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BaFuxa
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[*] posted on 13-4-2018 at 09:15
Problem with aqua regia


Today I attempted dissolving some platinum in aqua regia.

It did not work : I used 27% HCL and 42.5% HNO3. I am fully aware that you have to use concentrated acids but I thought I could get away with the right stochiometry. Obviously not.

What is perplexing me is that I got the exact same deep yellow/gold color and Nox fumes that was observed in the procedure I was following. The platinum did not dissolve at all.

So, before I go further and concentrate my acids, is it really the concentration that is causing this to fail or is the "platinum" ? the seller sold it as pt but I bought that off ebay so it could be something else.



[Edited on 13-4-2018 by BaFuxa]




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[*] posted on 13-4-2018 at 10:34


Aqua Regia dissolves just about any metal, but it does take quite a while to completely dissolve Pt.



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[*] posted on 13-4-2018 at 10:47


I have to agree with Aga. Even thought it is pretty strong oxidizing agent it takes long time to dissolve bulk platinum metal.



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[*] posted on 13-4-2018 at 10:48


Hmmm... Nobody tried dissolving Pt using electrochemistry? While I am aware that it's resistant as anode because it probably forms layer of PtO2 which is conductive, in presence of some catalyst or reducing chemical or lower voltage it should work.
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[*] posted on 13-4-2018 at 11:33


Quote: Originally posted by RawWork  
Hmmm... Nobody tried dissolving Pt using electrochemistry? While I am aware that it's resistant as anode because it probably forms layer of PtO2 which is conductive, in presence of some catalyst or reducing chemical or lower voltage it should work.


it could be done but it's not as easy as just waiting for the Pt to dissolve in aqua regia.
i've just read that Pt has been electrolyzed in perfluoroalkylsulfonic acid or in an ionic liquid.
plain aqua regia is much more straightforward

[Edited on 13-4-2018 by Ubya]





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[*] posted on 17-4-2018 at 18:37


BaFuxa,

Not enough information. You did "reflux" that Platinum in Aqua Regia, Right?

Might take a very long time to dissolve, without heat.

https://www.youtube.com/watch?annotation_id=annotation_37723...

[Edited on 18-4-2018 by zed]
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[*] posted on 20-4-2018 at 09:53


No I did heat it up to a brief boil, I was following Astral chemistry on the Ostwald process- catalyst preparation. I did as he did. How long does it usually take, hours, days ? ( fixed, watched your link).

In the video he says dissolution should happen in minutes. Note that I am using a small Pt wire while he uses a flake, so more surface area certainly.

Anyway my acids were not concentrated, I made some 38% HCL and am working to increase my HNO3, I will try again.

[Edited on 20-4-2018 by BaFuxa]




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[*] posted on 20-4-2018 at 15:22


I put it in a pressure vessel (had a pressure release valve that should release before the vessel would burst), set that on a hotplate, put the hotplate inside a shed, then plugged it in outside the shed and watched the reaction through the shed window. The solution got quite brown, I remember.



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[*] posted on 20-4-2018 at 15:29


In Nurdrage's example, there appeared to be no visible reaction, when he left his Platinum in cold Aqua Regia overnight.

Also Nurdrage, but utilizing Cl2 Gas! Pretty easy. https://www.youtube.com/watch?v=JFNtP2N2Eho

Chloroplatiinic Acid, by electrolysis of Platinum via HCl. https://www.youtube.com/watch?v=oqh3d8FbiKk

I have read procedures that utilize HCl + H2O2 + Reflux.
Ah, seems HCl +H2O2> produces some Cl2. The Cl2, attacks Platinum.

Here's an oldie! https://www.ideals.illinois.edu/bitstream/handle/2142/.../in...

I'm not wild about using Nitric Acid. Especially in bulk. Oh, I'd do it. But, I would prefer something cleaner. Oxides of Nitrogen are dangerous gases. I know Chlorine seems worse, but Chlorine sets off alarm bells in your body; it drives you out of the room. Oxides of Nitrogen, may be insidious....

Another oldie. https://www.youtube.com/watch?v=5aH7NsW5QfI





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[Edited on 21-4-2018 by zed]

[Edited on 21-4-2018 by zed]
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[*] posted on 20-4-2018 at 17:15


Quote: Originally posted by zed  

I'm not wild about using Nitric Acid. Especially in bulk. Oh, I'd do it. But, I would prefer something cleaner. Oxides of Nitrogen are dangerous gases. I know Chlorine seems worse, but Chlorine sets off alarm bells in your body; it drives you out of the room. Oxides of Nitrogen, may be insidious....


