Ted Mooney, P.E. RET
- Pine Beach, NJ
The authoritative public forum
for Metal Finishing since 1989
-----
Sulfite gold plating -- Problems & Solutions, Q&A
Q. My Question to Maurice:
Were you successful to find an antioxidant to slow down the oxidation of sulfite to sulphate in your alkaline gold plating bath?
Process Engineer - Massachusetts
May 20, 2023
⇩ Related postings, oldest first ⇩
Q. I am conducting a feasibility study on production of Potassium Gold Sulfite. I need some information about the following:
a. how to produce Potassium Gold Sulfite
b. uses of Potassium Gold Sulfite
c. cost per gram
Thank you very much,
Chunyi ChangFoxconn - Taipei, Taiwan
2003
Q. Hi all, I was wondering if someone could help me. I have heard that it's possible to gold plate jewelry without the use of cyanide, using something a lot less toxic, and an anode instead of the gold in the solution. I would appreciate any feedback.... thanks
Chris Kalogerasopal trading - Adelaide, S.A., Australia
2006
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.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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. 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: - Cerovski vrh Croatia |
Multiple threads merged: please forgive chronology errors :-)
Q. Hi,
I am involved in a research on gold plating for a Temple project in India.
I am radio electronic inventor, and can deal with chemistry as well.
We will have to gold plate a big area of the roof of the temple here. But it will be very difficult to control the quality of gold plating here in India if we order some. So, I am looking for Your help and advise for how I can prepare my own chemicals for electroplating from metallic Gold and any other chemicals (all available here in the market)
But I have heard that cyanides are very poisonous. So, I would prefer Chloride or DiPhosphate of Gold.
Can it be prepared? It will be also cheaper, I believe.
Hope this meets You in good health.
inventor, monk, minister - Vrindavan, UP, India
2007
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.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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!
- Cerovski vrh Croatia
on AbeBooks
or Amazon
(a reprinting of a 1924 book)
(affil links)
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?
Monterrey, NL, Mexico
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!
- Cerovski vrh Croatia
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.
Derek BlaisGraduate Student - St-Lambert, QC, Canada
Q. CYANIDE-FREE BATH?
Hi I just came across this forum. I am looking for cyanide free bath recipes for gold electroplating. I know there are some products readily available, but I did the math: even on eBay if you calculate the cost per gram of gold content costs least twice of the regular gold price. Plus shipping. Some companies do not ship to my country.
So I decided to do it myself. I studied science like a quarter of a century ago, I am not afraid of chemistry.
Here are the choices:
1) Cyanide containing bath solutions - out of question
2) potassium ferrocyanide
⇦this on
eBay
or
Amazon [affil links]
+Potassium carbonate+gold chloride
I know ferrocyanide is not very toxic, BUT once you prepare the mixture I think you exchange the Fe ions with K ions , you filter the iron oxide and you will have a KCN solution or least K[Au(CN)2] . The product is just as toxic as Type 1. Does anybody know the equation?
3) Gold sulfite
I did some research and I see it is not easy to produce at home
4) Fizeau's solution: 1g gold chloride + 4 g sodium hyposulfite in 1 litre of water. Did anybody try this? pH? Temperature?
5) :
1 gm gold chloride
75 gm sodium phosphate
2,5 gm sodium sulphite
1 lit water
stainless steel anodes
Did anybody try this one? pH, temperature?
6)
Chloride base bath:
gold chloride.......25-40 gm
HCl(1,19 gm/l)......20-50 gm
sodium chloride.....10-30 gm
H2SO4(1,84 gm/l)....10-20 gm
water................1 lit
temp.................70 °C 8-10 A/dm2
Who used this one?
7) Gold chloride itself. Some old books say it should work as well. What are the problems with that?
8) Any other CN-free options ?
Thanks a lot
- Toronto, Ontario, Canada
June 1, 2013
Multiple threads merged: please forgive chronology errors :-)
Q. We're running a small tank (4 gallons) of bright gold bath. We recently purchased a new bath. Always in the past they have worked fine right out of the bucket. This bath, first test piece, after 1-2 minutes in the tank (spec 3 ASF, good temp & agitation) looks a godawful reddish-brown, so I pulled it. I start checking, new bath is below spec on pH (8.6 vs 9-10), so I adjust, try again. Same thing, piece looks ruined after 1-2 minutes.
We keep working with the process and eventually find that a piece will look ruined at 1-2 minutes, but if we stay in the tank for 5-6 minutes it begins to brighten and ends up looking Okay. When the piece starts to brighten, the plating voltage drops from say 0.8 to 0.65 Volts (running a constant current supply at 200-300 mA, for 3 ASF)
I'm working with tech support but they've never seen this before. I've never seen this before, running the same process for 10 years. This is a sulfite gold with arsenic brightener.
Has anybody seen this kind of thing, know a probable cause?
Thanks,
Plating manager / plater - Charlottesville, Virginia, USA
January 8, 2009
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.
