# Oxalic Acid... to try or not to buy



## Acid_Bath76 (Aug 18, 2011)

I've been looking into using Oxalic Acid for my next batch, but the prices are a little steep. I see that it is also used as a wood bleach. Has anyone tried this wood bleach version of Oxalic or know of an affordable dealer? Thanks in advance.


----------



## dtectr (Aug 18, 2011)

Interesting that you start this thread, as I just did this yesterday.

I bought a gallon of Iron Out Out (that's the real name) for outside use, MSDS says it is 7-13% oxalic acid. There are no other chemicals listed, and it says its pH is less than 1.

I tried a small amount as is, with some 1st run auric chloride, that still had some base metals contamination. Both solutions were hot, the oxalic near boiling. The other metals precipitated out, leaving clean auric chloride which I then dropped with SMB.

On further reading, I needed to raise its pH to around 6 - 6.5 with ammonia or sodium hydroxide. I have also considered evaporating it down to concentrate it further. Still experimenting at this stage, however.


----------



## Acid_Bath76 (Aug 18, 2011)

Maybe I misunderstood you. The oxalic acid dropped the base metals leaving clean auric chloride? I thought oxalic was a very selective gold precipitant. Am I wrong?


----------



## dtectr (Aug 18, 2011)

When used properly, it sounds like it is.

However, if I was trying to use it properly, I made several mistakes:
1. I used a contaminated solution
2. The oxalic pH was untouched - so still around 1.0; its use calls for its pH to be raised to around 6, I believe.
3. The solution may be too dilute to be as effective? I don't know on that.

BTW, the precipitate was insoluble in concentrated HCl, so maybe metastannic acid?? Not sure.

I wish the chemist types would jump in & give us some direction, don't you?
EDIT: spelling


----------



## Acid_Bath76 (Aug 18, 2011)

Hoke didn't make it sound all that complicated. If you have the PDF, just search "oxalic", and you can hop right to the topic. Or just read below. Check it out, and tell me what you think. 

*PRECIPITATING GOLD WITH OXALIC ACID*
Oxalic acid is often used to precipitate gold from aqua regia solutions,
but since it is relatively expensive, it is usually saved for occasions
where the purpose is to obtain specially pure gold; that is, when the gold
you start with is already nearly pure enough for your purpose. Thus,
suppose you had your gold content down to 6-k or less, had dissolved
out the base metals with nitric acid, and had obtained a gold residue that
was almost pure enough to be used again. This in turn you had dissolved
in aqua regia, and you are now ready to re-precipitate it. This
aqua regia solution therefore contains only small amounts of other metals.
This is a case where oxalic acid might well be chosen as the most suitable
reagent.
You will observe that copperas and sulphur dioxide were employed on
solutions that contained (in addition to dissolved gold) much dissolved
copper, zinc, and possibly nickel and other metals.
Oxalic acid has this advantage over copperas: it does not introduce any
iron or other material which, if melted in with the fine gold, could hurt
its quality. The disadvantages are its cost, and the fact that when using
it you must heat your solutions, both the aqua regia solution and the dissolved
oxalic acid that is added to it, preferably to the boiling point. Hot
solutions, especially when bulky, are awkward to handle, and may be
dangerous. For these reasons, as we said, oxalic acid is usually reserved
for jobs where the quantity of solution is relatively small, or when you
are working with a solution that contains little or no dissolved base
metals.
The method of using it, therefore, will be described in Chapter XV
on the special purification of gold. You should become acquainted with
this method, even though you do not plan to use it regularly.

***************

For each Troy ounce of gold you will need at least a Troy ounce
of oxalic acid crystals. If you failed to expel all the excess nitric
acid, you will need more.
Dissolve the oxalic acid in hot water. Add a little ammonia, say
a teaspoonful or less to an ounce of oxalic acid; add slowly to avoid
spattering.
Do not use too much ammonia the mixture should not smell of
ammonia after being stirred. If it does, add enough additional
oxalic acid to kill the ammonia smell. If you have litmus paper,
use it here, and let the mixture show a slight acid reaction. Have
the mixture in a big beaker or jar that will withstand heat.
Next, heat your aqua regia solution to boiling. Also have the
oxalic solution hot boiling if possible. Pour the gold solution
slowly into the oxalic acid solution. Gold comes down at once with
hot oxalic acid, but takes hours or even days if it is cold.
Some people pour the oxalic into the gold; we think it is better
to pour the gold into the oxalic.
There will be bubbles and fumes, so work slowly or the liquids
may boil over when they come together. If you failed to expel all
the excess nitric acid, brown fumes will appear here; if this happens
you may have to add much more oxalic acid in order to kill all the
nitric acid.
Let the mixture stand for some time, keeping it hot if possible.
The gold comes down as bright clean crystals, often very beautiful,
which usually settle promptly.

