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Fluoride determination in HF/Nitric titanium etches
Lab services 
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 unfortunately 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 nucleophilic than fluoride (ie nothing!) or an ion chromatograph might be powerful enough.
I think the problem may be the definition of free fluoride....
testing services - Nottingham, UK
First of two simultaneous responses -- ++++++
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 Orion's 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 temperatures. 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.
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Dave Wichern - The Bronx, New York |
Second of two simultaneous responses -- ++++++
You do not want to analyze any of the complexed fluoride. The only fluoride 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 fluoride. 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.
- Navarre, Florida
July 29, 2008
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, New Jersey USA
September 12, 2011
Hi,
I've been working on trying to obtain a split of the HF and nitric acid compositions for our acid pickling baths (titanium and stainless steel).The method I've been using is obtaining a total free acid value of both acids by titration with 1M NaOH and then following this up with a second titration using an Fluoride electrode and reference electrode and Lanthanum nitrate as a titrant. The titrator we use then takes the fluoride value off the total free acid value to give us the Nitric value. We've had success only a few times when tanks are quite fresh, however once contamination level build up its very difficult as no end point on the second titration is picked up.
Recently my attention has been drawn to the Millivolt/concentration calibration curve method; using standard solutions of HF to obtain a range of MilliVolt readings. I've also notices that when I start the second titration the mv reading on our titrator is a negative reading; my questions are therefore is this negative reading due to free fluoride ions in solution and if so will this change on varying levels of HF? If this is so can I just use this initial millivolt reading without doing a second titration to obtained a relatively accurate reading of HF; by using a range of these initial millivolt readings of HF to obtain a calibration curve similar to that mentioned above?
Any help would be much appreciated as I'm a non-chemist.
Thanks,
James
Metallurgist - Halifax UK
September 14, 2011
Tough problem! I've wrestled with it myself. Approached it three different ways:
1) Did total acidity via titration with NaOH; determined HNO3 conc. via titration with FeSO4 in chilled conc. H2SO4. Determined HF, HNO3 concs by arithmetic.
2) Did total acidity via titration with NaOH; determined F- conc via electrode using the method of additions (the math gave me a headache, but it is possible) then determined HF, HNO3 concs by arithmetic.
3) Did total acidity via titration with NaOH; tried to do what you did; La(NO3)3 titration with electrode as endpoint.
Results: (1) and (2) gave totally different results. No useful endpoint observed with (3)
If I wanted a (4), I'd propose determining F via a distillation procedure. Then, there would be a third answer, likely different from the first two. Closer to (2) than (1), probably.
What's the "right" answer? HA!
My conclusion: what are we trying to measure? Once the bath is put in service, the HF starts to break down; the F is present as transition metal complexes with Fe and Ti. It is now less effective as an etchant...it's "unfree" HF.
My suggestion: think up an empirical test on samples that are as similar to the actual work as possible (if practical, use real work pieces) and do some kind of test to see how effective the bath is at doing what you want it to do. Cut the sample of the bath that is up on the line with fresh HF, and/or fresh HNO3, and do the same test on these solutions. (Watch phantom variables, like temperature; try to replicate process conditions)
Now, do your lab tests on your line bath; and, on your lab concoctions. Look for a correlation. Find one. Now you have your test.
Sound like a lot of work? It is. But if you follow through you'll solve the problem.
My motto: Science is a business of empiricism.
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Dave Wichern - The Bronx, New York |