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HF analysis using ISE
Q. My company is currently trying to switch to the ISE Fluoride analysis for our HF/HNO3 To milling tanks. The ISE electrode will calibrate fine using a three point calibration with NaF standards, but I am have trouble with the sample measurement drifting significantly (+ 50 ppm F) over e period of 1 hr. I'm in contact with the electrode manufacturer, but they can't seem to determine a reason either. (Note: the sample is buffered with TISAB III) My question(s) for all of you that use the ISE for fluoride analysis in this type of solution: how do you accurately determine the F of these samples with such a large drift? Or do you even see a drift? Is this a matrix interference? Any suggestions on how to reduce this?
Elaina L [last name deleted for privacy by Editor]Aerospace Industry - Terre Haute, Indiana, USA
2004
A. Part of that will depend on which technology electrode that you are using. A conventional electrode will drift and should not be left in the sample any longer than it takes for the mV to drop to a point that it reverses and starts climbing. That will give you a very repeatable analyses. Use the valley reading. The standard and the sample also have to be diluted to a region where the TISAB can swamp out the background noises. Also needs to be in a linear range. I forget which TISAB I used, but it also made a difference.
James Watts- Navarre, Florida
2004
A. What is the pH of your sample with TISAB? If below 4 pH the species in solution is a HF complex and not free fluoride ions. ISE's only can measure free Fluoride ions. HF is used to complex iron among other things forming fluoride complexes. Why are you doing the measurement? To see if available fluoride is still there for complexing or other? Adding TISAB typically is for samples where the pH needs to be brought down so hydroxyl ions do not interfere. With Nitric and Hydrofluoric ions around I don't think that's the issue. Are you diluting your sample with water? Why are you trying to make this measurement?
Burdick,Janeinstrumentation - Woonsocket, RI, USA
2005
A. Use of ISE for HF is OK if their are no interferents such as iron. I analyse HF in pickling tanks and need to remove Fe, Cr, Mo as interferents by using a complexing agent. I hope this helps.
Paul Gillespie- Melbourne, Victoria, Australia
2006
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