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Ted Mooney, P.E. RET
Pine Beach, NJ
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for Metal Finishing since 1989
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Measuring alkaline tin plating bath chemical concentrations
Q. I am a quality engineer for an automotive industry supplier. My facility has an alkaline tin plating process in which we are using a chemical immersion bath to tin plate aluminum products. The tin bath chemical is mostly potassium stannate.
My questions are about measuring the tin metal concentration within the bath. Currently, I am doing wet chemistry, which includes an oxidation reduction titration to determine the tin concentration. This test consists of centrifuging the bath sample and mixing a small portion with sulfuric and hydrochloric acid along with some lab grade pure aluminum. After the reaction has occurred I titrate with potassium iodate using a starch indicator. The method is sensitive to air (oxygen) contamination.
Are there any alternative methods to measure the bath's tin metal concentration, other than wet chemistry? I have been told this can be done using X-ray Fluorescence (XRF) spectroscopy. Will XRF measure the same tin that the titration does? Are there any online probes which could be inserted into the tin bath that can monitor tin concentration? (Ex: ORP, pH, or Conductivity probes?)
Quality Engineer - Parma, Michigan, USA
December 1, 2009
A. Hi Mike, I use a method described below. it helps prevent the oxidation of the tin before the titration has been completed:
Take 2.5 mls of sample, 100 mls water and 50 mls c. HCl.
Heat to 70 °C.
Add 3 g of degreased iron wire; allow to react for 30 mins.
Add another 100 mls water.
Add marble chips then take out the wire with a magnet and titrate QUICKLY against 0.1M Iodine --make sure that you keep adding marble chips until the titration is finished. Also use starch as indicator. To check accuracy of the method do another one with 5 mls of sample. Multiply titre by 5.978 for 2.5 ml sample.
This works fine for me, its quite reliable.
Best regards
- Isle of Man Great Britain
December 3, 2009
Q. Mark,
Thank you for your advice. I will try adding marble chips.
Do you happen to know which way the oxygen contamination affects the test? Does it bias high or low readings?
Thank you.
automotive supplier - Parma, Michigan, USA
December 3, 2009
A. Hi Mike, the oxygen reacts with the tin, therefore giving a lower result on your titration.
Mark Lees- Isle of Man Great Britain
December 14, 2009
Problems with Iodimetric Titration of Tin Metal
Q. We are using an Alkaline Tin plating bath and are in the process of moving the bath analysis in house. We are using the following method with terrible results:
x Pipette
[pipettes on
eBay or
Amazon [affil links]
5 mL sample to 250 Erlenmeyer
x Dilute with 50 mL of DI water
x Add 20 mL approx. 6N HCl slowly
x Add 2 mL Starch indicator solution
x Titrate with 0.1N iodine
It only takes 1 drop of iodine to produce the blue endpoint. Are we not reducing the Tin IV to Tin II, and therefore getting a zero result? I was under the impression that the addition of heat with either aluminum or iron was only to minimize oxidation by oxygen from the air, but it is it also to complete the reduction of Tin IV to Tin II? I would not expect a zero result unless there is absolutely no conversion to Tin II.
Any help with this would be much appreciated!
Paul
Chemical Manager - Minneapolis, Minnesota
May 9, 2014
A. Alkaline tin? The Sn is in the +4 state to begin with! (sodium stannate) What you need to do: add some iron powder; acidify with HCl: heat to a light boil for five minutes: filter: then do your titration. That'll work.
Dave Wichern
Consultant - The Bronx, New York
May 10, 2014
A. The titration you have shown is what I have used as an acid tin bath analysis. Perhaps that is why your results are off.
Unfortunately, I do not have an alkaline tin titration available....
You definitely need to find the proper method.
- Seattle, Washington USA
May 12, 2014
We tried boiling with aluminum foil yesterday and got a decent result. I'm thinking there was no reduction of Tin IV to Tin II before the titration when just HCl is added. As you say it takes heat and the addition of Fe or Al. Thanks for the confirmation!
Chemical Manager - Minneapolis, Minnesota
May 14, 2014
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