# Purification of the crude Platinum from Dental Alloys



## garage chemist (Oct 3, 2011)

Hi Everyone, 

I have refined dental gold alloys several times in the past, my method has been translated from German and posted here as "Stefans Dental Gold experiment". My username is "garage chemist" on the Sciencemadness board as well. I decided to post my experience here since there seem to be many members with knowledge and experience in PGM separations here. 
In these separations I have always precipitated gold from the denoxed AR solution with SO2, then reoxidised the PGMs with chlorine, boiled to reduce Pd(IV) back to Pd(II), precipitated Pt with excess ammonium chloride and finally Pd with chlorine, always storing the final solutions due to their possible residual PGM content. 
Many different alloys were refined this way, and I know for a fact that these generally contain not only Pt and Pd (both at 4-8%), but also Ir, Rh and Ru in amounts of up to 1% each, which is now causing problems in the final purification of the platinum.

Now I have combined all the ammonium hexachloroplatinate precipitates, which had varying shades of color ranging from muddy yellow over brown to greenish, calcined them in a quartz dish to crude platinum sponge, washed this well with hot aqueous HCl and, after drying and weighing, redissolved it in AR. There was a vigorous initial reaction with the first portion of AR, a much less vigorous reaction with the later portions, and finally there remained a small amount of black powder (perhaps 2-5% of the initial weight) that was entirely insoluble in boiling AR. 
The solutions were denoxed by repeated evaporation with HCl (all of this done in a distillation setup with additional absorption of HCl gas to avoid corroding the fume hood fan more than necessary), filtered through a porosity 4 glass frit, and precipitated with an excess of NH4Cl solution (calculated such that the solution contains about 1 mol/L excess NH4Cl after precipitation). The muddy yellow precipitate was immediately filtered at the pump through a Por. 4 glass frit and washed with cold aqueous NH4Cl solution. 

Now the unexpected thing is the deep brown coloration of the filtrate, which looks a lot like Pd(II) to me. I filtered immediately to avoid contaminating the Pt with the time-delayed precipitation of iridium, but now after 2 hours, absolutely nothing is precipitating in the deep brown filtrate. How could so much Pd get into my platinum precipitates?
I have attached pictures of the Pt precipitate on the glass filter, the black insoluble powder and the dark brown filtrate in the round-bottom flask. 

The platinum precipitate is currently drying in an oven at 120 degrees centigrade. 

I have a book that says that the dirty yellow coloration of the platinum salt indicates contamination with rhodium, which generally co-precipitates with platinum in the NH4Cl precipitation. 
To remove rhodium, it is recommended to reduce the impure hexachloroplatinate in a stream of town gas and then repeatedly fuse the platinum sponge with fresh portions of potassium bisulfate at a dull red heat until the salt no longer takes on a yellow powder. If iridium has been previously removed by the lead fusion method, this procedure is said to give pure platinum. 

Now, another book (Hoke) says that iridium doesn't even dissolve in AR when dissolving dental alloys, and stays as a black powder with the silver chloride. I find this statement doubtful at least (insoluble residue from my AR dissolution of crude Pt sponge and brown coloration of some hexachloroplatinate precipitates), and the types of dental aloys have surely changed a lot since Hoke's book was written. 

Now I am pondering possible methods of proceeding with the purification of my platinum. 
I am planning to remove rhodium from the Pt sponge with the bisulfate fusion. Does it make a difference in Rh extractability whether the Pt sponge was produced by calcining, calcining in a reducing atmosphere, or by zinc reduction? 

Concerning Ir removal: Is there a difference in solubility of the Ir in AR when the Pt sponge is produced by calcination or reduction with zinc? How much of the Ir and how much of the Pt is left behind in the insoluble black powder when calcined Pt sponge is redissolved in AR? 

My current plan is: calcine the hexachloroplatinate, extract Rh by bisulfate fusion, redissolve in AR (noting amount of insoluble residue), denox and reprecipitate with NH4Cl, noting color of precipitate and filtrate. Anything wrong with that?

Thanks for all educated answers and suggestions!


----------



## lazersteve (Oct 3, 2011)

Welcome Stefan.

A one primary question:

1. What are the results of stannous tests on the powder and liquid?

On the separation of Ir and Rh from Pt. Try covering the crude Pt sponge in 32% HCl and adding 35% H2O2 a few ml at a time until nothing else dissolves. Your should achieve an orange solution which precipitates nearly pure Pt with a saturated KCl solution at 10C or less.

