduced

But - that redox (or ion exchange) will not take place until the "free acid" - capable of oxidizing (dissolving) the lower reactive metal is "loaded" with those lower reactive metal ions

here is the reactive series of metals ------------

https://en.wikipedia.org/wiki/Reactivity_series

Kurt

 

[niels-chemist]

You did not add enough zinc

In order for the cementation process to work the "free acid" that is capable of dissolving metal needs to be used up before the metal higher on the reactive series of metals can do it's job of cementing out the metal(s) lower on the reactive series of metals

Cementation is a redox reaction wherein metals higher on the reactive series of metals are oxidized (dissolved) thereby allowing the metals lower on the reactive series of metals to be reduced

But - that redox (or ion exchange) will not take place until the "free acid" - capable of oxidizing (dissolving) the lower reactive metal is "loaded" with those lower reactive metal ions

here is the reactive series of metals ------------

https://en.wikipedia.org/wiki/Reactivity_series

Kurt

I was already wondering why it would work because the Zn will first quickly react with H+ before it reacts with Pd2+. That would suggest that starting at pH = 3 would require less Zn for it to work than at pH = 1?

 

[anachronism]

I was already wondering why it would work because the Zn will first quickly react with H+ before it reacts with Pd2+. That would suggest that starting at pH = 3 would require less Zn for it to work than at pH = 1?

Seems to me that you're looking too hard at theoretical chemistry whilst ignoring practical chemistry.

 

[Yggdrasil]

I was already wondering why it would work because the Zn will first quickly react with H+ before it reacts with Pd2+. That would suggest that starting at pH = 3 would require less Zn for it to work than at pH = 1?

Just to emphasize what Jon said.
Stochiometrically/Theoretically speaking one need 1 ml of Nitric to dissolve 1 gram of Gold in Aqua Regia.
In practical terms it can take everything from 0.5 to 2 or more ml of Nitric to get it done.
It all depends on the conditions.

 

[niels-chemist]

Just to emphasize what Jon said.
Stochiometrically/Theoretically speaking one need 1 ml of Nitric to dissolve 1 gram of Gold in Aqua Regia.
In practical terms it can take everything from 0.5 to 2 or more ml of Nitric to get it done.
It all depends on the conditions.

Yes of course, but there is still theory there ;-) If you first react away your acid with the Zn and you add more Zn you effectively add Zn to a (somewhat) neutral solution. Zn is not soluble around pH = 7, so it will react with Pd2+ instead of with H+. That means that it is a surface reaction between Zn surface and Pd2+?
What does it look in practice? Do you end up with a mixture of Zn(0) and Pd(0) that you have to clean with acid?

And how does this look with copper? I ordered 100g of pure copper, 300 mesh.

 

[Yggdrasil]

Yes of course, but there is still theory there ;-) If you first react away your acid with the Zn and you add more Zn you effectively add Zn to a (somewhat) neutral solution. Zn is not soluble around pH = 7, so it will react with Pd2+ instead of with H+. That means that it is a surface reaction between Zn surface and Pd2+?
What does it look in practice? Do you end up with a mixture of Zn(0) and Pd(0) that you have to clean with acid?

And how does this look with copper? I ordered 100g of pure copper, 300 mesh.

It is an ion exchange process, Metal (0) goes into solution as Pd ions picks up the extra electron and becomes Pd(0)
It can not be neutral since there is nothing to drive the reaction then.
It need to be either low pH or high pH, as in Cementing Gold Cyanide on either Zn or Al.

What Jon describes was slightly simplified, since there is more then one thing going on at the same time.
As Zn/Cu/Al/?? will be dissolving there will be "competition" for the extra ions and some will just output H₂ as some contribute with electrons to the Pd/Pt/Au/?? exchange.

At least that is how I see it.

 

[Yggdrasil]

Yes of course, but there is still theory there ;-) If you first react away your acid with the Zn and you add more Zn you effectively add Zn to a (somewhat) neutral solution. Zn is not soluble around pH = 7, so it will react with Pd2+ instead of with H+. That means that it is a surface reaction between Zn surface and Pd2+?
What does it look in practice? Do you end up with a mixture of Zn(0) and Pd(0) that you have to clean with acid?

