How to prevent colloidal gold formation in L.-P. Clerc gold-thiourea toner ?

[largo]

To tone my VanDyke brown and kallitype prints, I use a gold-thiourea toner solution made from 1% gold chloride, 1% thiourea, a pinch of tartaric acid, and some sodium chloride. This is the gold-thiourea formula from Louis-Philippe Clerc, published in his book Photography - Theory & Practice, 3rd Ed. (1954), page 396.

The recipe is as follows:

- Gold chloride (1% solution) : 50 ml
- Thiourea (1% solution) : 50 ml
- Tartaric or citric acid : 0.5 g
- Sodium chloride : 20 g
- Water to make 1000-2000 ml

I do prepare the minimal amount of solution needed to tone an image of a given surface/size in cm². After toning, I pour the used solution back into a bottle. After a while (days), I observe a red-purple deposit at the bottom of this bottle. I do think this is colloidal gold and at this point the toner is wasted.

To explain things more in details (with my limited chemistry knowledge ), what happens when I mix the 1% gold chloride solution with the 1% thiourea solution is I think a redox reaction, leading to the reduction of yellow gold(III) to colorless gold(I), while thiourea gets oxidized and forms a stable complex with gold(I).

The added NaCl helps to stabilize this gold-thiourea complex (preventing Au(0) formation) I think. The role of the tartaric acid remains unclear to me, but I think it helps to stabilize the toner as well.

When the print gets toned, the gold-thiourea complex is reduced to metallic gold, which deposits on the silver image. This is for sure. Now whether the silver gets replaced or covered by gold is not clear to me, I have read both possibilities. If silver is replaced, then some silver compounds would end up in the solution. In any case, what is certain is that as the gold-thiourea complex is reduced to metallic gold on the image, the used toner solution becomes exhausted in aurous/auric gold cations, and excess thiourea is now present. This is the state of the used toner solution which I collect in a brown glass bottle. Note that it's still colorless at this point.

Excess thiourea present in the used toner solution helps further reduction to metallic gold (Au(0)) over time (several days), leading to the formation of gold nanoparticles or colloidal gold, which is what I observe as a red-purple deposit at the bottom of the bottle. If already a bit exhausted but still usable, the toner now becomes even more exhausted over time, because of the formation of this colloidal gold. This is what I'd like to prevent, because I think the toner is still usable. And because once the Au(I) gold has been reduced to colloidal gold, it is absolutely no longer usable for toning and there is no way to turn it back to its aurous or auric forms (unless using aqua regia ?).

(I think the excess of thiourea favors the formation of colloidal gold, but you would still get this colloidal gold deposit if you would leave a freshly prepared gold-thiourea toner solution in a bottle for a few days, even if you don't use it to tone a print...)

So, to summarize: the used toner solution is still usable for toning (even if less performant) as long as you prevent the formation of colloidal gold in the bottle by stabilizing the used toner solution. The question is: how to stabilize it and prevent the colloidal gold formation in the bottle ?

I thought of the following options:

- Lower the pH slightly to slow thiourea decomposition ?
- Store in a full, airtight bottle to minimize oxygen exposure ?
- Add more sodium chloride to help maintain gold in a soluble complex ?
- Refrigerate the exhausted toner to slow down thiourea breakdown and gold reduction ?
- Filter before reuse to remove nuclei that maybe trigger further precipitation ?

(I don't want to add some more 1% gold chloride solution to the used toner to replenish it, I would actually like the remaining gold to stay in solution and not precipitate as colloidal gold.)

What do you think of these options ? Are they valid ? What would be in your opinion the best way to prevent colloidal gold formation in the toner ?

Now, in cases where it's too late, and colloidal gold has already formed (I have several bottles in this case, even though I try to prepare the minimum amount of toner each time), would it be possible to recover the metallic gold from these deposits? Do you think this is feasible? If so, how can I do it ? What would be the steps to recover the gold from the used solution ? Simple filtration, wash and drying ? I guess there would be impurities, like sulfur/thiourea residues... Further chemical reduction ? Electrochemical recovery with a copper electrode for instance ?

I am not a chemistry expert, and also I wonder if recovering the gold is really worth the effort. Gold makes up approximately 50% by weight in gold(III) chloride. I bought 1 gram of gold chloride, so at most I would recover 0.5 grams of gold from the used toner solution. But in reality, much less since most of the gold has been deposited on the toned prints. Let's say I recover 0.1 grams of gold from the used toner solution, it is worth about 9€

Your feedback and ideas are welcomed. Thank you in advance for your help and advice!

 

[revdoc]

The obvious question: was the toner exposed to light? Light exposure will create colloidal gold.I had this happen with gold acetate toner. Not much light was needed.

My approach to using Clerc's is to make up the main solution (thiourea, acid, salt) without gold. When I need toner, I measure out enough solution for one print, then add gold with a dropper. One drop of 1% gold solution per ml. I use the result once and discard.

 

[koraks]

My approach to using Clerc's is to make up the main solution (thiourea, acid, salt) without gold. When I need toner, I measure out enough solution for one print, then add gold with a dropper. One drop of 1% gold solution per ml. I use the result once and discard.

This is more or less what I do, too.

What do you think of these options ? Are they valid ?

I think what @revdoc and myself do is really the most sensible/practical solution - make as much toner as you'll need and discard the more or less spent toner afterwards. You can use this toner until it's virtually entirely inactive, and then make up some new if your print hasn't finished toning yet. If you work like this, the fraction of gold left in the waste will be so small that it's not going to be economical to try and recover it - not even if it relies on cheap chemicals and you don't put a price tag to your time.

Of course, from an academic viewpoint, your questions are interesting, so by all means see what comes up. I do recognize the formation of colloidal gold in spent gold/thiourea toner; I've seen it several times as well when I still kept the spent toner around. It's kind of pretty, in a way. Note that it takes only a tiny, tiny amount of gold to make quite an intense red color, so the little bit of orange strands you get in a spent thiourea toner evidence an amount of gold that may be down in the low microgram regions.

- Filter before reuse to remove nuclei that maybe trigger further precipitation ?

I think if you start determining the required filter mesh, you'll realize this isn't going to work. You'll be working with mesh sizes below that of regular molecular sieves. I think the cross-section of a single gold atom is something like 0.1nm. Based on this, let's say you want to filter out anything above 10nm. That's not a practical proposition.