Check out my website for a video of the precipitation.

Steve


----------



## freechemist (Oct 3, 2011)

In addition to lazersteves question: Did you test the brown, filtered solution with dimethylglyoxime? - If there is no Pd, judging from it's colour, it may be Ir and/or Ru, but that is only speculation. 

freechemist


----------



## garage chemist (Oct 3, 2011)

I do not have any DMG. I would have normally tested this solution by gassing with chlorine or adding sodium chlorate while hot- the voluminous red precipitate would have proven the presence of palladium. 

I am preparing some stannous chloride solution right now to test powder and solution, but I have no reason to doubt that the yellow powder is ammonium hexachloroplatinate. 

The yellow precipitate is now dry, I'll post a picture once I have made the stannous tests.


----------



## garage chemist (Oct 3, 2011)

Here is the dried yellow precipitate, it has turned brownish on the surface but becomes a yellow powder once crushed. 



Stannous tests, left hand: the brown filtrate (very deep blue/green color with stannous, almost black), right hand: the yellow precipitate, clear brown color, positive for platinum. 



I also swabbed some of my dichlorodiammine palladium with a stannous chloride soaked q-tip and oddly got no reaction. I take it that this is due to the insolubility of the compound?


----------



## Lou (Oct 3, 2011)

> I also swabbed some of my dichlorodiammine palladium with a stannous chloride soaked q-tip and oddly got no reaction. I take it that this is due to the insolubility of the compound?



It should still test.

If your ammonium hexachloroplatinate shows the slightest green coloration, it is likely due to rhodium contamination and it is brownish/burnt ochre if it is from iridium or ruthenium contamination.
Iridium is more prone to co-dissolve than ruthenium. You should be able to get a clear to slight yellow solution in concentrated ammonia. KCl gives a pure platinum precipitate at the cost of more platinum remaining in solution. 

I would not recommend the bisulfate fusion unless you have reason to believe it is the principal component. Hot 70% sulfuric acid will readily put the rhodium powder into solution.


----------



## garage chemist (Oct 3, 2011)

I boiled a bit of my dichlorodiammine palladium with HCl to dissolve it, and the resulting light brown solution now gives the same dark blue/green color with stannous as the brown filtrate. The plain dichlorodiammine palladium simply does not give a positive test with stannous, even after several hours. 

So it is confirmed that my crude platinum precipitates contained a lot of palladium, which is now in the filtrate. I will boil it down somehwat, cool to recover some more platinum, and then gas with chlorine to get the palladium.
The ammonium hexachloroplatinate has no hint of green color, but it is a "dirty" yellow, not canary yellow as in Steve's videos and pictures. "Burnt ochre" describes the shade of color quite well. 
What do you mean by solution in concentrated ammonia? Should I try to dissolve some of the hexachloroplatinate in ammonia? 

Steve, so when I dissolve the Pt sponge from calcination of the ammonium salt in HCl/H2O2 (at which temperature? hot or cold?) most of the Ir and Ru is left behind? Any Pd that is still contained in the Pt will co-dissolve, but will partially be reduced upon boiling of the solution and reveal its presence due to the color of the filtrate after precipitation of the Pt and dark blue/green color of the stannous test, I think.


----------



## Lou (Oct 3, 2011)

Stefan,

The very purest ammonium hexachloroplatinate is a very very light yellow (9995+ metals basis), so pale. Canary yellow indicates 99%+. The separation of platinum and palladium via the sodium chlorate/chlorine gas trick is not quantitative. Firstly, ammonium hexachloroplatinate is not nearly as insoluble as say AgCl, although having a 1.5 molar excess of NH4Cl precipitant will suppress its solubility. There will always remain a positive stannous result for Pt in the filtrate, but never less than about 40 ppm by ICPAES. I get that Pt via ion exchange or Zn cementation. The Pd is showing up in your ammonium hexachloroplatinate because 1.) always some Pd solution will get encapsulated onto the (NH4)2PtCl6, 2.) unless one is very careful there is always free chlorine present such that some Pd will co-precipitate. Unfortunately, there exists no reductant for Pt that will not reduce Pd as well. 