And how does this look with copper? I ordered 100g of pure copper, 300 mesh.

Copper is better but needs vigorous stirring.
Copper will only drop PMs plus Mercury and Tungsten which will not be in solution here.

 

[kurtak]

I was already wondering why it would work because the Zn will first quickly react with H+ before it reacts with Pd2+. That would suggest that starting at pH = 3 would require less Zn for it to work than at pH = 1?

You "kind of" answered this in your first post this thread

For unknown reasons I haven't succeeded in precipitating Pd(0) from my waste solutions. They contain a bunch of sodium salts (from adding sodium carbonate and hydroxide)

In other words - in precipitation chemistry - when you raise the Ph you "may" (or not) cause the precipitation of unwanted carbonates/hydroxides

That said - you are NOT going to achieve the 5N+ Pd you are looking for through "standard" precipitation chemistry using standard "lab grade" chemicals &/or in a standard refining lab set up

At best - with such - achieving 4Ns is difficult & that is especially true with the PGMs

Though 4N can be achieved though precipitation chemistry (if "great" care is taken) 4N is generally achived through electrolysis (electrolytic cells)

Going from 4Ns to 5Ns (or greater) EVERYTHING in the process has to be ULTRA clean - from the air in the lab to the reaction vessels to the reagents

Here are some old threads I looked up concerning refining ultra pure metals (5Ns+) & please note what Lou has posted concerning producing ultra pure metals

https://goldrefiningforum.com/threads/selling-ultra-pure-metals.21183/#post-218337

Cleanliness is critical when you take a 60+ element look at something. The material is melted, doesn't touch a mold, and isn't handled with one's hands or manipulated in a dusty atmosphere.

https://goldrefiningforum.com/threads/method-for-ultra-high-purity-gold.3079/

Thread by Lou

https://goldrefiningforum.com/threads/de-ionized-versus-ro-water.1289/#post-10798

If you're going after anything above 99.99% 4N or 5N5 (puratronic and about 3X or more the spot price from Alfa)

A procedure I use for consistent 99.995% gold (though it usually is 5 9s) is for the digestion in quartz using ACS grade chemicals, then precipitation with Ultra High Purity SO2, I think the cylinder is from Matheson. That gold is redissolved in PTFE beakers using Spectra-grade acids (IIRC, I think it's ppb levels of 32 metals), boiled, concentrated, diluted with ~18.7MΩ UHP water, then precipitated with Mallinckrodt ACS trace metal oxalic acid. If further purity is required, the gold can be grown into crystals by decomposing the AuCl3 formed by taking that already pure gold and reacting it with electrolytically produced chlorine that has been treated to remove impurities. That gold is then melted in either quartz or glassy carbon, in vacuo or sold as the crystalline form in which it deposits. It's a bother to do but you can get some very nice crystals from it.

EDIT: Should also mention that all of the glassware and utensils used are either tefzel/teflon or quartz and they all get a boiling in red fuming nitric, aqua regia, and nitric-perchloric mix. Also should say that zone refining is cool stuff, it's usually done with induction heating...I have a few zone refined pieces that I will have to find and take pictures of; if I can't find mine, I'll ask my friend if he's send me a few photos of his collection.

For 99.9% gold, I don't think tap water would make a hill of beans difference.

I've been meaning to show the difference between AuCl3 and HAuCl4* here on this board for a couple weeks. I see people use AuCl3 a lot and I occasionally get confused by it thinking they mean gold (III) chloride when they really mean HAuCl4, tetrachloroauric acid. I'm going to try and get these photos of my digital camera.

D'Oh edit again: those solutions below are really darn concentrated and are probably carrying a couple grams of gold each.


Lou

Kurt

 

[niels-chemist]

Copper is better but needs vigorous stirring.
Copper will only drop PMs plus Mercury and Tungsten which will not be in solution here.

So cementing with either Zn or Cu will work under acidic conditions at room temperature? What pH do you consider sufficiently acidic?