Pat Mentone
St Paul, Minnesota
by Reid & Goldie
(hard to find & expensive; if you
see a copy cheap, act fast)
on eBay or
AbeBooks
or Amazon
(affil links)
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
T.K. Mohan
plating process supplier - Mumbai, India
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
Plating manager / plater - Charlottesville Virginia USA
Fighting sudden undesired plateup in sulfite-based gold plating bath (Techni Gold 25E)
Q. Hi everyone,
My name is Dominic Frutiger. I am with the Institute of Robotics and Intelligent Systems at ETH Zurich, Switzerland. It is an university and not industry, but my question might be general enough to be of interest to others.
We produce some of the currently most successful microrobots, i.e., little mobile agents that are only a few widths of a human hair -- video illustrating this:
For one part of the robot we use electroplated gold in high-aspect ratio trenches (e.g. 6 µm wide, 35 µm high, plating 25 µm of gold into it). So far we have been relying on Techni Gold 25E and it gives good results for a while, say 6 months or 20 wafers -- but then it invariably "fails". Failing here does not mean it stops plating. The deposits may still look nice and shiny. It just starts to exhibit significant plateup on photoresist walls which is really a show stopper for us (say 5 µm plateup for 8 µm of normal, smooth deposit).
We checked all the parameters and cleaned the bath. We added gold and other required ingredients. We are not aware of any pollutants, but then again that is hard to know. The supplier apparently has no further advice to offer.
Now we are clearly more like your garden shack type platers with little professional know-how when it comes to electroplating gold -- but there must be people out there that have experienced a similar problem (probably with the same product even) and that might be able to help with pointing us to say a particular leveling additive or a different product or with some other general piece of advice.
We are of course looking into different options and processes in the meantime that do not include gold. However, this is an important and expensive piece of equipment and it would be great to be able to "recover" it and make it usable for this particular purpose again.
Thank you
Dominic
microrobotics researcher - Zurich, Switzerland
October 7, 2009
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
Geoff Smith
Hampshire, England
Structure of the Gold-Sulfite complex
by Reid & Goldie
(hard to find & expensive; if you
see a copy cheap, act fast)
on eBay or
AbeBooks
or Amazon
(affil links)
Q. Aurous ion forms a complex with sulfite ion. I gather that the SO2- ion is shaped like an equilateral triangle, with each of the 3 'O's at the vertices. The aurous ion (I think) has an empty 's' orbital, and a completely filled set of 'd' and 'f' orbitals.
How does one ion "grip" the other?
Dave Wichern
Consultant - The Bronx, New York
June 7, 2012
A. Solvation Energy?
Hamilton Solidum- Mays Landing, New Jersey, USA
Thanks for replying.
Sure, the thermodynamics of the complexation are the cause of them sticking together. But, what I meant was; what is the geometry of the complex?
Dave Wichern
Consultant - The Bronx, New York
How To Extend The Life Of A Gold Sulfite Bath
Q. Gold Sulfite Electroplating Baths are known for their instability as the AuroSulfite Complex is not very stable in an aqueous solution. The uncomplexed Gold(I) cations disproportionate, forming metallic Gold and Gold (III) cations. Apparently, one of the undesirable reactions in the bath is the oxidation of Sulfite anions to sulphate anions. This cause the density to rise. The vendor suggests that the bath be changed when the density reaches 25 degrees baumé. Can someone suggest a way to slow down the oxidation of Sulfite to sulphate or how to use the bath after the density reaches 25 degrees baumé? The operating temperature of the bath is 50 °C and the operating pH is 8.0
Maurice MasonConsultant - Danbury, Connecticut, USA
May 7, 2014
A. Hi Maurice,
One way is to add in antioxidant like catechol to slow down oxidation of sulfite to sulphate.
Regards,
David
David Shiu
- Singapore
Q. Hi David,
Have you ever used Catechol yourself as an antioxidant? Antioxidants are usually reducing agents, and reducing agents could reduce the Gold(I) cations in the bath to metallic Gold. Dithionite is a reducing agent produced from Sulfite at the Cathode which reduces the Gold(I) cations to metallic Gold in the bath. If you used Catechol, what concentration did you use?
- Danbury, Connecticut, USA
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
David Shiu
- Singapore
Hi David,
Thanks for your response. I will check with my bath supplier to get their thoughts on Catechol. My concern is that Catechol, a reducing agent, could reduce Gold(I) to metallic Gold. Dithionite, formed by cathodic reduction in the Gold bath, actually reduces Gold(I) to metallic Gold as well.
Regards,
Maurice.
- Danbury, Connecticut, USA
Q. Hi David, I checked with the Chemical supplier. The response was that Catechol is used in an acid Tin bath not in an alkaline Sulfite Gold bath. Did you ever get it to work in an alkaline Sulfite Gold bath?
Thanks,
Maurice.