The use of hydrochloric acid and hydrogen peroxide is a very well known and old method. I've used it for the last five years when I didn't want alkali contamination from sodium chlorate (it's convenient to simply boil to a syrup).
Seeing Steve use it has shown it to be very useful for parting rhodium and iridium from platinum with excellent selectivity--I really only used it for dissolving relatively pure metals so I was surprised. We, like the other PGM refiners now use Cl2 from the cylinder because it's cheaper than peroxide. I think it works the best on the worse the quality of Pt with more Ir and other PGM contamination. It is not as fast as 80-85*C aqua regia and suffers from poor reagent economy because as it heats up, the chlorine exits the solution. Works a treat in a FEP autoclave though! Room temperature works, but increasing temperature increases attack rate, decreases selectivity, and decreases purity

By solution in ammonia, I mean if completely pure, it will dissolve in ammonia which can then be filtered and treated with hydrazine sulfate to yield pure platinum. Platinum is extremely stable to base hydrolysis unlike the other platinum group metals (although this is ligand dependent, particularly for Pd). 

If you have extensive Pd contamination with rhodium present in your Pt sponge, then consider boiling in 30% nitric acid (yellow then dark yellow-brown) and then boiling in 80% sulfuric after the nitric is removed (this gets the Rh, the solution will go a dark brown). The nitric solution can be re-used multiple times. The Pd is then dropped with DMG/DMSO. After that, then re-dissolve in acid peroxide, boil to remove Cl2.


----------



## garage chemist (Oct 3, 2011)

Thanks Lou, that was quite helpful. So I will try dissolving my Pt sponge in cool or moderately warm HCl/H2O2 and see how the color of the Pt precipitate and filtrate changes after this. 

In Hoke, I read about the use of alcohol as a selective reductant for Pd(IV) to Pd(II) that supposedly leaves Pt(IV) untouched. If the filtrate of the next Pt precipitation still contains appreciable amounts of Pd I might try this. 

How does ammonium hexachloroplatinate react with aqueous ammonia? Does acidifying the solution precipitate a platinum compound? If so, then the purification of Pd via dissolution in ammonia and reprecipitation as Pd-dichlorodiammine would not remove any platinum. I was under the impression that Pt either remains undissolved in ammonia (I think I even read that somewhere) or does not reprecipitate upon acidifcation. 

Initially I considered trying the bromate hydrolysis for purification of my platinum, but if the HCl/H2O2 method turns out to do the job almost as well, even better. Thanks for this advice!


----------



## samuel-a (Oct 3, 2011)

Lou said:


> Unfortunately, there exists no reductant for Pt that will not reduce Pd as well.



Maybe in this case it worth considering to work the other way around ?
Precipitating the Pd from hot solution with DMG, thereby removing the Pd from the equation.

I have read in some patent (sorry, no refference. i can't remember where), that Pt(DMG)2 could be later selectively precipitated (blue ppt) by boiling acidic solution of Pt(IV) for several hours under reflux.

If i'm mistaken, flease forgive me.
In any case, i think it will be wise if you (garage chemist) add DMG to your testing repertuar.


----------



## Lou (Oct 3, 2011)

garage chemist said:


> Thanks Lou, that was quite helpful. So I will try dissolving my Pt sponge in cool or moderately warm HCl/H2O2 and see how the color of the Pt precipitate and filtrate changes after this.
> 
> In Hoke, I read about the use of alcohol as a selective reductant for Pd(IV) to Pd(II) that supposedly leaves Pt(IV) untouched. If the filtrate of the next Pt precipitation still contains appreciable amounts of Pd I might try this.
> 
> ...



You ask good questions, my friend. 
The Pd(II) to [Pd(NH3)2Cl2] (upon reacidification) transformation is not terribly effective in removing platinum if it is in great quantity. Otherwise, it works fine because the solubility of the platinum remaining is many orders of magnitude higher than that of the relatively insoluble Pd. It's a very loss-ridden procedure. I try to avoid it unless the Pd is quite pure, and even then, some Pd is left in the raffinate. It's best to remove the Pd from the Pt as early as possible to separate the feed streams. 

I can give you Gilchrist's procedure. What determines whether or not the Pd goes out as a hydrous oxide is the oxidation state. 4metals detailed the bromate hydrolysis procedure, and I have my procedure for 400 ozt lots of which I may share some details. There are several modifications that one may make, mostly in how one flocs the undesirables (temp control is important). 

See Brauer's collection, page 1571 for details on dissolving ammonium hexachloroplatinate by digestion in ammonia. This operation will give you [Pt(NH3)4]Cl2, Reisert's first chloride with excess ammonia and temperature (this salt may be driven out of solution by adding alcohol to yield clear crystals of the monohydrate). It proceeds via _cis_-[PtCl2(NH3)2], Peyrone's chloride. Not enough ammonia/long enough heat will give you Magnus's green salt. There aren't too many advantages to such high basicity of the solution, as most of the rhodium hydroxides/oxides are precipitated quickly before pH 9. 

