Sciencemadness Discussion Board

Oxidising gold by mistake

Caesarius - 28-10-2013 at 04:00

I think I oxidised some gold but discarded the oxide by mistake. I'll spare you the flood of emotions. Mostly.

I had separated the gold plate from some gold-plated copper etc by electrolysis, and had filtered the anode sludge, but I had used filter paper, let it dry, and getting the very thin pieces of gold off was proving difficult. Probably much less than a gram of gold. So I started on an increasingly desperate process to separate the gold from the paper.

I discarded any paper that clearly had nothing on it, and burnt the rest (3g of paper?). I presumed that I had gold, carbon and other paper-related stuff, and possibly some copper oxide.

I added conc H2SO4 and boiled until I got 50ml of thick blackened syrup, so the carbon was still there.

I added water to wash out soluble stuff, and could see the gold on the bottom of the beaker.

I added NaOH to neutralise, and I must confess to throwing in a calcium supplement pill, which had CaCO3 and a few other things.

With hindsight, at this point I should have tried panning it. But I didn't.

I added water, let it settle, decanted, and added a little conc H2SO4 and a few g of K2Cr2O7, thinking to oxidise the carbon. I could see orange dichromate when I shined a bright torch through a small sample of the blackened brew. I simmered it for a day, and the orange had turned to green (as expected), but there was clearly lots of carbon left. So I added 100ml conc H2SO4 and 40g K2Cr2O7 and simmered for two days.

I neutralised with NaOH and then with Na2CO3. Now I had a milky orange brew, so the carbon was gone, and there was still plenty of dichromate left. I presumed that the gold was obscured by other precipitate. I filtered.

The filtrate was much darker than I expected, a deep red, and, being twitchy about dire warnings about dichromate, I let it run away. Then I washed the residue, but found no gold at all, and got a terrible sinking feeling.

So, do you think I oxidised the gold? That would account for the deep red colour and no gold. I only ask because I cannot find a detailed procedure to oxidise gold, only references to "highly reactive chemical environments", for which I might just qualify...

deltaH - 28-10-2013 at 04:20

Quote:
I had separated the gold plate from some gold-plated copper etc by electrolysis, and had filtered the anode sludge, but I had used filter paper, let it dry, and getting the very thin pieces of gold off was proving difficult. Probably much less than a gram of gold.


Are you sure you had significant amounts of gold to start with. Typically electroplated items contain nanometer thick layers of gold, I doubt there's enough for a recovery. Perhaps you should calculate a quick back-of-the-envelope estimation of how many milligrams of gold you would have from a rough estimate of the surface area of your items and a thumb sucked value for a typical gold electroplating thickness.

If this is so, then you can rest easy... :P

UPDATE: According to this website, the average decorative gold plating thickness is about 381nm... so you can work with that.

[Edited on 28-10-2013 by deltaH]

phlogiston - 28-10-2013 at 05:02

Did you discard the dichromate via the sewer? That's not good, you really should keep it next time and dispose of it in a more environmentally responsible manner. If you had done so, you would also still have had the solution containing your (probably very) small amount of gold.

Quantity of gold

Caesarius - 28-10-2013 at 07:12

I work with an estimate of gold thickness of 1 micron, e.g. see
http://www.artisanplating.com/articles/platingthick.html
The raw source is gold plated pins from good quality electrical connectors. Pins say 0.5mm across would then have 0.8% gold by volume and 1.7% by weight. The gold in question was from about 10g raw, so that's 0.2g gold.

I'm not so upset about 0.2g of gold (there's more where that came from), but I am disappointed to lose an interesting compound!

But: Was it gold oxide? And how would you prepare it?