 

[kurtak]

So cementing with either Zn or Cu will work under acidic conditions at room temperature? What pH do you consider sufficiently acidic?

I have always done it Ph 0 - 1

Read this thread it may help you -----------

https://goldrefiningforum.com/threads/fuzz-button-interconnects-need-some-advice.22203/#p232030

Kurt

 

[anachronism]

Copper is better but needs vigorous stirring.
Copper will only drop PMs plus Mercury and Tungsten which will not be in solution here.

Ygg, not to mention the fact that you would be introducing another metal into the process, and that's counterproductive.

 

[Yggdrasil]

Ygg, not to mention the fact that you would be introducing another metal into the process, and that's counterproductive.

That is correct, so in my eyes the Formic acid approach will probably be best here.

 

[niels-chemist]

That is correct, so in my eyes the Formic acid approach will probably be best here.

I'm clueless why it doesn't work. I synthesized an organic palladium complex that looks good. Based on the yield I have a flask left with several hundred mg of palladium (at least) and I'm trying to convert that back into Pd(0). Dissolving the solids into 36% HCl, increasing the pH to 3 and then adding formic acid should definitely do the job, correct?

 

[anachronism]

Apologies the thread is getting somewhat longwinded but can you remind me of the pH you were using with the Sodium Formate?

 

[Yggdrasil]

Apologies the thread is getting somewhat longwinded but can you remind me of the pH you were using with the Sodium Formate?

He said 3 in the post over yours

 

[anachronism]

Ive had a long think about this. I can see why Neils is wanting to use Formate.

He said 3 in the post over yours

Thanks if he's reducing with Sodium Formate/Formic acid he should be hitting closer to pH 6.

Edit: What I don't get is why use AR. It would be much simpler to use Nitric- drop as Hydroxide - dissolve in HCl then formate reduction.

 

[anachronism]

This is how it should look when finished. Another point to bear in mind Neils is that if you're trying to do this with under a gramme of Pd then your losses when trying this will be a lot greater and harder to spot. Excuse the leaf... it snuck in there.

 

[niels-chemist]

For now I'm working on small scale. I'm doing tests in vials with only a few mg of Pd in there, just to see if it works. I have a reaction mixture residue of which I took a tiny bit of solid and dissolved it in 36% HCl. Upon addition of 50% NaOH I got a white precipitate and tried adding formic acid: no result. I heated it: no result.

Then I added sodium borohydride and *instantly* got black precipitate. This is proof there id Pd in there. Then why on earth is formic acid not working?

And why HNO3 instead of HCl? Well, HNO3 is a lot more toxic and expensive than HCl, and if you precipitate the hydroxide it doesn't matter if you add NaOH to AR or to HNO3?

 

[anachronism]

For now I'm working on small scale. I'm doing tests in vials with only a few mg of Pd in there, just to see if it works. I have a reaction mixture residue of which I took a tiny bit of solid and dissolved it in 36% HCl. Upon addition of 50% NaOH I got a white precipitate and tried adding formic acid: no result. I heated it: no result.

Then I added sodium borohydride and *instantly* got black precipitate. This is proof there id Pd in there. Then why on earth is formic acid not working?

And why HNO3 instead of HCl? Well, HNO3 is a lot more toxic and expensive than HCl, and if you precipitate the hydroxide it doesn't matter if you add NaOH to AR or to HNO3?

The simple answer to that is that most Pd that's being dissolved doesn't come in fine powder form in the "real world" and HCl is limited to fine powders hence Nitric is used. That given you're already using AR so just using the Nitric doesn't change much at all.

 

[niels-chemist]

The simple answer to that is that most Pd that's being dissolved doesn't come in fine powder form in the "real world" and HCl is limited to fine powders hence Nitric is used. That given you're already using AR so just using the Nitric doesn't change much at all.

Yes it does. 5% nitric acid in 38% HCl is a lot cheaper than pure 63% nitric acid. I agree that it's all cheap relative to Pd, but still...
I have tried using 63% HNO3 in the past but it is much slower to dissolve Pd sheet than AR...