Hoke is indeed right, you can use alcohol to take Pd(IV) --> Pd(II), but it must not be terribly basic, or some reduction to metal will occur. 


If you want pure palladium with relative ease, invest in dimethylglyoxime. It works very well for taking decent quality Pd to very high purity (4N+) material, if you can deal with the voluminous precipitate that is quite tenacious in sticking.


----------



## Harold_V (Oct 4, 2011)

garage chemist said:


> I will boil it down somehwat,


At the risk of offending, please do NOT use the term "boiling down" when you mean evaporate (that's a good way to lose values). You send the wrong message to these guys, who take your words literally.

Harold


----------



## Lou (Oct 4, 2011)

I'm 100% confident that he can boil it down without losing it: he's using a full reflux setup and he's a professional chemist (not merely a garage chemist).

But I do agree with the message, Harold, it's best not to boil any solution containing values because the money will go off as droplets all over the place!


----------



## garage chemist (Oct 4, 2011)

When I say "boil down" in the context of evaporating water or acid from solutions of value then I mean distilling it off in an all-glass distillation setup where I can test the distillate before pouring it away. Not that the distillates ever contained values when the solution didn't boil over, and if it boils over then I simply pour the distillate back and repeat the process. 

It's good that you mention this Harold, I was somewhat confused when I read here that you shouldn't boil the solutions- I had always denoxed my gold and PGM solutions by vigorous boiling and still do, and I even acquired a powerful Meker burner to hasten the process since denoxing would always take a full day with my old little bunsen burner. 
I am quite sure that any loss of values from openly boiled solutions is due to formation of fine droplets and aerosols from the bubbling. If you look closely you can see these fine droplets hitting the flask wall in the distillation setup. But generally, no values are found in the distillate. 

I know someone who concentrates his value solutions in a rotovap- probably the ideal method, but more expensive since an oil bath and lots of electricity is needed for for heating. I prefer to use propane burners whenever possible. 

I am not a professional chemist (I don't earn my money with chemistry) but I have taken "garage chemistry" quite far in the past.


----------



## Lou (Oct 4, 2011)

I thought you were already done and finished with your chemistry degree.

As far as I'm concerned, making oleum safely at home qualifies you as highly competent. Or ketene. Or a host of other dangerous experiments you conducted.


----------



## Harold_V (Oct 5, 2011)

garage chemist said:


> I am quite sure that any loss of values from openly boiled solutions is due to formation of fine droplets and aerosols from the bubbling. If you look closely you can see these fine droplets hitting the flask wall in the distillation setup.


Your assessment is right on the money. I had read (in Hoke) that one should not evaporate too quickly, but what does that mean? How would one describe "too quickly"?
I found out when I covered the side of my travel trailer with the droplets of which you speak, turning the trailer purple. Distinct spots were apparent where it wasn't concentrated. 

By the way, I spent an afternoon hand rubbing the trailer, to remove the staining. Lesson learned. 

Thanks to both you and Lou for explaining evaporation in ways that are not the norm for those of us that are not well versed in chemistry. I was not aware of the methods described. I expect that the vast majority of the home refiners won't have that kind of apparatus at their disposal, so they're well served to understand the need to evaporate without boiling. 

Harold


----------



## Lou (Oct 5, 2011)

Low and slow is definitely better to avoid physical losses.

Boiling is a very violent process.


----------



## garage chemist (Oct 9, 2011)

Update on my experiences: I tried calcining the platinum precipitate by slow heating in an electric oven (one with protected windings) but was greeted by dark smoke when I lifted the lid, so I immediately stopped this process and tried out the ammonium formate reduction instead. 
I prepared the slightly acidic ammonium formate solution, heated this to a boil and added the precipitate one spoonful at a time, always covering the beaker with a glass plate to avoid loss by spattering. Every portion of heaxchloroplatinate produced vigorous gas evolution, and the beaker contents turned pitch black. 
After everything had been added, I boiled the solution for one hour, with occasional stirring, and then let the platinum black settle. I washed it by repeated decantation, addition of water, stirring and settling, and slowly dried it at low heat on the hotplate. 
Then I heated it strongly in a crucible to turn it into sponge and render the Ir and Rh less soluble (I read this in Hoke), changing the color to grey. 