PS Apologies for letting it go unprocessed. I won't make that mistake again. Not to detract from the posting about proper disposal, but it's ironic that the signature talks about not caring where a rocket lands: let's assume it was an ironic signature! ;->

deltaH - 28-10-2013 at 12:24

Thanks Caesarius, the reason I asked is so that you know it was there to start with and in what amounts, it helps for the chemistry riddle. As for the gold oxide, I cannot advice on that as I am not an expert in gold chemistry, I just know about chloroauric acid, cyanides and nanogold :)

I hope someone can shed some light on your mysterious compound.


[Edited on 28-10-2013 by deltaH]

Perhaps it's repeatable

Caesarius - 28-10-2013 at 16:20

I'll just have to try it again, but without the carbon, and using such quantities that avoid any accusation of "bucket chemist". (Actually, that's a throw-back to my school days: I imagine that you folks see no problem using buckets when the occasion demands it!)

phlogiston - 29-10-2013 at 02:10

Perhaps this paper has some answers for you. I can't access the full text from here (you can try to request in the references subforum), but the abstract suggests a layer of Au<sub>2</sub>O<sub>3</sub> forms on the gold. So, if your foils are very thin, perhaps a significant fraction would be converted to insoluble gold oxide.
Still, you may be able to recover that.

BTW, It is often a lot more difficult to work with small quantities and dilute solutions. (On the other hand, ofcourse, it is risky to use larger quantities before you have figured out a working procedure).
Quote: Originally posted by Caesarius  

PS Apologies for letting it go unprocessed. I won't make that mistake again. Not to detract from the posting about proper disposal, but it's ironic that the signature talks about not caring where a rocket lands: let's assume it was an ironic signature! ;->


No need to apologize, and it most definately was satirical. It was quoted from a song by the satirical singer/song-writer Tom Lehrer. It is about the late Wernher Von Braun who was the German engineer that devised the V1 and V2 missiles that were dropped on London in WOII. The song by Lehrer is about how his role in nazi Germany is downplayed in the US to allow him to work on rocket development in post-war USA.

Sorry to go way off-topic here but another great comedian, Mort Sahl, suggested a similarly amusing satirical subtitle for The movie "I Aim for the Stars", a biography of von Braun: "But Sometimes I Hit London".

[Edited on 29-10-2013 by phlogiston]

Interim Progress Report

Caesarius - 8-11-2013 at 07:02

An experiment, to see if oxidisation occurred in the manner I thought it might.

I put 50mg Au, 1g K2Cr2O7, 10g conc H2SO4 in a test tube. The gold is in pieces approx 1mm x 1mm x 1um. Very pretty.

First observation: the gold had clearly not dissolved yet, but the solution was already a deep red colour. This suggests the hypothesis that, in the previous experiment, the gold had been broken into much smaller pieces and was hidden in the milky sludge.

Next I heated the mixture over a low heat (test tube over a hot plate (1) at 180 deg C, not visibly boiling) overnight. No visible reaction.

Next I heated it over a low flame so that it boiled gently for 5 minutes. Red colour is very deep, but solid gold not visibly depleted. Left on hot plate as before.

Will report again later, maybe even with pictures.

(1) Yes, I know a test tube over a hot plate is awkward and inefficient, but that's what I've got, OK?!

Report

Caesarius - 13-11-2013 at 10:28

After a day or two, the colour of the liquid was deep green, consistent with chromium oxide / sulphate complex. Also very pretty.

The gold appears unaffected.

Had the gold disappeared, I should have known it was in solution, because the liquid is clear and so cannot hide smallish pieces of gold. Now that there is solid gold remaining, I have to consider how to find the gold content of the solution. I do not have sufficiently accurate scales with which to have weighed the gold beforehand, nor to weigh it now. Suggestions welcome.

I am guessing that there is no gold in solution, and that in my original experiment the gold was broken up and hidden in the milky precipitate.

Now I have learned to dry any precipitate and try it with aqua regia to see if I get any gold out of it. I still run into problems if there are obscuring "impurities", as above.

(The electrolysis purification method is working well on the gold-plated pins: I'll have to report on that later. All sorts of gotchas!)