Experiment precipitating gold out of solution 3 ways

[g_axelsson]

Yes you could slightly change a pH with water, but you will have to add ten times the amount of water to the acid to change the pH by one-pH unit, so to move from a strong acidic solution of a pH 1 to a strongly acidic solution of pH 3 we would have to add about twenty volumes of water, thus changing hydrogen H+ ion concentration and changing the pH with this large volume of water.

Butcher, I agree with you on all but one tiny detail... pH 1->2 takes 10 times volume (add 9 parts water), pH 2->3 also takes ten times (also add 9 parts) but now we're starting with the larger volume... 10*10 or 100 times the volume to go 2 steps (9/10 + 90/100 parts water, 1 part acid = 99 times the volume water added).

This is of course a simplification as we don't count the pH 7 of the water we are diluting with so actually we need even more than the 99 parts. But for strong acids or bases a dilution by ten moves the pH one step towards neutral is a good enough rule.
Anyone interested in the exact solution can look up the logarithmic laws and the definition of pH.

I know, I'm a besserwisser (a know-it-all for you English speakers)! :mrgreen:

Göran

 

[butcher]

Göran,


Thank you again for the clarification and the math involved with the pH change and the volumes of water needed to make the change, I have not tried it but I have read where they take an acid and add ten volumes of water to raise the pH by one, then take a portion of that and add ten times water to this diluted acid raise that one more value in pH.

I guess I kind of generalize in my mind when I add water to a solution I am basically just diluting the ions in solution, and it has basically little to no effect on pH (although in reality there is a very slight effect on pH).

I have to admit you do keep me on my toes, and keep me thinking or rethinking my ideas, this also very good to help us both learn more or at least to help apply some of the things we learn.

I know I am not a besserwisser (Know it all), it is hard for me to see past what I think I know, until I can learn differently.

I also do not think you are a besserwisser, other wise you would not have learned as much as you have.

 

[Rob2069]

This weekend I tried an experiment with precipitating gold in a couple different ways. I have easy access to SMB, but was looking for a second way to more selectivity isolate the gold and get better purity. So, in my searching I found a couple different methods. I think the obvious one that comes to mind is oxalic acid precipitation. But after looking around a bit longer I decided on trying three different approaches... one of which i think was very interesting because there is not much info out there on the method. I will do my best to describe the conditions of each experiment

Standard Oxalic Acid precipitation (heating both solutions to near boiling and then mixing together)
starting conditions: once refined gold dissolved in HCl/bleach
method: auric chloride was produced, diluted, filtered. It was then heating on a hot plate until bubbles started to form in solution. A separate solution of oxalic acid was produced (using 1.5ounce of acid crystals to 1ounc AU ratio). The ph of this solution was adjusted to 6 using sodium hydroxide. This solution was then heated to near boiling as well.
observations: upon adding the heated solutions together (AND SLOWLY) there was a lot of fizzing but no change to the solution. no gold was precipitating. I then prepared a sodium hydroxide solution by adding crystals to water (careful, this solution becomes hot). I added this to the hot auric chloride solution (VERY SLOWLY AS THE REACTION BETWEEN THE TWO IS VERY VIGOROUS!!!) until a reaction took place. It took a while, but when the gold finally started to precipitate, it did so very rapidly... i was rather amazed
results: happy! actually, the gold powder that precipitated was the lightest tan I have seen so far. After washing the powder, the gold was melted and the button had no visible signs of tarnishing at all... definitely the purist gold I have produced so far. as an interesting side note, when the gold was molten, i noticed the mythical green hue that i have read about but never witnessed before


Oxalic Acid precipitation in conjunction with SMB
starting conditions: once refined gold dissolved in HCl/bleach
method: after the auric chloride and oxalic acid solution was added in the above step, I set aside about 500 mL to precipitate with this method. No other ph adjustment was used, I simply added the standard ratio of SMB to the whole solution (1 to 1 by weight).
observations: precipitation was immediate. another observation was that the left over liquid was the clearest i have ever seen it... no hint of any color what-so-ever. also, the powder formed was the same tan color as the traditional oxalic acid drop. all rinses of the resulting powder were crystal clear as well.
results: happy again! The gold powder was indistinguishable from traditional oxalic drop. so was the gold button produced....and the green hue during melting was present once again.