I do not consider Oxides of Nitrogen to be insidious....

N2O would probably not be generated in any significant quantity, and is not too hazardous,
NO changes to NO2 on exposure to the atmosphere,
NO2 is very visible and extremely irritating to eyes and especially lungs
- you will move quite quickly if you inhale just a little.

OTOH chlorine gas is not nearly as visible.
Wikipedia on chlorine:
Coughing and vomiting may occur at 30 ppm and lung damage at 60 ppm. About 1000 ppm can be fatal after a few deep breaths of the gas. The IDLH (immediately dangerous to life and health) concentration is 10 ppm
Wikipedia on NO2:
10–20 ppm can cause mild irritation of the nose and throat, 25–50 ppm can cause edema leading to bronchitis or pneumonia, and levels above 100 ppm can cause death due to asphyxiation from fluid in the lungs

so the toxicity of NO2 and Cl2 are similar,
and I do not enjoy inhaling either :D




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[*] posted on 21-4-2018 at 06:24


Would anyone believe that H2SO4 + some NaCl at 135 C works in a matter of hours?

See 'Leaching platinum-group metals in a sulfuric acid/chloride solution' by M. H. H. Mahmoud with extract at https://link.springer.com/article/10.1007/s11837-003-0086-y .

[Edited on 21-4-2018 by AJKOER]
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[*] posted on 21-4-2018 at 07:55


Awesome find AJKOER.

I thought that the reaction went:

H2SO4 + 2NaCl => 2HCl & Na2SO4

or

H2SO4 + NaCl => HCl & NaHSO4

... and that it went quite fast, so it's hard to see how the procedure in that paper works.

Got to give it a try i guess.




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[*] posted on 23-4-2018 at 16:42


OK. The link to the clever 1920s Platinum Paper(above) failed to work.

I think I found a working link. https://archive.org/details/interactionofpla00shaf
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[*] posted on 25-4-2018 at 06:27


@Zed

Thanks a lot for the links.

I tried the electrochemical process and saw a nice formation of Chlorine at the cathode but I stopped because my set up was rickety, I had leaks, my alligator clip got attacked by the HCL and I could not get the entire platinum wire in the HCL solution.

So today I tried Nurdrage's method, in contrast to the electrochemical one the chlorine concentration in the solution is much less pronounced, I only get a pale yellow. Another thing is that I have to disolve a platinum wire and not a pt powder. So I think I am going to let it sit for the rest of the day and retry the electrochemical method if I am not seeing any hexachloroplatinic acid by then.

In terms of efficiency the electrochemical method is unbeatable in my opinion as chlorine is generated in situ directly at the pt cathode. I have a pt anode as well. I will report back in a few days.





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[*] posted on 25-4-2018 at 07:57


I would be tempted to keep the wire as wire, and buy whatever Pt is reqd.
as wire shape is significantly more expensive than e.g. bar or coin form of solid platinum.




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[*] posted on 25-4-2018 at 13:39


Sulaiman, This is good reasoning. Platinum wire is expensive and subject to counterfeiting.

My concern about the electrochemical method, is the re-plating out the dissolved Platinum at the opposite electrode.

I don't know if it will present a problem.

Some folks suggest utilizing a semi-permeable membrane, to physically prevent the dissolving Platinum from migrating to the opposite electrode.

But perhaps, I am overthinking.
-------------------------------------------------------
I just surveyed Platinum wire prices. Very, very expensive. At least, from what I have seen so far.

Platinum prices in general, while high, aren't THAT high.... Considering the small quantities usually needed, and its recycle-ability.

I recently bought 1/10 of an ounce, in the form of a coin. A little over spot price, but not bad. 3.1 grams of Platinum should produce +6grams of Chloroplatinic acid. A lifetime supply for the small experimenter.

https://www.ebay.com/itm/2016-1-10oz-Platinum-Noble-Coin-999...





[Edited on 26-4-2018 by zed]

[Edited on 26-4-2018 by zed]

[Edited on 26-4-2018 by zed]
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BaFuxa
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[*] posted on 26-4-2018 at 12:20


The deposition of platinum at the anode is a fair point. I just finished preparing the apparatus now I am on the fence. The experimenter in the video you linked to managed to get H2PtCl6 so who knows maybe I will run it for 4-5 hours see what happens.

