Letter 27051

Silver plating was very dull and not mend [Thailand] 

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I have a cyanide silver bath. Last month it dont work. I made Hull Cell test and result of it very dull middle current to low current and high current was soft dull. I added silver cyanide to Hull cell bath then result its worked. But now, after I made Hull Cell test result it dull similar as last month. I added silver but it not worked. I think higher carbonate in bath. But I have not instrument for analysis of carbonate. I have been wanting silver plating is work . I dont know how to addition or edit for bath. I was to venture add brightener but added very , dull was very.

Help me please. Thank you very much.

Tawan
lab supervisor - Bangkok, Thailand

Tawan,

To analyse for Carbonate content you can use the following method: Pipette a 10ml sample of plating solution into a 600ml squat form beaker, add 300mls of de-ionised water and heat on a hot-plate with a boiling rod. *DO NOT BOIL*. Add 10% Barium Chloride solution until no more precipitate forms then filter the precipitate through a Whatman Number 40 filter paper.Wash the filter paper and beaker thoroughly with hot de-ionised water until the final washings no longer turn pH indicator paper blue.Transfer the filter paper back to the original beaker, pulp up the filter paper and add 200 mls of de-ionised water. Add 5 drops of Methyl Orange indicator solution the titrate with 1.0 Normal Hydrochloric Acid while stirring the pulped filter paper with the boiling rod.

End-point is: ORANGE to RED

Calculation: Titre mls of 1.0 Normal Hydrochloric acid * 5.3 = grams per litre Sodium Carbonate

Titre mls * 6.91 = grams per litre Potassium Carbonate.

Nigel D Gill, B.Sc. MIMF AIEMA MRSC
- Glasgow, Scotland

Tawan,

Be very carefull with adding brigther in your solution. Try to dilute some of the solution of the silver solution in a hullcell. I belive your brigther is to high. Also what you mentioned abouth the carbonate can also be to high.

Good Luck.

Anders Sundman     surface finishing engineer Sweden

Perhaps I am wrong, but there are two other questions posted in the section from Tawan from Thailand and they both relate to silver cyanide baths.

If I am wrong, I apologise ahead of time. I just wonder if Chris Owen was on the right track in his response in 26978, which asks about chloride contamination. It is possible that the chloride comes from the make-up water, but you should only use pure water, not tap water, when making up plating baths. 27119 talks about nitrate contamination. If you have nitrate contamination, you have a problem because virtually all nitrates are soluble so they cannot be precipitated out. The only other route may be to electrochemically reduce the nitrate, but this is very unpredictable. The reduction products can range between ammonia, nitrogen, nitrite, nitrosamine etc and the final product will depend on a wide range of operating parameters, including the type and structure of cathode being used.

There is now a suggestion the problem lies with carbonate. It is well known that cyanide will produce carbonates and these are not conducive to good cyanide plating, so this is quite possible. Your comments in this submission suggest there is an inbalance in the cyanide, so I would suggest you get a complete analysis of everything in the bath and tell us what it is. Also tell us whether the silver cyanide is a pure commercial grade or if some-one has made it up on the cheap. Once we all know what you actually have, perhaps someone can advise you on how to get good silver plating.

Trevor Crichton R&D practical scientist - UK

You definitely win's this month's "read between the lines" award, Trevor, thanks!

I hadn't noticed it but you are correct that Tawan has posted 3 questions which seem to be related to the same issue, and his problem may not be the need for removal of nitrates, or the need for removal of chlorides, or even the need for removal of carbonates, but a clearer understanding of exactly what may be wrong with his bath.

Ted Mooney, P.E.  finishing.com Brick, NJ

You're probably right, Trevor. I could very well be on the wrong track. But, coincidentally, the 2 strangers that Tawan asked about, chloride and nitrate, just happen to to be the 2 that are involved in a AgNO3 to AgCl to AgCN makeup process. Tawan, what is the source of the suspected chloride and nitrate contamination?

Chris Owen
- Houston, TX, USA

Chris,

I think you could be on the right track with your suggestion that dodgey silver salts are being used. I was only offering the water scenario as an alternative source. I am just wondering what Tawan is actually doing and what he has really got.

Trevor Crichton R&D practical scientist - UK

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