Metal finishing Q&As since 1989

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Determining HF concentration in Nitric acid baths

1999

I'm looking for alternative methods in determining Hydrofluoric Acid concentration in Nitric Acid Baths. We use this process to etch titanium parts for aerospace applications. We currently use a method published in the Metal Finishing Guidebook which calls for the use of Thorium Nitrate. Due to its radioactivity, we are searching for another method. Any info, advise or references would be appreciated.

Thanks.

Leroy Samuels

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1999

Pratt and Whitney Aircraft has a procedure for testing that exact solution. It is a poor procedure, because it tests for the nitrate and subtracts it from total acidity. Since the nitrate is the major component, the test is a total pain with several sources of error, it yields massive errors in the resultant fluoride result.

I used a fluoride selective ion electrode from Orion, established 3 standards, and ran on a good machine( it was a 901). This gave extremely repeatable results for the HF. Run a total acid and subtract the HF value. It is not perfect, but it gives results that are relatively quick, safe and more than accurate enough to control the tank. The thorium titration is more accurate, but totally not necessary.

Procedure was derived and modified from a couple of references listed in Orion's Handbook of electrode technology.

James Watts
- Navarre, Florida

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1999

The procedure with ion selective electrode is accurate only in new solutions. Solutions that contain aluminum (most nitric + fluoride solutions are used as aluminum deoxidiers) give a non accurate result as part of the fluorides create a complex with aluminum. I am not sure about titanium but I think it acts the same way.

The best way to check fluoride in nitric+fluoride etchers is by observing the etching rate.

Sara Michaeli
Tel-Aviv-Yafo, Israel

The real beauty of the procedure developed is that it only measures free fluoride which is what you are after. It works, new or old solution. A secondary portion of doing the total acid is precipitation the titanium out which when total acid is subtracted, a proper fudge factor used, it gives the titanium content. Practical purposes had a concentration of about 0.8 grams per liter if I remember correctly. I think that PWA limit was 1.2, a point where etch rate was terrible, even with their max concentration of HF.

James Watts
- Navarre, Florida
1999

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This sounds very useful. How do you precipitate out the titanium? And what total acid method do you use?

Thanks,

Wayne VanTassell
- Ogden, Utah, USA
2002