My pt is a cheap 1 cm long 9.76$ wire I bought on ebay from a chinese seller. Not sure if that is actually a normal price to pay for platinum :D, far from what you bought.

Anyway, I found an interesting paper here from the ancient times, oxidation ammonia over copper : https://pubs.acs.org/doi/abs/10.1021/ed011p575?journalCode=j...

Definitely straightforward, you need neat NH3 and O2 streams though. I tried this with the classic ostwald set up and it did not work, I should get myself an O2 generator, thinking of using and HHO so that I can control the flow rate of O2 via the power supply. The stochiometry of O2 to NH3 is important, not enough O2 and you get nitrogen and water.

I have found other papers where other earth abundant catalysts are used, like Cu and Ce. I am not sure the platinum is worth the fuss after all.





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[*] posted on 26-4-2018 at 14:11


Well, Chloroplatinic Acid is highly colored, so its physical migration can be directly observed. It also figures to be fairly dense and slow to migrate.

Were the cathode in a separate container, connected to the anode container, only by a "salt" bridge (perhaps a coil of fine, flexible plastic tubing filled with HCl), it might be possible to completely prevent Pt from reaching the cathode.

But, perhaps I am overthinking, and anticipating possible problems, that may not manifest.

I'll know in a few days. One thing I do have, is lots of electricity!

P.S.

Prices on Platinum wire are pretty extreme. Folks are charging big bucks for small amounts of fine gauge wire. They make no reference to weight, probably because the weight is miniscule, compared the the pricing. It is a specialty item.

Jewelers often make their own wire, to avoid such problems.

Slices of metal can be drawn or pulled through a "Draw plate" to become wire.
Requires periodic annealing during the process, but if you need a quantity of exotic material wire... Well, you gotta do, what you gotta do.

P.P.S. Seems to me, the "sacrificial electrode" should be the anode. Remembrance of electrochemistry is pretty fuzzy,but our process is analogous to "Anodizing" Aluminum. Excepting that, the reacted metal of the anode is soluble, and dissolves as Chloroplatinic Acid, whereas Aluminum Oxide is insoluble, and it passivates an Aluminum electrode.

https://www.youtube.com/watch?v=aY9HJpXboBY









[Edited on 27-4-2018 by zed]
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[*] posted on 27-4-2018 at 15:48


Well, I can't edit this into my former post, it timed out.

Interesting Patent on Electrolysis of Platinum in aqueous HCL.

https://patentimages.storage.googleapis.com/fb/c1/27/0c060e4...
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[*] posted on 1-5-2018 at 06:31


Well sadly I was away for the last days and when I came back my electrolysis set up for platinum was all over the place because of the strong winds we have had so this is postponed for now.

Anyway thanks for the additional links Zed, yes I do think an anionic only membrane or a salt bridge should be used as a precaution but that is not something I have at the ready. Regarding electrochemistry, your suggestions make me think that it would be possible to make HNO3 simply by electrolysing a solution of a nitrate salt, the nitrate radical gets protonated at the anode, like in the electrochemical synthesis of H2SO4, this would be much more tranquil than the salt distillation method. Where it gets even more interesting is that I think it is possible to make HNO3 directly from ambiant air via splitting of N2 and O2, with platinum or platinum-like electrodes.

Regarding platinum I think it may be a better plan to switch for another Ostwald style HNO3 synthesis and to indeed keep that pt wire. I have linked some papers up the thread and I am going to try the oxidation of ammonia with MnO2 and Co3O4 instead. Another member going by the name of Chemetix tried this with success about 2 years ago. It makes much more sense to use another oxidizing catalyst as even platinum gets worn out eventually and I personally dislike working with rare earth metals. I want an HNO3 generator that I can use for years without having to worry about reagent costs. The nitrate salt way is too time consuming and gobbles sulfuric acid by the gallon, not to mention that nitrate salts do cost some money, the classic ostwald way will end up too costly in terms of pt, birkeland is too unefficient from what I have read, unless a microwave generated plasma can be tamed but from what I have read no one around these parts has managed to do that.





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[*] posted on 1-5-2018 at 16:45


Umm. Initially, I think I'm going to try H2O2 and HCl, to produce Chloroplatinic Acid. This may not work, but it would be great if it did.

I did receive my 1/10 of an Ounce of Platinum.

I was surprised. The coin seemed very, very, small.

That is because Platinum is very, very, dense.

21.45 g/cm3.... ~1.45 ML per OZ.


[Edited on 2-5-2018 by zed]

[Edited on 2-5-2018 by zed]
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