Precipitation with Hydrogen Peroxide
this was the most interesting in my opinion, and the one I was really looking forward to trying. I found the reaction on Wikipedia but almost no information on anyone actually doing it. here is the equation:
(2)AuCl4[-] + (3)H2O2 + (6)OH[-] --> (2)Au + (8)Cl[-] + (6)H2O + (3)O2
so, here is what i did:
starting conditions: unrefined gold (clean foils, black powder)dissolved in HCl/bleach
method: auric chloride was produced, diluted, and filtered. To this, I added the same volume of 3% H2O2. (i would be cautious about using strong h2o2). to this, i added sodium hydroxide in flake form. This was continued until the solution dropped the gold.
observations: when the flakes were added, they would turn black immediately from a reaction taking place. as more was added, the solution would turn dark brown and gold would precipitate, but get dissolved back into solution. after continuing to add the sodium hydroxide, the solution eventually gave up the gold without putting it back into solution. the solution too a while to settle, probably due to how dilute it was.
results: so-so. while the gold did precipitate, it ended up dark brown. (i can post pics if anyone is interested) This gold would have to be refined probably a couple times to get it as pure as the other methods. To me it seems as if SMB is much more selective as to what is reduced in the auric chloride solution. because of this and the fact that SMB is relatively cheap, I don't see any advantage to using peroxide over SMB. One plus I did observe was there was no nasty odors from this precipitation method...but this was still done under a hood.


Just thought i would share with you guys... if you have any questions about the peroxide drop, please ask away and i will do my best to answer what i can

Mike

By chance could I get you to please share the link to the Wikipedia article where you got the information about the hydrogen peroxide precipitation from?

Edited by moderator, removed double quote.

 

[Hossein Mazaheri]

It all depends on the content of the original solution. Obviously the addition of hydroxide to a solution of metals will precipitate most other metals as hydroxides too, not only gold.

So whatever you start with together with the gold will end up in your melt.

Göran

I also got some gold hydroxide from AR with sodium hydroxide. How to turn it into metal؟ And how do I separate the gold hydroxide mixture from palladium hydroxide? Is there a chemical method besides the melting method؟ I asked several times but the committee didn't respond. Thank you mazaheri***@mailatsome.server

 

[Yggdrasil]

I also got some gold hydroxide from AR with sodium hydroxide. How to turn it into metal؟ And how do I separate the gold hydroxide mixture from palladium hydroxide? Is there a chemical method besides the melting method؟ I asked several times but the committee didn't respond. Thank you

We do not use our phones or emails publicly so I will remove it for you
All replies shall be in the open forum anyway.

 

[Yggdrasil]

I also got some gold hydroxide from AR with sodium hydroxide. How to turn it into metal؟ And how do I separate the gold hydroxide mixture from palladium hydroxide? Is there a chemical method besides the melting method؟ I asked several times but the committee didn't respond. Thank you mazaheri***@mailatsome.server

Welcome to us.

What was your material and how did you manage to make hydroxides?


Here is for your studies:

We ask our new members to do 3 things.
1. Read C.M. Hokes book on refining jewelers scrap, it gives an easy introduction to the most important chemistry regarding refining.
It is free here on the forum: Screen Readable Copy of Hoke's Book
2. Then read the safety section of the forum: Safety
3. And then read about "Dealing with waste" in the forum: Dealing with Waste

Suggested reading: The Library

https://goldrefiningforum.com/threads/gold-refining-forum-rules.31182/

 

[g_axelsson]

I also got some gold hydroxide from AR with sodium hydroxide. How to turn it into metal؟ And how do I separate the gold hydroxide mixture from palladium hydroxide? Is there a chemical method besides the melting method؟ I asked several times but the committee didn't respond. Thank you mazaheri

Gold hydroxide dehydrate at 140 C and turns into steam and gold oxide.
Gold oxide decomposes at 300 C and turns into metallic gold and oxygen

So in theory, just heat it up to over 300 C and you get back gold powder. But since no one is doing it this way you are on your own. Tell us what happened.

https://en.wikipedia.org/wiki/Gold(III)_hydroxidehttps://en.wikipedia.org/wiki/Gold(III)_oxide
To separate gold hydroxide from the palladium hydroxide, dissolve it in hydrochloric acid and use standard methods to precipitate the gold first and then the palladium. Read Hokes book and check the library for methods, just as Yggdrasil suggested.

Göran