Oxalix acid versus other precipitants

[bswartzwelder]

I am not questioning IF oxalic acid works as a precipitant, I know it does. I am also aware that it probably produces some of the purest gold when using real AR for the final refining process. But, when we deal with HCl/Chlorox, we have to make sure there is no excess chlorine before we precipitate. When using AR as a recovery process, we must make sure all excess nitric is gone. When we use oxalic acid, we must adjust the pH of the solution to about 6 using NaOH or else the gold will not drop completely. The NaOH turns the strong acids into weak acids because a pH of 7 is considered neither acidic nor alkaline. What I don't understand is: why can't we just use HaOH in ANY acidic solution (AR, poorman's AR, etc.) to destroy ALL the acid? Wouldn't that force the gold to drop? I guess that while I can understand many chemical processes, changing pH and it's actual effects is something that eludes me. Is this too involved to explain easily or in terms that can be understood?

 

[solar_plasma]

That is not correct:

The NaOH turns the strong acids into weak acids

strong acids stay being strong acids. A strong acid dissociates almost completely:
HCl + H2O => Cl- + H3O+

A weak acid will only partly dissociate:
CH3COOH + H2O <=> CH3COO- + H3O+

to destroy ALL the acid?

NaOH will not destroy anything. The problem is an excess of Cl and NOCl which has to be used up by reaction with the gold or driven of as gas.

 

[bswartzwelder]

Thank you. It had been my understanding that something which has a pH of 1 is a strong acid, while something with a pH of 13 is a strong base, but I see the term "strong acid" does not necessarily relate to the pH of the acid. I guess the proper terminology would be that something with a pH of 1 is strongly acidic while something with a pH of 13 is strongly basic (or alkalinic). NaOH to destroy an acid. Once again, the terminology has been my enemy. However, if you have an acid (any acid), once you add NaOH, the resultant (pH) will become less acidic by changing to a pH that is closer to 6 or 7. Continue adding NaOH and the solution will become neutral with a pH of 7. Even more NaOH and the solution will become basic with a pH that is above 7.0. The acid that you started out with is no longer there. It has been transformed into new compounds and the original acid is simply not there any longer. This was my understanding of how mixing acids and bases worked.

I did not see what adding NaOH did to the AR solution which had oxalic acid other than changing it to a solution which is more neutral. I knew the AR/oxalic acid solution had to have a pH with a number less than 6 and that we were trying to get the pH as close to a range of 6 (I'm guessing most anything from 5.5 to 6.5 shoul work) as possible. To my way of thinking, we were getting rid of the acid. I know from reading Hoke that many people (usually the first time they try it) add too much nitric acid to their solutions of AR. That having been said, it has been said that you need to destroy the excess nitric with evaporation being the preferred method. However, (I believe it was Hoke) also states that sodium carbonate (which is basic) can be used to neutralize a slight excess of nitric (what difference doe it matter if you use NaOH in place of the sodium carbonate?). Much of this is confusing since it has been a long time since I have taken a formal chemistry class. If sodium carbonate works for a slight overabundance of nitric, why wouldn't it work for a large overabundance?

 

[solar_plasma]

Thank you. It had been my understanding that something which has a pH of 1 is a strong acid, while something with a pH of 13 is a strong base, but I see the term "strong acid" does not necessarily relate to the pH of the acid. I guess the proper terminology would be that something with a pH of 1 is strongly acidic while something with a pH of 13 is strongly basic (or alkalinic). NaOH to destroy an acid. Once again, the terminology has been my enemy. However, if you have an acid (any acid), once you add NaOH, the resultant (pH) will become less acidic by changing to a pH that is closer to 6 or 7. Continue adding NaOH and the solution will become neutral with a pH of 7. Even more NaOH and the solution will become basic with a pH that is above 7.0. The acid that you started out with is no longer there. It has been transformed into new compounds and the original acid is simply not there any longer. This was my understanding of how mixing acids and bases worked.