Redissolution was done with HCl/H2O2. I made the mistake of adding too much H2O2 at the beginning. The solution was turning red, but then violent fizzing started. I thought that the H2O2 had started to decompose, but the gas wasnt oxygen, it was chlorine! The atmosphere in the 1L beaker (covered with a glass plate) instantly turned green and I had to move the beaker into the fume hood. 
With the next portions of acid, I added the H2O2 gradually in small portions, and it worked just fine. This is a better dissolution method than AR- it requires no heating, releases no chlorine if done correctly, and no denoxing is necessary. 

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. 

Today the platinum was again precipitated with excess ammonium chloride and filtered by suction. This time the filtrate was only a light brown color and tested positive for both Pt and Pd. Gassing it with chlorine produced a rather small amount of red precipitate (Pd). 
The Pt precipitate didnt look much different from the previous time, perhaps somewhat lighter. Upon drying at 120°C, it again slightly browned on the surface of the clumps, but became a purely yellow powder when crushed. 

Then I gassed the (dark brown) filtrate from the previous Pt precipitation, and this time, a copius amount of very ugly brown precipitate was produced. This seems to be very dirty ammonium hexachloropalladate. Which contaminant coprecipitates with palladium upon gassing with chlorine, and gives it that nasty brown color? There is really no red color in this precipitate, just brown.


----------



## butcher (Oct 9, 2011)

Stefan (garage chemist),

I just wanted to say welcome, I have read a lot of things you wrote on another(science) forum, and feel you will like it here, it is good to have you here were we all can learn from each other.

I have almost no experience with purifying PGM, and am just a back yard refiner, so take that in the light of my thoughts:

seems to me you have recovered the platinum and Pd and possible Rh as fairly pure salts with some contamination of the others.

would washing these in a hydroxide solution, washing well to remove salt waters, maybe even a hydrogen peroxide rinse, and incinerating (to remove the previous salts of the acids in your metals and oxidizing the metals), (possibly calcining to blacks of the metals, here i am just guessing what they call it), then adding dilute nitric to dissolve Pd from PGM powders, wash powders, then re-dissolve the PGM (hopefully leaving behind the ones that do not go into aqua regia easily) then de-noxx solution concentrate and precipitate the platinum with NH4CL.

Basically, just re-refining your previously refined product. it seems to me each re-refining brings up purity considerably, and just like re-crystallizing salts we can get very close to pure.

without knowing how much other base metals (or silver)you may have in your starting stock, and being salts of previous acids, I think it may effect the outcome of further steps without the tools of driving off these acids in incineration.

I really feel like I could not offer any help as you are much more advanced in this field than me and with Lou helping you I am just speaking from the dark corner. Pardon me


----------



## freechemist (Oct 11, 2011)

Hi Stefan,

Having read your experiences update, I want to add a few comments.

_"I prepared the slightly acidic ammonium formate solution, heated this to a boil and added the precipitate one spoonful at a time, always covering the beaker with a glass plate to avoid loss by spattering. Every portion of heaxchloroplatinate produced vigorous gas evolution, and the beaker contents turned pitch black."_

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.

_"Redissolution was done with HCl/H2O2. I made the mistake of adding too much H2O2 at the beginning. The solution was turning red, but then violent fizzing started. I thought that the H2O2 had started to decompose, but the gas wasnt oxygen, it was chlorine! The atmosphere in the 1L beaker (covered with a glass plate) instantly turned green and I had to move the beaker into the fume hood."_

If both, HCl and H2O2 are concentrated enough, mixing them directly can lead to chlorine-evolution. It can happen by pouring together 32% HCl and 35% H2O2.

_"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.

_"Today the platinum was again precipitated with excess ammonium chloride and filtered by suction. This time the filtrate was only a light brown color and tested positive for both Pt and Pd. Gassing it with chlorine produced a rather small amount of red precipitate (Pd)."_

This would have been the moment, to precipitate Pd quantitatively with dimethylglyoxime, or to cement both together, to keep them as metals, to be worked up in a different batch.

_"Then I gassed the (dark brown) filtrate from the previous Pt precipitation, and this time, a copius amount of very ugly brown precipitate was produced."_

My speculation is, that Ir can lead to such discoloration.


Regards, freechemist


----------



## Lou (Oct 11, 2011)

freechemist said:


> 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 (Oct 12, 2011)

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 (Oct 12, 2011)

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 (Oct 13, 2011)

garage chemist said:


> 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 (Oct 13, 2011)

garage chemist said:


> 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 (Oct 13, 2011)

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 (Oct 19, 2011)

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


----------

