Sulfite gold plating -- Problems & Solutions, Q&A

A. Hi Chris. Acid gold cyanide is not nearly as toxic as sodium cyanide itself, but there are also gold sulfite and other gold plating baths which are cyanide-free.

I am not aware of any gold plating bath that uses gold anodes, but this is probably not due to technical limitation so much as the general impracticality of buying and safely securing hundreds of pounds of gold.

A. What's the big deal with cyanide, anyhow? It's used by the ton, hot enough to be molten, in some metal heat treatment processes. If you're using a solution of a cyanide in water don't drink it just as you don't drink any other poisonous liquid, and don't add excess acid to the solution just as you don't indiscriminately mix other substances. Bill Reynolds [deceased] consultant metallurgist - Ballarat, Victoria, Australia We sadly relate the news that Bill passed away on Jan. 29, 2010. A. You can use next solutions: 1 gm gold chloride 70 gm sodium phosphate 2,5 gm sodium sulphite 1 lit water stainless steel anode,only grease and oxide free objects can be plated Or you can use ferrocyanide based solution (0,5% of potassium cyanide toxicity): 2,65 gr gold chloride 15 gm potassium ferrocyanide ⇦this on eBay or Amazon [affil links] 15 gm sodium carbonate ⇦this on eBay or Amazon] (water free) 1 lit water Stainless steel anodes Dissolve ferrocyanide in boiling water; when dissolved add carbonate, and when it is dissolved add gold chloride. Boil it 30-180 minutes, add evaporated water, filter it and you can use it. Good luck! Goran Budija - Cerovski vrh Croatia

Usually available on eBay;

sometimes on
AbeBooks or Amazon

(affil links)
free pdf is currently available from academia.edu

A. Hello Alexander. I don't really agree with the premise that formulating your own gold plating solutions is necessarily more economical; and I think it may be difficult to control the quality of the gold plating solution if you don't order it from a qualified supplier (although a researcher in an advanced facility with state of the art instrumentation and good access to technical info has a fair chance of preparing an acceptable gold plating solution).

And when dealing with precious metal plating chemistry, the precious metal is so expensive, and waste becomes so costly, that any value-added cost for the process chemistry is marginal. Your money, your choice, but personally I'd pick a brush plating supplier and order their equipment and chemistry.

Ideally you should get some plating textbooks, but if that proves impossible then you can view the Metal Finishing Guidebook on-line, which has some formulas for gold plating baths as well as some instruction. Best of luck.

A. Two cyanide free plating formulas (both work as ordinary plating solutions but can be used as brush plating solutions):

No 1:
1 gm gold chloride
75 gm sodium phosphate
2,5 gm sodium sulphite
1 lit water
stainless steel anodes

No 2. 2,65 gm gold chloride
15 gm potassium ferrocyanide ⇦this on eBay or Amazon [affil links]
15 gm sodium carbonate ⇦this on eBay or Amazon]
1 lit water
stainless steel anodes,
(dissolve ferrocyanide in boiling water,when dissolved add carbonate, then add gold chloride. You can make your own: dissolve gold in aqua regia, boil 1,5 -2 hours; when cold, add evaporated water and filter it). Test it first on some metal scrap.
Hope it helps and good luck!

A. Alexander,
I agree with Mr. Mooney. Trying to develop your own chemicals and process just to find out something is going wrong over and over again with such an extremely large and complicated project, especially when one of the most expensive metals is involved, may prove to be not just a technical but a financial disaster. Get professional equipment and products and expert advise.

Mr. Budija
Regarding the formulas given, do you know what their pH range is? Is there any pretreatment necessary to obtain adhesion or to avoid attack to, or diffusion into the substrate?

A. Dear Guillermo!
These two baths are old 19th century formulas and both work -- bath no .2 (from Langbein's handbook, but this type of formula is much older/1844./) can be used on brass, copper, nickel, and iron, tin and pewter without any special pretreatment (oxide and grease free surface). That's all I know.

Best and simplest solution for that job is leaf gilding.
Hope it helps and good luck!

A. I don't know about the use of stainless steel anodes, I would figure that it would cause a cementation of gold chloride into metallic gold. I would recommend a platinum electrode for the low overpotential.

A. Gerry

Sulfite gold baths are very sensitive to voltage. I would check the anodes and make sure they are clean before you begin to plate and not covered by an insulating layer of sulfur or sulfite breakdown products.

"Gold Plating Technology"
by Reid & Goldie
(hard to find & expensive; if you
see a copy cheap, act fast)


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A. Hi Gerry,
I think there is deficiency of initial brightener addition in the bath. Try adding 3 mls/ltr and see if there is any improvement. You may need to add up to 10 ml/ltr or more.
Please let us know the result.
Good luck

Q. Thanks for the interesting replies. We're still working on this, our vendor is issuing an RMA for faulty product. Your comments, if we did a brightener add like that it would drive the bath way high on arsenic, per analysis done on this bath and spec. But a plater who used to work here with that process would have done exactly that (his personal approach was "always add brightener") and who knows, it might have fixed it? We have done small adds of conducting salt and brightener with no positive effects.

Comment on the anode is interesting too. We started with the "old" anode which did have a coat of yellowish stuff (sulfite or gold?), then switched to a newer clean anode set. We run constant current supplies and didn't note any unusual voltages or voltage drops in the system.

Do anodes have a life expectancy?

Thanks, Gerry

A. Hi Dominic

A few ideas to work on --

Sulphite golds tend to be very sensitive to pH. I have seen an entire large bath "fall apart" and deposit all the gold on the bottom! More commonly there can be bridging of gold between adjacent conductors which was traced to cleaners trapped under masking tape leaching out to locally change the pH.

