Purification of the crude Platinum from Dental Alloys

[Lou]

1.) Be careful, working this way. Hexachloroplatinates(IV) are potent allergens. Thus avoid every direct contact with these materials, be it in solid form or dissolved.
2.) This fast reaction indicates to me, that your ammonium salt still is quite impure. Very pure ammonium-hexachloroplatinate often reacts with formiate-solution only after a long induction period, where apparently nothing happens. This induction period can last as long as 30 minutes, until the mixture abruptly foams up and becomes black. Only after this complete reduction of the very first small portion, you can continue adding platinum salt like stated above, without risk of nearly explosive overfoaming, leading to platinum-losses.

Point one is very important. Many people foolishly think that noble metals are quite safe. Wrong! Gold, platinum, and rhodium salts are all quite poisonous! Platinum is every bit as potent an allergen as nickel or cobalt.

Point 2 means that it was contaminated with palladium. I have noticed that even slight Pd contamination of the platinum will cause a much more rapid reduction. You are very correct in asserting that high purity ammonium hexachloroplatinate has an induction period before reduction. Depending on the heat, some times it is possible to get a sandy material not unlike that from hydrazine and its salts.

 

[garage chemist]

Good to know that contact with Platinum salts should be avoided. I've always been handling hexachloroplatinates without any special precautions, sometimes getting a little bit of solution or precipitate on my fingers, or maybe breathing in a tiny amount of aerosol from fizzing solutions. I should probably wear gloves from now on, though I never noticed any symptoms.
I knew about the toxicity of Cisplatin, the cytotoxic chemotherapeutic that makes all your hair fall out at doses of around 50mg, but didn't think that simple hexachloroplatinates had significant adverse effects.

"The solution was heated to boiling briefly to expel chlorine and destroy residual H2O2. Overnight, an extremely small amount of fine black powder settled down, too little to isolate or weigh, and was filtered off."

If Ir is present in low concentrations, cristallization of sparingly soluble ammonium-hexachloro-iridate(IV) is very slow, forming tiny, nearly black cristals.

There was no ammonium chloride in the solution at this point, this was the solution obtained from the Pt sponge and HCl/H2O2. Any insoluble black powder must have been metallic Ir or Rh.

Interesting point about the induction period of the formate reduction, freechemist and Lou!
I am almost sure that Pd is the only remaining principal impurity in my Pt, since the salt is nice and yellow, and Ir and Rh were present only in very small amounts that should have been removed by the three redissolutions and precipitations that my Pt has gone through.
For now, I'll reduce my yellow Pt precipitate again with ammonium formate and keep it that way. I can possibly soon get it tested by a friend (though he can't measure any other PGMs than Pt, Pd and Rh).

I could also use some DMG to test my Pt for Pd content. I was planning to synthesize some myself and I have the chemicals for that, but it's quite a lot of work and I don't have the time for that at the moment.

Again, my important question: Are there any other reducing agents than ethanol (which is only mentioned by Hoke) that selectively reduce Pd(IV) to Pd(II) and leave Pt(IV) as it is? If yes, which?
Since I always use SO2 for the gold precipitation, reoxidation of the Pt(II) with chlorine is absolutely necessary after this, and this also oxidises the Pd to Pd(IV). Even prolonged boiling only reduces a part of the Pd(IV), leading to the very poor separation of Pt and Pd that is characteristic of my process.

The brown ammonium hexachloropalladate is a mystery to me. People have written about the different colors of ammonium hexachloroplatinate when contaminated by other PGMs, but I find almost no information about the colors that the hexachloropalladate can assume when contaminated.

 

[HAuCl4]

Would most of these difficulties be bypassed by just separating the blacks into the "lower" and the "upper" with hot sulphuric acid, leaving Ag, Pd, Rh in solution, and Au, Pt, Ir in solid form?.

Not a perfect separation, but simple and expedite. The problem would be greatly simplified, in my eyes (enthusiast/hobbyist eyes), with less handling of mixed salts. :?:

If you make DMG at the home lab, please post the procedure!. I'm sure will be reading it. Cheers. 8)

I think DMG is best left for analysis and/or scavenging Pd, and not only because of costs.

