Sciencemadness Discussion Board

HAuCl4 + NaHCO3

terrax753 - 3-5-2011 at 10:03

Hello. I was trying to purify some gold. Not having access to nitric acid, I decided to make due with what I did have.

I have this 31.45% hydrochloric acid which I purchased at a hardware store. I took a small amount of it and mix it with some potassium nitrate. To test it, I added an old gold-plated S-video connector.

It appears to have worked. The gold is gone. Of course, so is the copper onto which the gold was plated.

To neutralize the acidic solution, I started adding sodium hydrogen carbonate until it stopped fizzing. I also added some more water to ensure that the sodium and potassium compounds would not saturate the solution.

I could quickly see precipitation of copper carbonate. As the blue substance collected at the bottom, the above liquid began to turn green, then yellow, then eventually a yellowy-brown.

Upon checking it today (about twelve hours later), I found there to be a yellow-orange precipitate. Not what I was expecting.

I am new to chemistry. My understanding of it is quite lacking. What has happened? The liquid is mostly clear, but has a slight blue tinge.

I cannot find anything about a "gold carbonate" compound. I was expecting the chloroauric acid to stay in solution. By precipitating out the copper, I was hoping to then filter it away, gently heat the HAuCl4 to decompose it to AuCl3, then add some aluminium to precipitate out the gold.

What has gone wrong? Also, that you guys can think of, are there any problems with my subsequent plans?

I appreciate any help you guys can give me. And please, be patient with me. I am still learning, and have only just begun to explore the world of chemistry.

cyanureeves - 3-5-2011 at 10:16

one connector? i couldnt drop nil with a bunch of connectors and pins. unless you got a significant amount you wont see much gold. try bleach with your hcl acid,it works o.k. then maybe you can drop with iron out.sodium bisulfite instead of sodium metabisulphate but i dont know. the connector would be mostly steel wouldnt it?

ScienceSquirrel - 3-5-2011 at 13:19

I do not know how much gold there would be on an S-video connector but I would guess at not a lot.
Maybe tens of milligrams, over 100 milligrams or a tenth of a gram if you are lucky and it is a very expensive and old connector.

Arthur Dent - 3-5-2011 at 13:44

The recipe you're talking about is poor man's AR, an alternative to Aqua Regia...

Normally, you'd use sodium nitrate, which is less expensive than KNO<sub>3</sub>, but I guess the potassium salt should work. Dissolve it in hot water, and then add conc. HCl. Boil the solution until it turns intense yellow/orange. Then add your gold-plated metal to the hot solution. A lot of Nitrogen Dioxide (aka angry orange fumes) will be evolved so do this outside of course.

A couple of hours should strip all the gold off the parts. check occasionally by pulling out a part to see if there is still gold left on the components. You need the heat for the solution to be efficient. cyanureeves' suggestion of using bleach is also a good technique of stripping gold, a bit slower though but somewhat safer (no boiling stuff).

At the end, the metabi will do a good job precipitating your gold.

Robert

terrax753 - 3-5-2011 at 13:53

First off, this was just a test. The only reason I used an S-video connector was because it was the only old, gold-plated cable I had laying around. It probably used to be connected to a VCR or something.

Ultimately, I only need a very small amount of gold. I am talking maybe a couple of dozen milligrams. But that is besides the point.

Basically, I am wondering what exactly happened. Clearly, there is too much of this golden-orange precipitate for it to be a gold compound. Then again, I have no idea what it could be. Beneath it, there appears to be a bit of copper carbonate.

I can kind of understand simple single displacements like Zn + CuSO4 => ZnSO4 + Cu. But beyond that (such as the complex reactions that occurred with this potassium nitrate, hydrochloric acid, gold, copper, sodium hydrogen carbonate, and water mixture)...

I mean, why is the water slightly blue? If anything, I would have expected a slight orange tinge, due to the HAuCl4. Though, due to its low concentration, not much of one.

