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Thumbrule for checking sulfuric acid conductivity during anodising by its color or appearance

Q. Dear sir,
I am a home type anodiser doing small scale sulfuric anodising process for labels.
After some time, say 2 to 6 months, it seems that throwing power of solution is reduced considerably. Is there any technique by which we can confidently say that yes solution has become weak and needs to be replaced. Or, to say it another way, any thumbrule by which we can judge that solution is to be discarded say by observing its colour, or specific gravity etc. Or is it possible that we add fresh acid in the above solution and it will be useful and will give results same as fresh solution.
Thanks

S Tambe
- Raipur, Chhattisgarh, India
December 21, 2011

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December 21, 2011

No, it will not be as good as a relatively fresh solution.

Simple chemistry titration will tell you if you need to add acid. A less simple titration will tell you if you need to trash the solution because of excessive aluminum in it. Testing for copper and other alloy components is far more complicated.

You can extend the life of your solution by doing a simple acid titration mentioned above. Do not use specific gravity because it also includes dissolved metals giving you a false reading.

Depending on the haz waste treatments required in your country, it may be cost effective to use it as long as the product is OK and then dump 95% of the tank.

James Watts
- Navarre, Florida