- Danbury, Connecticut, USA
May 19, 2014
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
David Shiu
- Singapore
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
- Mays Landing , New Jersey
June 26, 2014
Q. Hi Hamilton,
Excellent suggestions. I will try these low potential oxidizing agents and see how they work. As long as they will not oxidize Sulfite to sulphate they will work. I measure the specific gravity in baumé units on a daily basis to give an indication of the formation of sulphate. sulphate is formed by oxidation of Sulfite mainly form atmospheric oxidation as well as anodic oxidation. Do you know if bath efficiency as well as conductivity are affected by sulphate formation? I suspect that bath efficiency drops as the bath ages leading to more Hydrogen evolution. What are your thoughts on this?
Thanks, Maurice
- Danbury, Connecticut USA
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.
- Mays Landing, New Jersey USA
Q. My Sulfite Gold efficiency decreases as the bath ages resulting in increased Hydrogen evolution leaving circular voids called "fish eyes" in the deposits. I believe increased TOC levels is a contributor. Can someone give me a reference to some literature on how TOC affects Sulfite Gold bath efficiency and bath deterioration?
Thank you very much.
- Bloomfield, Connecticut USA
November 27, 2015
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,
David Shiu
- Singapore
Q. Hi David, Thanks for your response. The Organics in an aging Sulfite Gold Bath are usually derived from The Photoresist interacting with the Gold Bath solution owing to the alkaline nature of the bath. The pH is approx. 8.0 and the Photoresist belongs to the AZ "family" of Novolac Photoresists. (Phenol Formaldehyde copolymer) - Danbury, Connecticut, USA December 24, 2015 A. Good day Maurice. Aerotek Mfg. Ltd. - Whitby, Ontario, Canada |
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?
Geoff Smith
Hampshire, England
December 31, 2015
Q. Hi Geoff,
Thanks for your response. I am not looking for Sulphite Gold, per se. What I really need is Non-Cyanide Gold mainly because of Environmental, Health, and Safety issues as well as the ability of Cyanide to attack the Novolac Photoresist I am using. I chose Sulphite Gold mainly because it was recommended as a suitable alternative that would meet my requirements for throwing power, fine grain structure, etc.
Maurice
- Danbury, Connecticut, USA
January 3, 2016
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?
Geoff Smith
Hampshire, England
Q. Hi Geoff,
Thanks for your response again. I totally agree with you about This "Cyanophobia". Unfortunately, both Upper Management and The Environmental, Health, and Safety folks dictate what chemical processes are used in their facility. No way will they ever approve any chemical process that uses Cyanide. Therefore, I must find a non-Cyanide alternative. Can you help me with the Sulphite Gold Electroplating process?
Maurice.
- Danbury, Connecticut, USA
Hi Maurice. Hopefully someone will help you! If Geoff will help you do something he disbelieves in, he's a better man than I :-) I can't tell you how many people have absolutely insisted that I instruct them how to eliminate phosphating before painting, after I've clearly said I think it's a sustainability disgrace that (with transportation between almost every step, no less) we re-mine, re-smelt, re-roll, re-fabricate, re-paint, re-package, re-sell, and then landfill millions of tons of steel items that now last 2 or 3 years instead of 20-30 when we've known for 75 years how to easily fix that environmental travesty with an ounce of phosphate per ton of steel :-) Although we can't beat city hall, our bosses, & society at large when they make decisions, we 'experts' should at least raise a sufficient stink to insure that their attempts at plausible deniability fall flat :-) Regards, Ted Mooney, P.E. RET Striving to live Aloha finishing.com - Pine Beach, New Jersey |
A. Good day Maurice. Aerotek Mfg. Ltd. - Whitby, Ontario, Canada January 5, 2016 Ed. note May 2023: www.scielo.br is still functional but that article is no longer available at that URL. A. Hi again Geoff Smith Hampshire, England |
Q. Hi Ted, Geoff.
Management and EHS have already decided that they do not want a Gold Cyanide process. This is entirely outside of my control. Geoff has made his position on Sulphite Gold baths clear. Apparently he is not in a position to help me. Geoff,
Is this correct? I was unable to access the article you sent me.
Hi Eric,
Thanks for your response. Here is my question again.
"My Sulfite Gold efficiency decreases as the bath ages resulting in increased Hydrogen evolution leaving circular voids called "fish eyes" in the deposits. I believe increased TOC levels is a contributor. Can someone give me a reference to some literature on how TOC affects Sulfite Gold bath efficiency and bath deterioration?"
Thanks,
Maurice.
- Danbury, Connecticut, USA
January 6, 2016
Q. Is Gold potassium sulphite better than Gold potassium cyanide chemistry to plate a 20-30 micron thick gold layer on copper or brass?
Parameters important to us are hardness (wear & tear), porosity, colour (our target is to achieve 24K gold colour), and purity.
Percept - decorative coatings - cairo, egypt
April 25, 2019
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