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pH determination of zinc plating solution which kills pH meters

December 23, 2010

Hi,
My factory uses a mixed solution of Zinc Chloride and Ammonium Chloride. pH is one of our control parameters. Unfortunately, our pH meter gets poisoned by the solution after a while and the probe needs to be changed frequently. I also heard that even top-range pH meters will get poisoned, and they only last slightly longer than normal pH meters.

Therefore I would like to ask if there's any other way to determine the pH to the accuracy of 0.1 or 0.2? Our pH range is from 3.5 to 5.5.

Thank you!

Regards,

CS Leong
Galvanizing Plant - Employee - Selangor, Malaysia

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First of two simultaneous responses -- December 23, 2010

You can control it with a proper colorimetric indicator, but that is a bit of a pain.
I will guess that you are using a silver- silver chloride type pH meter. Switch to a different indicator.
Gel type indicators will stand more abuse than conventional liquid units. High dollar units that are typically ground glass joints (no frit) use a lot more solution in the tube. but it tends to keep the intruding solution out of the unit. Also, they are very easy to dump and refill, thus prolonging their use.
Next, keep the probe in a pH 4 solution and not in the tank except when you need a reading. Leaving it in the tank is nice, but it is darn expensive.

James Watts
- Navarre, Florida

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Second of two simultaneous responses -- December 24, 2010

Sir:

If you are referring to zinc ammonium chloride galvanizing flux solution, then I have had exactly the same experience with pH meters. Namely the probes are destroyed AND they do NOT display a pH that is correct.

The useful way is by bromophenol blue [on eBay or Amazon] , sodium salt and proper titration methods.

Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA

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December 29, 2010

Dear Sirs,

Thanks alot for your response! I shall follow your advices and do some trial & error testings.

Best Regards,

CS Leong
- Selangor, Malaysia

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Use a double junction probe. My probes last typically > 1 year and I am measuring the same solution.

Trent Kaufman
electroplater - Galva, Illinois
January 4, 2011

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January 10, 2011

Dear Mr Trent,

Thanks for your advice!
I shall discuss with my lab equipment supplier

Regards,

CS Leong
- Selangor, Malaysia