 

[Harold_V]

Again, my important question: Are there any other reducing agents than ethanol (which is only mentioned by Hoke) that selectively reduce Pd(IV) to Pd(II) and leave Pt(IV) as it is? If yes, which?

My knowledge of chemistry leaves a great deal to be desired. So much so that I tend to stay out of conversations such as this. However, in the back of my head I keep thinking that I had read that palladium oxide is soluble in HCl, whereas palladium is not. So then, assuming it is, would a wash in HCl remove the oxidized palladium to any degree? Might not be the solution you seek, but once back in solution, it would be easy to recover. (Let me know if I'd be better served to listen instead of talk. I won't be offended!)

Harold

 

[HAuCl4]

For now, I'll reduce my yellow Pt precipitate again with ammonium formate and keep it that way. I can possibly soon get it tested by a friend (though he can't measure any other PGMs than Pt, Pd and Rh).

After you do this, you can put the blacks in sulphuric acid. That will dissolve the Pd. The powder left is nearly pure platinum, that you can dissolve separately in AR. After you do this, a pinch of DMG will get all the remaining Pd contamination out of the Pt solution, then ammonium formate will get you nearly pure Pt.(If you want also you can do the hydrolysis here to really get all the contaminants).

The Pd sulphate solution, you can cement it with zinc, then dissolve the Pd powder in AR, and do the ammonia/HCL flipflop iteratively to get nearly pure Pd salt, filtering out any Ag, Pt, etc salts from there, then ammonium formate it to nearly pure Pd.

A strong magnet helps settle the blacks faster when applied to the outside of the beaker. 8)

All of this sounds like a lot of work, but the cost of reagents is kept low.

 

[Lou]

Whether or not PdO is soluble in any acid is a function of how hot it was heated and for how long. It can be "dead burnt" to the point where it won't dissolve expeditiously.

Typically HBr and HI (stronger and softer acids) are used for PdO.

 

[freechemist]

Hi Lou, you wrote:

"Depending on the heat, some times it is possible to get a sandy material not unlike that from hydrazine and its salts."

This meets my own observations. To me the formation of a much denser coarse and grey, essentially metallic material is mainly dependent on formiate concentration, and thus on the pH of the reduction mixture. In a constantly buffered medium, e.g. at pH 2, about 1% only of the total formic acid added exists as formiate anion, leading to much better controlled reaction and finally to a heavy grey metallic precipitate, which can be melted by induction directly after drying, in contrast to Pt-black formed by reduction with ammonium-formiate.

To garage chemist:

"I could also use some DMG to test my Pt for Pd content. I was planning to synthesize some myself and I have the chemicals for that, but it's quite a lot of work and I don't have the time for that at the moment."

Dimethylglyoxime is not as expensive, as you may think. From Omikron as an example you can get 10 grams for EUR 11,25, which will cost you much less, than synthesizing it yourself. If you do a Google-search, entering "Dimethylglyoxim 10 g" (or a different weight) you certainly will find other offers.

"Again, my important question: Are there any other reducing agents than ethanol (which is only mentioned by Hoke) that selectively reduce Pd(IV) to Pd(II) and leave Pt(IV) as it is? If yes, which?
Since I always use SO2 for the gold precipitation, reoxidation of the Pt(II) with chlorine is absolutely necessary after this, and this also oxidises the Pd to Pd(IV). Even prolonged boiling only reduces a part of the Pd(IV), leading to the very poor separation of Pt and Pd that is characteristic of my process."

I do not know any selective reducing agent for Pd(IV) in HCl-solution, which does not affect Pt(IV), too. But probably you will be more successful, using H2O2 in place of Cl2, to reoxidize Pt(II) to Pt(IV) without affecting much Pd(II). If you hold the free HCl-concentration not much higher than about 1 mole per liter, it can be done.

"The brown ammonium hexachloropalladate is a mystery to me."

Are you really sure about (NH4)2PdCl6? - (NH4)2PdCl4 is not very well soluble in water, especially at lower temperatures. As a solid it consists of deep brown needle-like cristals, being able to grow as long as 1 cm, if cristallizing slowly.

Regards, freechemist