** test with stannous to make sure you have all the gold..


----------



## qst42know (Aug 18, 2011)

As wood bleach (technical grade power) it's about $9.00 for 12oz. at Ace Hardware. Better prices are found here:

http://www.chemistrystore.com/cart.cgi?group=49922&child=49928


----------



## Acid_Bath76 (Aug 20, 2011)

Have you used their product? How do YOU use it? THanks for the link!


----------



## dtectr (Aug 20, 2011)

qst42know said:


> As wood bleach (technical grade power) it's about $9.00 for 12oz. at Ace Hardware. Better prices are found here:
> 
> http://www.chemistrystore.com/cart.cgi?group=49922&child=49928


I bought a gallon Iron Out OUT for $8.99 US at Menard's (7-13% Oxalic acid).


----------



## qst42know (Aug 20, 2011)

dtectr said:


> qst42know said:
> 
> 
> > As wood bleach (technical grade power) it's about $9.00 for 12oz. at Ace Hardware. Better prices are found here:
> ...




In my opinion I think iron out will be too dilute. It may work, but you need a large vessel near boiling and oxalic creates bubbles. An oversize beaker helps contain any losses to spattering.

Oxalic powder is very soluble in water and doesn't add any excess volume to create losses. I use two ounces of oxalic in 100ml of water and that should be plenty to drop an ounce of gold.


----------



## qst42know (Aug 20, 2011)

Acid_Bath76 said:


> Have you used their product? How do YOU use it? THanks for the link!



I have used the wood bleach but when it's gone I will order some from the link.

Hoke describes it's use on page 173, in the e-book the page is 182. 

Because I don't inquart for lack of proper nitric acid, I use it to precipitate the second time when palladium is detected.

It is fascinating to watch like an insane snow globe. Champagne bubbles and crystals forming from nowhere. You will however have purple specks all over your work space even if you are very careful.


----------



## Alentia (Oct 4, 2012)

I wonder if anyone has good success with Oxalic Acid precipitation on a regular basis. I am at it again and it seems the only solution to drop pH level of gold solution to around 3-4 with Na2CO3 or NaOH for for full precipitation. Anyone, any thoughts, experience?

Would be nice for step by step process to be posted. Hoke does not describe it very well in all the details.


----------



## samuel-a (Oct 4, 2012)

Alentia said:


> I wonder if anyone has good success with Oxalic Acid precipitation on a regular basis. I am at it again and it seems the only solution to drop pH level of gold solution to around 3-4 with Na2CO3 or NaOH for for full precipitation. Anyone, any thoughts, experience?
> 
> Would be nice for step by step process to be posted. Hoke does not describe it very well in all the details.



http://www.goldrefiningforum.com/phpBB3/viewtopic.php?f=61&t=10029&hilit=oxalic#p96880

Now days i do things slightly different, namely adding room temp' oxalic/oxalate soultion to the boiling gold solution rather then the other way around. i find it more convenient.
I'm waiting for new lab glass setup, when it arraives, i'll probably make a video of the entire process.


----------



## Alentia (Oct 4, 2012)

Thank you, Samuel!

I am normally dealing with half a kilo or more of material at a time for purification and my vessels are 6L, 13L, 22L and 50L reaction vessels. My minimum concentrated gold solution is at least 3 liters. It might be fun to use pipette...

I never get 100% drop with Oxalic... There is always gold left in the solution and I have to use different tricks to get it out. It is time consuming and not practical. Last time I had to raise pH of gold solution to about 4 for complete drop. My digital pH meter is not working now, need to replace probe. May be I am doing something wrong...

*Do you dilute gold solution with water, before pouring it into Oxalic solution?*

*What is your ratio of Gold Solution to Oxalic solution?*


One more thing... in your process, you indicate following:



> Wash gold powder: 3x water, 3x HCL, 3x D. water.



You need to replace HCL with HNO3 (weak solution should do the trick if purifying). The reason being, Oxalic drops tin and copper with gold. HCL will not dissolve copper.

Wash with HCL for SMB drop and with HNO3 when you drop with Oxalic.


----------



## samuel-a (Oct 5, 2012)

Alentia 

I'm not sure in what part of the process you are using the oxalic ppt. Is it the first refining or second?
I'm using oxalic ppt only as second refining cycle following SMB drop and washes, the first refining cycle produce well over 99.5%.
What follows the gold with SMB usually is Ag and Cu and Pd.

Pd and Cu are not easly reduced with oxalic as far as i know. Ag more so (and Tin).
I have no objection to wash with HNO3, though it will require an inciniration as you still have chlorides mixed in.
btw, hot HCl will definatlly dissolve Cu and Ag in given enough surface area (fine powder for example), so i would think HCl will suffice.

I thrive for a 2 Ozt per liter when using oxalic. the oxalic/oxalate solution is as concentrated as possible (which is hard given the low solubility of sodium/ammonium oxalate). the reason i say 'thrive' for this conc' is, that i don't have the luxury of haveing large glass.

