Cyanide in Brass/Copper plating solution analysis

A. Hello Rebecca. In your case, it looks like "pale yellow" should not be the endpoint. The endpoint is turbidity. You want to stop at the earliest sign that the turbidity is not disappearing because permanent turbidity is the indication that you have passed the equivalence point. All that is needed is a bit of patience in those last few drops of silver nitrate. If you think you may have gone too far, it is always a good idea to repeat the titration and were more careful the second time.

The method I used for free sodium cyanide in a copper strike bath is a variation from published methods in that I increased the sample size in order to get a more accurate result for free NaCN range of the bath that I was using (~6g/L).

a. Pipette [pipettes on eBay or Amazon [affil links] 25.0 mL of bath sample into a 250 mL Erlenmeyer flask.
b. Add about 50 mL deionized water and about 5 mL of 20% potassium iodide.
c. Titrate with 0.100 N silver nitrate to a faint permanent turbidity.
d. Free Sodium Cyanide g/L = 0.392 x mL of 0.100 N silver nitrate.

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A. Another method that does not need filtering is as follows:
Take 5 mls of plating solution into a flask.
Put in a fume cupboard, add 10 mls of H2SO4, Hydrogen Cyanide gas will go up the fume cupboard. Put the flask on a hot plate and heat until white SO2 fumes appear after the water has all boiled off. Remove from the hot plate, add 100 mls of water. Then add conc. ammonia ⇦ this on eBay or Amazon [affil links] until a deep blue colour just appears and remains. Add a few drops of 1-(2-Pyridylazo)-2-Naphthol (PAN) as indicator, then titrate to a green end point with 0.1 M EDTA. The colour may go back to purple a couple of times before you get the green end point to remain with addition of further EDTA.

Cu g/l = Titre X 3.169.

A. Your method requires modification. It is necessary to destroy the cyanide first but then you must get rid of the nitric acid as it is an oxidant and will effect the thiosulphate endpoint. It is usual to add sulfuric acid and nitric acid and then boil until you see the white fumes of sulfuric acid when all the nitric has been also boiled away. This MUST all be done in a fume cupboard. Also fluoride needs to be added at this stage to complex any iron contamination If you don't have a fume hood find a method using EDTA and PAN indicator where the cyanide is destroyed with hydrogen peroxide.

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A. We use a similar method, which is very accurate and repeatable. Probably the differences lie in the details. Here is our method:
a) Pipette 10 ml sample into a 250 ml Erlenmeyer.
b) Add 5 ml Sulfuric conc. and 1 ml Nitric conc. Acid while shaking under a hood. WARNING! dangerous HCN gas is evolved.
c) Boil until dense, white sulfur trioxide fumes are evolved, to eliminate all nitric acid.
d) Cool and add 100 ml deionized water.
e) Add Ammonium Hydroxide conc. until the solution is colored dark blue and a definite ammonia odor can be noted.
f) Boil for 15 minutes or until all excess ammonia has been vaporized.
g) Add 2 gr Ammonium Bifluoride and 10 ml acetic acid (5N) and at this point the solution turns to light blue.
h) After cooling at room temp. add 25 ml Potassium Iodide (20%) and shake.
i) Titrate with 0.1N Sodium Thiosulphate until the brown color of the solution begins to turn yellow.
j) Add 2 ml starch indicator solution, 2 g ammonium thiocyanate ⇦ this on eBay or Amazon [affil links] and continue titration until the blue color disappears and does not return for about 1 minute.
k) Let A ml be the volume of Sodium Thiosulphate consumed.

CALCULATION. Copper Cyanide, g/l= A x N (of sodium thiocyanate [affil links]) x 8.94

A. Avel Berroya, you can have the analysis from metalfinishing.com, then go to chemical analysis of plating solutions or control, analysis and testing.

A. That's not hard. Combine 5.0 ml of plating bath with about 100 ml of distilled water in a flask. Add about a gram of potassium iodide, and swirl the flask until it is dissolved. Then, titrate the mixture with 0.1 N silver nitrate solution. The endpoint is a persistent yellowish white cloudiness. Then, multiply the mls silver nitrate used by 0.347 for "free" potassium cyanide, in ounces per gallon.

A. Pipette 2 ml sample
Add approx. 1 g ammonium persulphate Warm to decompose CN
Add 10 ml of ammonia and approx. 100 ml DI water
Add PAN indicator and titrate with 0.1M EDTA.

1ml EDTA = 0.00637g Cu

A. Mr. Whitelaw offers sound advice, as usual. The only thing I'd do different is to use H2O2 for prelim. CN destruct.

A. 10ml sample solution dilute to 100 ml
get 5 ml sample
add 50 ml water
add 10 ml ammonium hydroxide
add 2-3 ml of muroxide solution
endpoint purple

6.35 x F x V

Hi Babasaheb. We appended your question to a thread which may answer it, but please get back to us with any further questions.

Regards,

A. Hi Bruno,

There are some really useful volumetric methods in this publication by Charles Rosenstein and Stanley Hirsch. You will find most of what you are looking for:
www.clarksonlab.com/caps.pdf

A1. Hi Karen. On the thread we appended your inquiry to, Dave Wichern has told us that Source 2 is the correct one. Further, the Source 1 formula sounds like a very awkward way of expressing the situation, so I'd guess it's just a typo or transcription error; my bet is they meant to say multiply the mL of silver nitrate titrant times its normality times .173 -- which would make it agree with Source 2 and with Dave.

A2. I don't think there is any difference between "free cyanide" and "free potassium cyanide". Some cyanide plating baths are based on sodium cyanide rather than potassium cyanide, in which case I don't think there would be any difference between "free cyanide" and "free sodium cyanide". But I'm no chemist so corrections are certainly welcome.

Regards,

A. For NaOH you titrate to a sulfo orange ⇦ this on eBay or Amazon [affil links] endpoint with 0.1 N HCl. Look up the factor.

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A. Hi Asmaa. Copper cyanide, CuCN, is 71% copper. But a plater might buy soluble K2Cu(CN)3 or Na2Cu(CN)3, thus about 34% or about 29% copper. Plating shops generally buy process chemicals from trusted sources, they don't simply find chemicals somewhere and then test for every possible contaminant which might be problematical -- they'd have no time for plating :-)

Copper cyanide plating is the subject of 20-page chapters in many plating text books, so it's hard to say how to use it in a paragraph or two... maybe you can be a bit more specific with your question? But in the meanwhile, the Metal Finishing Guidebook has a pretty good explanation of the subjects of your inquiry. Good luck.

Regards,

A. Hi Lamberto. Google the Mohr method, or Fajans method. Each are relatively simple to perform, I typically use them for our acid chloride zinc bath. Do a bit of digging and ensure that it will function in an alkaline environment but I believe it should serve your purpose. As always when dealing with cyanide, take the necessary precautions; I am sure someone more knowledgeable on the subject may be able to offer better advice, but this should give you a place to start.

- Jordan

 Thanks for the directions, for now I tried with a colorimetric method that gave good results; I will try to compare them with those suggested.
Thank you

A. Thinking about this further, those methods may work, but the silver nitrate is going to bind with the cyanide in solution as well. Not sure which would be preferential, I'd guess the cyanide, but if you did try it you could start measuring the chloride content once the precipitate appeared. Glad you got some results though, if you don't mind me asking, where do you think your source of chloride may have come from? Did it seem to cause any adverse affects on the plate?