Letter 40027

Fluoride determination in HF/Nitric titanium etches  

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Hi there

I know that many postings have been made on similar topics, I just wanted to express my problems in measuring this type of solution.

We test titanium etch tanks belonging to a leading aero manufacturer, who use tanks around 2-5% HF and about 20% HNO3.

Current methods used include:

direct measurement with standardised fluoride ISE, which is fairly time consuming and not really stable;

indirect method by measuring nitrate by IC and subtracting from total acidity. This involves a large dilution, which adds error, and is also very time consuming.

We are testing a new method which involves titration against lanthanum nitrate, which we obtained from Eutech. This works well for the standard, but unf we can't get an end point for our samples!

I theorise that, instead of being in solution, the unreacted HF exists as a complex with titanium fluoride:

6HF + 2Ti -> 2TiF3 + ^3H2

3HF + TiF3 -> H3[TiF6]

As a result no amount of TISAB will free this fluoride from the complex. To liberate the fluoride from this complex I think you would need to use something more neucleophilic than fluoride (ie nothing!) or an ion chromatograph might be powerful enough.

I think the problem may be the definition of free fluoride....

Chris Nattrass
testing services - Nottingham, UK

What I do is time consuming, but works. I use the method of additions, plotting apparent concentration vs. spiked concentrations. I evaluate the apparent concentrations by making a four point standard curve with a standard fluoride solution, the points being 50, 100, 200, and 400 ppm. The samples, of course, must be diluted down into this range.
I don't use Orions version of the method of additions, with their table of "q" values - quite frankly, I never understood how it worked, anyway :)

Another alternative is to do a ferrous titration of the nitric acid in a matrix of sulfuric acid, at ice water tempeatures. This also works, but uses vast amounts of acid, and a titrant that must be standardized each and every time you do the test.

The "free" HF vs "total" HF question has never been raised by our customers (I work for a commercial lab) and I've never brought it up. Sometimes raising such questions creates problems that have more to do with salesmanship than science.

Dave Wichern - The Bronx, New York

You do not want to analyze any of the complexed fluoride. The only floride that does the work in your tank is the free fluoride.
With ISE, it takes a known dilution of the acid sample to get it into the area where the mV response is linear.(MANDATORY) Look at the curve that came with the ISE.
I made a Cal curve from 4 known samples of concentration vs mV and then tested the diluted sample. Enter the graph with mV that you read and you have a fairly quick and accurate analyses of the flouride. I forgot which tisab we used. The purpose of the tisab is a swamping solution to eliminate the random noise, not to dissolve the fluoride complex.

James Watts
- Navarre, Florida

I agree that what we want to measure is "free" or active fluoride. After all, the total fluoride hardly changes as titanium is etched, but the etch rate certainly decreases!My samples typically contain ca. 20 g/l free fluoride, and I use the simple method of diluting the sample 1:100 in 30 g/l NaOAc. Then I use ISE to measure the mv. Standards are made to bracket the unknown, and the rest is quite simple. The addition of ABF to raise the fluoride so as to show approx. 20 g/l in the bath always restores the etch rate to the nominal value, which in our case is approx. 3 mil/hr at ambient temps.

Mac McNeil
- Elmwood Park, NJ USA