Cyanureeves, I do not think there was any iron or steel in the connector. I had clipped off a piece, and, as I expected, beneath the plating, there was what appeared to be copper.

terrax753 - 3-5-2011 at 14:01

Quote: Originally posted by Arthur Dent  
The recipe you're talking about is poor man's AR, an alternative to Aqua Regia...

Normally, you'd use sodium nitrate, which is less expensive than KNO<sub>3</sub>, but I guess the potassium salt should work. Dissolve it in hot water, and then add conc. HCl. Boil the solution until it turns intense yellow/orange. Then add your gold-plated metal to the hot solution. A lot of Nitrogen Dioxide (aka angry orange fumes) will be evolved so do this outside of course.

A couple of hours should strip all the gold off the parts. check occasionally by pulling out a part to see if there is still gold left on the components. You need the heat for the solution to be efficient. cyanureeves' suggestion of using bleach is also a good technique of stripping gold, a bit slower though but somewhat safer (no boiling stuff).

At the end, the metabi will do a good job precipitating your gold.

Robert


Well, I have been using potassium nitrate because it is what I have. I can get it locally for just a few dollars.

How critical is the concentrated hydrochloric acid? As previously stated, mine is only 31.45%.

And could you please elaborate a bit more about the sodium metabisulfite? So it will precipitate the gold but not the copper?

[Edited on 3-5-2011 by terrax753]

cyanureeves - 3-5-2011 at 14:28

sorry terrax753 but i was thinking that an s connector was something like a speaker jack. if you use aqua regia or bleach and hcl acid both have to be rid of the nitric by either evaporating or denoxxing by adding urea until it stops fizzing..so evaporating is cheaper of course.when all is evaporated you will be left with a green scab looking stuff,at this point mix sodium metabisulphate in warm water and add it to the dry stuff. gold will drop as a brown mud and copper will stay in blue solution.

cyanureeves - 3-5-2011 at 14:53

terrax753 hcl and bleach does not produce nitric but a fellow on a gold recovery site says to do the same as when using aqua regia. bleach and hcl acid is used mostly by them to strip the gold of the substrate then aqua regia is used to dissolve the gold. denoxxing is a term used by them for driving off the nitric fumes.

Neil - 3-5-2011 at 15:25

The blue could be coming from the Nickel.

Even with old electronics that have thick juicy gold plating, the amount of gold is so trivial that it would be very difficult for you to gain a useful precipitate. You can purchase gold flake cheaply off of Ebay. If you really wish to make some I'd suggest looking for old hard drives, old modem cards and old processors. I've found some hard drives made about ten to twelve years ago used gold circuitry. I found this out after stripping down a pile to salvage the bearings and magnets, by then I had no way to identify the manufacture, sorry. I've found old modem cards with gold circuitry and old processor pins have thick plating on the pins and sometimes, locked in the toxic inner ceramic mesh, gold foils.


If you ever have the chance to melt Gold onto a piece of copper for an artistic affect, it will be sure to disappoint you. Even when cold, the gold slowly solves into the copper producing a dull oxide coated surface. To prevent this, electronics manufactures put a thin coating of Ni on the copper circuits, which is then plated with gold. you may have better luck dissolving the Cu and Ni leaving the gold as thin foil which can then be dissolved producing a more pure solution.


Edit: For my own tiny gold experiments I use HCl + and small amount of 3% H2O2.

[Edited on 3-5-2011 by Neil]

cyanureeves - 4-5-2011 at 11:33

terrax753 i was googling for a particular name brand's msds and up pops a science madness discussion about your poor man's aqua regia and my marc 55 drain cleaner as well. you know when the thread was written? about 4 yrs. ago! we're treading on ghost foot steps. eeery! but any way your hcl acid is plenty strong for aqua regia and the liquid you discribed turning yellow then brown is interestingly goldlike. i never had enough gold to risk experimenting but you report that by morning your auric solution turned copper blue and your drop was yellow. all with baking soda? now boil a piece of tin soldering wire in hcl acid, dip a Q-tip in the tin chloride and touch the yellow drop to see if your Q-tip turns a purplish color(gold in them thar hills). my tin chloride never shows bright purple because its contaminated with copper powder and i'm not sure if it tests for gold in every state either.