The photoresist could be a source of trouble. Positive resists are acidic in the exposed area. That is why they develop in alkaline solution. If the developing is incomplete, there can be local acidity. This is usually not noticed but you are working with very small dimensions.

So ... You need good pH control. If you are not familiar with pH meters you will need to learn to use one properly. The procedure in the plating industry is often dip it in and read. For critical work it is a little more complex.

Good exposure control and developing of resists requires thickness control, exposure step wedge, control developer chemistry, very thorough rinsing, etc.

It is also worth considering a drag in bath before plating buffered at the pH of the gold tank.

Hope this helps

A. Solvation Energy?

A. Hi Maurice,

One way is to add in antioxidant like catechol to slow down oxidation of sulfite to sulphate.

Regards,
David

A. Hi Maurice,

Au in trisodium gold disulfite is present as (AuS2O6)3- anion complex (with 3 Na+ cation) which easily be destroyed whenever disulfite is oxidized to become 2 sulphate ions. Weak antioxidant like catechol can help to stabilize (AuS2O6)3- anion complex to minimize presence of free Au+ cation to undergo disproportionate reaction to become gold metal. The amount of catechol or equivalent antioxidant has to be carefully controlled at proprietary chemical supplier recommendation.

Regards,
David

A. Hi Maurice,

Catechol can be used as antioxidant for tin MSA bath, gold sulfite bath and some other plating processes. As mention earlier, catechol or equivalent antioxidant can be a way to minimize presence of free Au ion to cause gold precipitation which is the main point I would like to express. You may check with chemical supplier for any equivalent antioxidant they recommend for gold sulfite bath.

Regards,
David

A. Hi Maurice,
US Patent No. 4,192,723 Mar.11, 1980. " EXAMPLE 6 Comparison between the Useful Working Life of the Invention and That of the Gold-Sulfite Baths of the prior Art

An extended comparative electrolysis was performed of the Bath A disclosed in Example 1 and of a bath of the prior art: Example 2 Of U.S. Pat. No. 3,475,292. With the latter, crystallization and precipitation of solids, namely sodium sulphate, occurred in the tank before plating a total of 200 g of gold metal per liter of the bath were achieved. In the case of bath A, it was possible to plate more than 500g of gold per liter of the bath with no observable crystallization nor precipitation of solids."

There is also good information on the cathodic reduction of sulfites to lower valency sulfur compounds such as for instance, pyrosulfites, dithionites, dithionates and tetrathionates. which are reducing agents, has a detrimental effect in the plating and possible formation of colloidal gold. Addition of an oxidizing compound, the oxidation-reduction potential of which is not enough for oxidizing the sulfites to sulphate, but, nevertheless, sufficient to destroy the divalent sulfur compounds is desirable especially when it is desired to operate under current densities higher than 0.5A/dm2. Examples of such oxidizing compound, p-nitrophenol, 2,4-dinitrophenol, 2,6-dinitrophen, 2,4,6-dinitrophenol. For having a noticeable effect, these compounds can be added at concentrations of about 5mg/l or more."
Note that BATH USES an AMMONIUM GOLD SULFITE COMPLEX

FYI, good luck

A. Hi Maurice,
Hydrogen evolution takes my thought to the diffusion layer (electrical double layer), where Faraday's Law no longer applies. As you deplete the gold from the layer, it is replenished by diffusion of gold ions into the layer. If the rate of replacement is lower than plating rate then the excess current will reduce/oxidize the next available compound based on its redox potential.

sulphate formation Vs. Efficiency, do not know. However, for me it is easy to check. Run Hull Cell at 0.5 amps for 2 minutes. Weight gain for Tri gold is 40 mg. Next add sufficient amount of sodium/potassium sulphate to raise baumé to a specific range you want to check. Run your Hull Cell and find your efficiency. If efficiency is low, increase gold content, add compatible wetting agent and run your Hull Cell. Check for improvement. If none, you need a totally different approach.

Hi Maurice,
Total organic carbon (TOC) unable to differentiate "good" and "bad" organic byproduct which impact a plating bath performance in different way. Some organic present in significant amount won't deteriorate plating performance (consider "good" organic byproduct) while some even present in ppm level could poison the bath (consider "bad" organic byproduct) hence we can't simply create a chart to show how TOC affects Sulfite Gold bath efficiency and bath deterioration.

What we can do is to base on each plating line situation (specific plating article, plating chemicals, materials, plating resist, etc) to study TOC vs plating bath efficiency/deterioration apply for that particular plating line only. However, once any major change on that particular plating line (e.g. plating article, plating chemicals, materials, plating resist, etc) then we need to re-study the relationship again as types of organic byproduct build up in plating bath would be different whenever a change of materials used.

Hope you understand the rationale behind.

Regards,

A. I am sure that gold sulphite baths are improved from when I last used them but they remain intrinsically far less stable than standard GPC based processes.
Where my standard baths worked for years with minimal maintenance, sulphite baths were a constant problem.
They are also much more expensive to run and I would only consider them where their increased throwing power is essential.

So my question is - why do you need sulphite chemistry?

A. Hi Maurice

It is a problem in our industry that the word cyanide generates instant panic in management and safety circles.
And that leads to replacing a well understood process with a much less understood substitute.
In practice, gold cyanide solutions have very little free cyanide and the gold content is arguably the more toxic element. It is worth asking your adviser to quote the toxicity of both the cyanide and sulphite baths - it is measured as LD50. My bet is that they do not know. And what is the first aid treatment for accidental ingestion of the sulphite bath (which often contains arsenic)?
The subject is too large to discuss fully but you will find a paper on the subject at
https://independent.academia.edu/GeoffSmith17/Papers

Problems with photoresists often boil down to poor exposure control. Do you use a step wedge?