pH is nothing else but a logarythmic scale on the concentration of H3O+. PH 3 contains ten times more H3O+ than pH 4 and so on. In the example HCl+NaOH there will be as many H3O+ as OH- at pH7. Noneof theions are gone or destroyed (edit: okay, now we have water which only has a weak autoprotolysis, so most of the H3O+ and OH- will find together to H2O, the rest will be Na+ and Cl- ions), unless you take them out of the system by evaporation or by dropping them as unsoluble compounds. So if you just dilute HCl with enough of water, lets's say a drop of HCl on 100ml water, you will obtain higher pH values

I did not see what adding NaOH did to the AR solution which had oxalic acid other than changing it to a solution which is more neutral. I knew the AR/oxalic acid solution had to have a pH with a number less than 6 and that we were trying to get the pH as close to a range of 6 (I'm guessing most anything from 5.5 to 6.5 shoul work) as possible. To my way of thinking, we were getting rid of the acid. I know from reading Hoke that many people (usually the first time they try it) add too much nitric acid to their solutions of AR. That having been said, it has been said that you need to destroy the excess nitric with evaporation being the preferred method. However, (I believe it was Hoke) also states that sodium carbonate (which is basic) can be used to neutralize a slight excess of nitric (what difference doe it matter if you use NaOH in place of the sodium carbonate?). Much of this is confusing since it has been a long time since I have taken a formal chemistry class. If sodium carbonate works for a slight overabundance of nitric, why wouldn't it work for a large overabundance?

I have not tried oxalic acid, yet. I read, that it will drive off some nitric by its own, which will cause unnecessary waste of oxalic, if there is too much excess. I don't know the reason, why the pH has to be adjusted to about 5-6.

 

[mls26cwru]

from what i have experienced, oxalic acid is selective in what metals it will precipitate (in the correct pH range), while NaOH is not selective at all and will drop almost any metal in solution. if you know that ONLY gold is in solution, you can use a NaOH/H2O2 method to precipitate it.

 

[bswartzwelder]

I am aware of the selectivity of oxalic acid. My questions were about the chemical reactions themselves and why certain things react as they do.

 

[solar_plasma]

Make sure you have read this thread:

http://goldrefiningforum.com/~goldr...&t=19168&p=195577&hilit=precipitation#p195577

After I read this (especiallly lazersteve's post at the end), the only reason for me to try oxalic is for educational purpose. I remember other threads, that make clear, oxalic isn't necessary to obtain highest purity, but proper washings have the greatest effect. After those threads I am not anylonger convinced that oxalic is selective, though I believed this from earlier readings. Please correct me.

 

[mls26cwru]

when i decided to use oxalic, it was more of an educational thing, but it did seem to really help clean up the gold that dropped... I found a place that I could get it cheap ( 2lbs of dry oxalic crystals for $6.70) so i figured why not try it. That being said, it is more of a hassle and working with nearly boiling acids and hot bases are much more hazardous than using just SMB. Good washing procedures and patience are much safer in my opinion, but the learning experience was worth it in my opinion.

 

[freechemist]

But, when we deal with HCl/Chlorox, we have to make sure there is no excess chlorine before we precipitate.

That is done easily, by heating to the boiling point and holding at this temperature, until no more chlorine is escaping. Usually excess elemental chlorine is driven off quantitatively in a short time, in contrast to NOx from a solution of gold in AR.

When we use oxalic acid, we must adjust the pH of the solution to about 6 using NaOH or else the gold will not drop completely.

In my hands, addition of lye was never necessary. Reduction to metallic gold from HCl-solutions, containing 50-70 g HCl/liter (pH less than 0) occurred always smoothly and quantitatively on boiling under reflux overnight.

from what i have experienced, oxalic acid is selective in what metals it will precipitate (in the correct pH range), while NaOH is not selective at all and will drop almost any metal in solution.

Oxalic acid reduces gold to the metal, and thus precipitates it. Pd and/or Pt are not reduced to the metals. Oxalic acid will precipitate copper(II), if present, as practically insoluble, pale-blue copper(II)oxalate, even from strongly acidic solutions. NaOH first neutralizes excessive acid, and then makes the solution alkaline, precipitating dissolved metals as hydroxides, without reducing them.
Oxalic acid brings no additional chemicals to the reaction mixture, because it's oxidation product is gaseous CO₂, which is driven off easily.