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How to prevent titanium precipitation when analyzing an etch bath with a F- ISE

Hello,

I am interning at a titanium foundry that pickles our castings (Ti 6-4) in a Nitric-HF bath. We are currently trying to implement process controls and optimization, but we are having trouble analyzing our F- concentration (with an ISE) when the dissolved titanium is above about 8 oz/gal (60 g/L). At this point either a orangeish flaky precipitation or a white-teal clumpy precipitation occurs when 2 mL of bath is added to 98 mL of 15% sodium acetate solution. The precipitation causes our reading to climb high or low respectively and prevents the sample from stabilizing until a new equilibrium is reached sometime later. I was wondering if anyone else has encountered this problem and if so, if they have found a way of preventing the precipitation or accurately measuring the fluoride concentration with an ISE at these Ti levels.

I have tried dilution with water to no avail (precipitation is prevented at about 70X dilution but measurement is inaccurate)

as well as dilution with a known quantity of HF. The dilution with HF works to a point but not very well.

Thank in advance for your help and advice.

Michael Ong
Employee - Portland
November 10, 2010

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Michael-
Have you considered raising the pH to about 8.5 to precipitate the titanium followed by filtration before you analyze by ISE? It's an extra step, but it should take care of the problem.

Jon Barrows, MSF, EHSSC
GOAD Company

Independence, Missouri

First of two simultaneous responses -- November 12, 2010

We have undertaken fluoride analysis with ISE for many years and have employed TISAB 3 (total ionic strength adjustment buffer) as part of the analysis. A sit contains CDTA (de-complexing agent) it might help your analysis. I would be interested in your findings if you did you this product.

Nigel Gill, BSc CSci CChem MRSC FIMF
- Glasgow, Scotland
Second of two simultaneous responses -- November 15, 2010

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Another option may be to simply control the etch rate in the bath.

Fluoride chemistry is tricky.

Terry Tomt
- Auburn, Washington
First of two simultaneous responses -- November 15, 2010

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Second of two simultaneous responses -- November 16, 2010

Thank you both for your quick responses.

Jon--

If we raise the pH to 8.5 and filter out the precipitate won't this throw off the Free F- measurement as some of the F- would precipitate out and some of it would be liberated from the titanium complex? Or, have you found that around stoichiometric amounts of titanium and F- precipitate out so the measurement remains fairly accurate?

In terms of standards would you recommended creating standards at 8.5 pH, or stay with the sodium acetate standards?

Nigel---

We do have some TISAB 3 on site that we could use, however we are concerned with the free (available) fluoride. By using TISAB 3 won't the sample appear to have a higher Free F- content that it actually does? If you don't mind me asking, do you use a calculation to correlate the decomplexed fluoride measurement with the amount of free fluoride in the bath?
Thanks again for all the help, it is much appreciated and very informative

Michael Ong
- Portland

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Don't worry, the fluoride will not precipitate out in that step. The titanium precipitates as a hydroxide and the fluoride remains soluble. You just need to make sure that you washing the precipitate with lots of DI water in order to ensure that you recover all of the soluble fluoride back into your flask.

Jon Barrows, MSF, EHSSC
GOAD Company

Independence, Missouri

November 16, 2010