Letter 40027

Fluoride determination in HF/Nitric titanium etches [UK] 

March 23, 2006

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 - Bronx, NY, USA

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
- FL

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