Proper method to sample a plating solution?  

letter 52025

April 29, 2009

I'm sorry if this seems very elemental to most of you, but here in Maine, there's apparently no such thing as "common sense".

For years, our chrome tanks were sampled by inserting a "ladle" on about a 3-foot shaft into the tank, as deep as it could reach, then withdrawn and poured into a sample bottle, which is then sent to the lab for analysis. The ladle hold about 3 times the volume of the sample bottle. Our tanks are roughly 2' by 3' by 10 feet deep.

I was taught to sample plating solutions using multiple "jabs" with a thief tube, long enough to get at least half-way to the bottom of the tank, making sure that at it took least 3 "jabs" to fill the sample container, the more "jabs", the more representative the sample. My "mentors", who, incidentally were real platers, unlike here, would have called me on the carpet if I sampled that way. Later on, the "thief tube" method is the only way I would ever allow my chemists to take a sample.

I'm trying to get some people here to realize that if the tank is not completely mixed and homogenous, the ladle method is not delivering anything close to a truly representative sample. Also, as the ladle emerges from the liquid, I suspect anything that's floating on the surface of the solution is picked up and poured into the sample container.

I would appreciate some feedback on this to show to my boss, who knows virtually nothing about plating, sampling, or analyses....

George Brackett
- Maine

I used to use a 4 foot length of pyrex glass tubing, 9 - 10 mm in diameter. I'd "stick" the plating bath in 12 - 15 places, all along the length and width of it. And, sometimes, to be extra sure, I'd mix it up manually beforehand.

Representative sampling is a crucial issue.

Dave Wichern - The Bronx, New York

Pardon my spelling, but your thief tube is technically referred to as a colwissa^^Coliwasa tube. I used a 1/4" CPVC tube for the sampling and took multiple samples into a beaker which I mixed and poured the sample into a bottle to take to the lab.
I did not consider it to be representative unless I had at least 3 depths and at least 2 areas of the tank.
EPA requires this method for sampling in many cases. Look up my word on the internet and some site will have a good explanation.
Your method is far superior to the ladle method which will only get a sample from one strata.

James Watts
- Navarre, Florida

Hey George,
I was taught to draw a sample the same way you were. Baths that have solution agitation, or adequate air agitation, heated to operating temp, sampling methods really don't matter as long as you draw 4-6 inches down from the surface of the solution. Years ago, I witnessed one of our competitors draw a refining sample from a 55 gal drum (stored for 30 days) without mixing the contents. The real bad news was that the lab / plating mgr witnessed this too. Needless to say, I gained the customer's refining business that same day.
With a cold, un-agitated solution there are certain chemicals that can crystallize and form on tank walls, anode bags, etc. Tell your boss he should look up the specific gravity of the make-up chemicals vs specific gravity of water. I'm sure then he will see your point. Good Luck to you.

Mark Baker
Fellow Plater - Syracuse, NY, USA

Thanks, James. I looked up "colwissa" ^^Coliwasa on Google, the only results were your references to it at finishing.com. Tried Yahoo search, as well. Did you make this up? I have to laugh at how much time and effort people take on analyses, but give almost no consideration to sampling technique.

G. Brackett III
- Maine

Taking a sample from 3' down a 10' deep tank is not remotely representative.

Someone needs educating

Geoff Smith Hampshire, England

 

Hi George,
No I did not make it up. It was from some spec that I had a long time ago.
did not claim to spell well, so try colawisa^^Coliwasa. It brings up one site. Try sampling tools for a lot of sites, with a few applicable. Try chemical sampling tools for hits on several specs, some (few) are applicable. I did not try it, but chemical sampling might be worth a try.
Jim

James Watts
- Navarre, Florida

Just in case that anyone is looking for it. The spelling is Coliwasa. According to a couple websites, it is an abbreviation for COmposite LIquid WAste SAmpler.

Kai Lorcharoensery
- Warsaw, Indiana