Metal finishing Q&As since 1989

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What test equipment is available for nitric acid and ammonium bifluoride for stainless steel pickling?

Hello, my name is Gyneth Jenkins. I work in the Northern KY area for a company that manufactures stub ends. All products are made from stainless steel, and most go through our pickling process. Our company uses and Nitric Acid and Ammonium Bifluoride combined with water for pickling. Our question, what test equipment is available for controlling/monitoring the concentration level of each chemical? We have been told the solution should be 87% Water, 10% Nitric Acid, 3% ABF. We are looking for testing equipment.
Thank you

Gyneth Jenkins
machining manager - Florence, Kentucky, USA
February 19, 2009

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February 23, 2009

It is not an easy answer. You can do a total acid by titration with 1.0N NaOH [1N NaOH on Amazon] , good agitation and either a good pH meter or an indicator that changes color at about pH 4 to 5. Avoid any higher than that as it will start to precipitate out some of the dissolved metals and give a wrong answer. The same goes for the NaOH, it should NOT be a concentrated one as it will cause localized precipitation of dissolved metals.

Total acid may be good enough to get by. To be more accurate, you need to titrate for Fluoride. This can be done with a titrant that is mildly radioactive, not really strong enough to be of great concern, but it gets EPA excited and also whoever handles your waste disposal, so I do not recommend it. There is another method that I used for a while, but is a long involved titration as temperature is critical. Results varied wildly, so I will not recommend it. I ended up with a Lab grade pH instrument that would work with a fluoride Selective Ion Electrode. This took a bit of testing to prove that it would work, as it is not a standard method. It required a particular TISAB additive as a swamping agent. If you are not highly familiar with it, I will suggest that you buy the probe from Orion as they have a procedure that requires a little modification. They gave me lots of information since I also bought their pH meter. They are now a Thermo company, and I suspect that customer service may not be as good, but do not know. Very important, you will be titrating for available fluoride and not total fluoride.

I will offer a wild guess that testing for total acid and making the fluoride addition in the ratio of the original tank make up, as you make the acid addition that you will be close for a fair while. Check to see if INCO has any information on pickling.

James Watts
- Navarre, Florida