Cadmium Analysis - Cadmium Cyanide Bath
A discussion started in 2012 & continuing through 2017February 29, 2012
We have a cadmium cyanide plating bath and I was hoping someone could help clarify my understanding of the solution analysis for cadmium metal.
We take an aliquot and add the following:
pH buffer 10 consisting of ammonium hydroxide, ammonium chloride, and water (~ 10 mL)
10% formaldehyde (~ 15 mL)
and Erichromte Black T
We then titrate with EDTA.
I have three questions:
1) What is the purpose of the pH buffer (or really the addition of the ammonium hydroxide)? Is it to help drive a reaction?
2) Can someone confirm that the formaldehyde is there to liberate the cadmium-cyanide complexes freeing the cadmium for the titration?
3) In the books I have been reading, some recommend adding a few drops of 25% HCl prior to adding the ammonium hydroxide, is this step necessary and if so, what is its purpose if the next step is to add a few mL of a alkaline liquid?
Thank you all in advance.
- Denver, Colorado, USA
March 14, 2012
I must say that whenever adding acid to a cyanide solution you are analyzing, it MUST be done in a fume hood. Deadly Hydrocyanic Acid is liberated. I used to use premixed 1:1 sulfuric acid dropwise until the aliquot just turned milky.
Ammonia/Ammonium Chloride buffer is added to make it alkaline.
Indicator used is Eriochrome Black T. (Mixed at 1 Gram of dye to 99 Grams Reagent NaCl. Correct amount of dye is a learned skill. Too much; and the color is too dark to see the endpoint. Too little; and the color might fade.
After the dye is added, and JUST before titrating, add about 10 ml. of 1:9 Formaldehyde. Titrate immediately. Titration times the "factor" will give oz/gallon of Cadmium. When you get to the endpoint, add 5 ml. more of formaldehyde. If it reddens, continue the titration to a Blue endpoint. Note that the endpoint may turn red in a minute or so. You have reached the endpoint, and use that for your calculation.
Why do we use 0.0575 Molar E.D.T.A. in plating? Long ago, a man named R.O.Hull noted that when analyzing zinc, that this Molarity gave a factor of 1. Other factors may be calculated. The Metal Finishing Guidebook might be a good source of the correct factor to use for your analysis.
Remember: Safety first. You are analyzing a cyanide solution, and all care must be used. I've done thousands of these cadmium titrations, and must say that you will develop the skill to "know," how the color will change. Some baths that are high in iron contamination will act differently and you need to know the individual characteristics. Practice and hone your skills.
July 29, 2013
Q. I have an aerospace plating spec that lists as a makeup for our cyanide cadmium plating bath the following:
Is just "Sodium Cyanide" the same as "Total Sodium Cyanide""?
and is "total Sodium Cyanide" the same as just "Total Cyanide"? Our cust. spec seems to differentiate as shown in the list above but I can't seem to get answers from labs whether I can test for all that's listed. Your help would be greatly appreciated.
- Monroe, North Carolina, USA
July 30, 2013
A. With Cd it doesn't make a lot of difference. The complex that it forms with CN- is quite weak and you're pretty much titrating it all.
Consultant - The Bronx, New York
November 21, 2017
Q. Hello, I am quite new to the electroplating industry and have been hired on as my company's chemist. Unfortunately there was no one in the position before I entered the scene and I am woefully uninformed on nearly all the analyses which need completed to ensure proper functioning of our process tanks. My current headache is my Cadmium metal content in our Cad / Cyanide plating tank. The only procedures which I have to follow or go on are several older metal finishing books and what I can glean from sites online such as this. That being said the current procedure I am attempting to follow is that spelled out in the metal finishing guidebook for Cad metal content analysis.
It goes as such:
Add 2 ml sample from Cad tank
Add 100 ml DI water
Add 10 ml pH 10 buffer
Add EBT powder
Add 15 ml 10% Formaldehyde
Titrate with 0.1M EDTA
Color change of Red to Blue
I am having significant problems. The first being recognizing the endpoint. From what I have researched online the color of EBT in an alkaline solution complexed with "metal" ions should be somewhat of a "wine red." After following the above outlined procedure and adding the EBT I am NOT getting any sort of red color, more of a purple that seems to be what I have seen as the color of the EBT as it nears an endpoint. In an effort to determine the endpoint more exactly I took a flask of tap water, added a small amount of of EBT / NaCl mixture (1/100) and a small amount of my pH 10 buffer and to my delight began to see the red color I was supposed to. I wanted it a bit darker and proceeded to add a bit more EBT mix and splashed in a significant amount of pH 10 buffer and to my dismay the solution turned a deep purple blue. This confounded me. The pH 10 buffer I have is 0.38% Sodium tetraborate, 0.05% NaOH, and 99.57% water.
I am now here asking for any help that anyone may be able to offer. I have no other resources available, no procedures to follow, no one to ask. We do not run our Cad with brighteners, and I have very little information on the bath other than its current pH. (13.2) If I can determine Cad metal, I can determine total CN- and then free.
Again any help would be more than appreciated, or any resources anyone could point me towards would be fantastic.
Thank you in advance.
plating - Huntington, West Virginia USA
November 22, 2017
A. Hi Jordan
Welcome to the frustrations of EDTA titrations.
A full answer is not practical here but there are several alternatives. These partly depend on what kit you have available; do you, for instance, have a fume cupboard to drive off the cyanide? (and incidentally do you know about cyanide first aid etc?)
Longer term you might try to get hold of Atomic Absorption kit. You will not need the latest bells and whistles so an old second hand unit might be cheap.
Your quickest way to get help is to contact your chemical supplier. You will find them very helpful and anxious to ensure that their processes are well maintained. Most have standard analysis methods for their products.
If you are new to cyanide plating, this may help
If you are new to plating, there is much to learn. You may consider the distance learning courses offered by e.g., http://www.materialsfinishing.org/
November 22, 2017
Q. Geoff, thanks for the response. I will attempt to answer your queries in the order which you dispensed them.
I do not have a fume hood, however the area is fairly well ventilated as is the shop. I am aware of the dangers of cyanide, the quickest routes of exposure, warning signs and slight first aid, though there are two large hospitals less than 2 miles from the shop.That being said, I appreciate the concern.
I have a copy of "the chemical analysis of electroplating solution" by T.H. Irvine although it is somewhat dated and preaches a sulfide method for metal determination.
I will have to look into an Atomic Absorption Kit.
As far as contacting my chemical supplier I would say that is an excellent idea, however, our Cad tank is run without brighteners, off a metal finishing handbook "recipe" which I have inherited and as such has no technical data associated with it. I guess there is nothing to lose in asking, although I have run into some issues with companies being wary of cyanide baths.
I will look into your additional links and hopefully they may give me some additional direction.
If I may place another question, what is the use of the 10% formaldehyde in the titration and would a different buffer solution provide better results? As I mentioned, I have yet to achieve the "wine red" color associated with complexed EBT in an alkaline solution and am attempting to fully understand the mechanisms by which everything is taking place.
Again, thank you for your quick reply and sources, glad to hear I am not the only person that experiences issues with compleximetric titrations.
plating - Huntington, West Virginia USA
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