I'm seeing same thing as you as far as incomplete precipitation inspite pH adjustments. Though, i can afford not to bother with it and just ppt the very last portion (which is usually less then 0.5 gram per liter) with SMB and put it in the next batch. Maybe you can do that too, it cuts alot of time and energy.


----------



## Alentia (Oct 7, 2012)

Samuel, you are good man!

Bringing Oxalic solution to pH6 worked very well for me. Out of 500gr of material, I had collected remaining 2gr with SMB. So basically, Oxalic drops 99.6% of gold.

I was using suggestion from Hoke, using 1 spoon for each ounce of dry Oxalic, in reality you need way more than that.

I use Oxalic for purifying (second refining). Bringing solution up to pH4-5 and as I did before, is very tedious work, as you need to drop about 50+ spoons of Na2Ca3 into gold solution, waiting about 2 min each time for bubble to disappear before next spoon.

For those, who does not use Oxalic... It is very nice to able to actually roll the gold onto small balls with bare fingers (use gloves though), which you can not do with powder after SMB drop. And there are salts and minor residue left in AR vessel after digestion from looking like pure gold after first refining. And the gold dropped after Oxalic second refining is almost canary yellow, versus orange-brown powder from SMB.

I hope you can afford large vessels soon!


----------



## leelandbullock (Oct 14, 2012)

this is the one that had me stumped..

but i think either comet or ajax cleaner contains oxalic acid..not sure if id use it though...lol


----------



## kadriver (Oct 17, 2012)

Wow this stuff makes some pretty gold. I used oxalic acid for the first time last night.

I inquarted with silver, then dissolved the gold in hot AR.

After denoxxing and filtering, I dropped the gold with SMB, then washed with HCl.

I then dissolved the gold powder a second time using HCl/Cl.

It started off good, but then slowed to a crawl, so I added nitric acid - one drop at a time. Most of the gold dissolved after adding about 4 or 5 drops of nitric acid.

I had just a few tiny pieces left to dissolve - the nitric was gone, so I added a little more chlorox.

The chlorox did the trick.

After filtering the gold chloride, I then dissolved 2 table spoons of lab grade oxalic acid in about 100ml boiling distilled water.

I then added several ml of 29% ammonium hydroxide to the oxalic acid. It took a little more ammonium hydroxide, but I got the pH up around 4 or 5 - I used pH test papers.

I put the gold chloride on one hot plate - heat on full blast.

I put the oxalic acid on the other burner and added lots of heat.

With both solutions boiling, I added the oxalic acid to the gold chloride solution.

At first nothing, but then the gold precipitated into a gelatinouss mass that reminded me of pond moss, only brown instead of green.

I allowed it to settle over night, than rinsed and washed the gold with HCl.

The gold came out very clean and very bright.

From now on, I will always use oxalic acid when I double refine my karat gold scrap.

kadriver


----------



## kadriver (Oct 17, 2012)

I bought some wood bleach from Ace Hardware.

I requested and received an MSDS for the wood bleach.

The MSDS states that the purity is "95% to 100%" - no other ingredients listed on the label of the product or the MSDS.

Next time I'll use the wood bleach instead of the lab grade oxalic acid to see if I get the same superior result.

kadriver


----------



## kadriver (Oct 21, 2012)

Here are some photos of the gold dropped with oxalic acid.

I used the lab grade again instead of the wood bleach.

I'll melt it into a bar and post a picture of that.

Total weight after drying was 47.0 grams of fine gold.

I weighed the dish at 295.2g and again with the dry gold at 342.2g

This is gold recovered from karat scrap and 500 grams of gold filled scrap.

kadriver


----------



## kadriver (Oct 23, 2012)

Here is the gold bar (and two gold beads for my personal savings) from the oxalic acid precipitation.

This was only my second time using it and I forgot to raise the pH before doing the precipitation.

I had a hard time getting the gold to come down, but if finally did after adding more oxalic with the pH properly adjusted (I think I had pH at 4 or 5 with Ph test strips).

The gold is visibly cleaner and brighter than any I have done in the past with just SMB precipitations.

It assayed at 99.93% - almost made it to the industry standard or 99.95%

NTR does an XRF before melting, then another after the gold is molten to see if there is a big difference. They will not let me witness the process.

kadriver


----------



## butcher (Oct 23, 2012)

That is beautiful, you only made one big mistake, you should have sold the small ones and kept that big one, it was too pretty to sell.


----------



## kadriver (Oct 23, 2012)

Hey Butcher, thanks for the compliment. I'm like you, I wish I could keep the big one. 

kadriver


----------



## qst42know (Oct 23, 2012)

I'd bet your gold was cleaner than the NTR xrf indicated. 

How did the dish look?


----------

