using NaHCO3 for neutralizing the aqua regia

[Harshana552]

Sir, i am new to the forum and my chemistry knowledge is very low.
So i went through lot of threads here saying do not neutralize the solution. I got that, But I really wanna know how the chemistry work during the addition of NaHCO3. how it affect the AuCl4- to prevent the dropping of gold.
if anyone can explain me about the chemistry here with reactions, I will be greatfull.
thanks very much

 

[orvi]

Sodium bicarbonate is base. It neutralize acids. Firstly, it does nothing to AuCl4- ion. You will be adding it to the spent AR solution till it will neutralize all of the acid to form SALTS - not destroying nitric acid or nitrates - they will still be there. As pH rise, base metals will start to precipitate as hydroxides. Gold will come out of the solution at the end of the neutralization as hydrated oxide, which upon standing on light or heating will decompose to elemental gold and oxygen. In theory, you can take advantage of the gold hydroxide formation to isolate it from the base metals, as they came out of the solution at far lower pH values. But if you try this with regular AR solutions contamined with loads of Cu, Fe, Ni, Ag and so on, you will realize that these hydroxides formed are nearly impossible to filter and resolution is not that striking, adsorption into vast surface area is very real issue and overall, you won´t be doing it this way

Au(OH)3 does exist but it is in a very heavily contaminated state--it is a mixed hydroxide/oxide, and this is the same for most of the platinum metal group and other transition metals. In this case, whether or not you get a high proportion of the oxide versus the hydroxide or vice versa, is dependent upon the thermodynamic stability of the product. In silver's case, Ag2O is more stable than AgOH.

When you have an HAuCl4 solution and you add base, you get Au(OH)3*xH2O and Au2O3.

Very, very gentle heating of Au(OH)3 will yield Au2O3, which under further heating, will yield gold.

Now as you can imagine, gold (III) hydroxide is unstable. It would prefer to be the oxide, and it would be happiest to be elemental gold. This is why it is light and temperature sensitive. This photosensitivity of Au(OH)3 is known to anyone who's ever looked into doing alternative photography.

Lou

Nice response from Lou on the subject.

 

[Yggdrasil]

First and foremost you need to know a few basic terminologies.
When we talks about neutralizing an acidic solution, we talk about bringing the pH up with some caustic element from (1-7) to 7 which is neutral
You can do the same to a caustic solution, but then we use an Acid to bring the pH down from (7-14) to 7

Then we have a few members talking about neutralizing the Nitric NOx in either Nitric Acid or Aqua Regia.
Then we stress that it is the wrong terminology, we call it destroying the Nitric or NOx because what we do then
has nothing to do with the pH but rather converting Nitric Acid or NOx to either (Nitrates by Urea, Not recommended),
to Sulfuric acid by using Sulfamic acid (this procedure is ok since it converts both Nitric and NOx without creating dangerous nitrates)
and the best method of not using too much Nitric in the first place.

Adding Sodium Bicarbonate will push the pH up and so end up with dropping Hydroxides of all sorts, unless you have very strict control of your pH.

We recommend reading Ms. Hokes book on refining precious metal wastes.

You will find a free PDF in Frugalrefiners signature line here:
https://goldrefiningforum.com/phpBB3/viewtopic.php?f=54&t=19798https://goldrefiningforum.com/phpBB3/viewtopic.php?f=54&t=17706#p179081

 

[Harshana552]

thank you soo much every body. i downloaded the hokes. i will start to refer that one.