DieForelle
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anybody familiar with the use of TISAB buffers?
This is a pretty far-out topic for this board. I might need to find a more specialized one, but that would mean going through teh rigmaroles.
Seeking some practical advice on the use of TISAB buffers. A meter like this one: http://www.extech.com/instruments/product.asp?catid=42&prodid=424 comes with TISAB buffer tablets. But there is only the most cursory explanation of their use IMHO, when I RTFM; it's not even clear how much solute to use
with them, but I assume it's one TISAB tablet for the 20 ml sample container included with the meter. (Do they effervesce? I'll find out! Otherwise
it seems like they will be a pain to dissolve in such a small volume.)
Here's my question: if I already have a rather ion-rich solution, do I still need to use them? Should I use less, like 1/2 a tablet? My
understanding is they serve 2 basic functions: increase ionic strength, and chelate interfering metal ions. For the former, it would make sense when
testing relatively hypotonic drinking water, which I suppose is what many people use a fluoride detector for. In my case, I will at times be testing
relatively ion-rich (i.e., salty) food stuffs. And as for metal ions, in some of the items I am testing, they will already be partly chelated by the
likes of citric acid or whatnot.
[Edited on 30-3-2012 by DieForelle]
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francis
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I have used TISAB in the past for fluoride ISEs. I always used a ratio of 1:1 TISAB/sample (by volume, since we had TISAB solution).
But for the tablets and powders you add them to the sample/standard.
For your question:
The third function of TISAB is to bring the solution to a constant pH (~5) because fluoride measurements are inaccurate at low pH.
The salty food stuffs you are measuring have other ions, which may affect the activity of fluoride. By adding the TISAB you can overcome these
interferent effects.
As long as your calibration solutions are matrix matched to the unknown (ie the calibration standards have the same amount of TISAB added as the
unknown) then you'll find the ISE to be fairly accurate.
For example, in the past we compared ISE precision, speed, selectivity etc, for measurement of vitamin/mineral supplements, to IC for the same
parameters. ISE was found wanting but so was, I think, our technique - for a quick field measurement it is remarkably good.
The fluoride electrode has an especially high selectivity for that ion compard with other iSEs. I think Iron(III) is an interferent for fluoride
measurements, even in the presence of TISAB it reduces the accuracy of the result.
Make sure you give the electrode time to adjust for the lowest standard readings since the ISE tends to have a high signal bandwidth.
Also: you can double check the data quality by using standard additions. Actually, you will get better (more certain) results using standard addition
rather than calibration standards/calibration curve for fluoride determination by ISE.
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francis
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Also, don't use half a tablet if you're using calibration standards, because then you'll be adding a different amount of TISAB to each standard and to
the unknown.
You could crush the tablet and used a weighed amount, as long as you ensure that the same amount is added to the samples and the standards.
If you are using the variable volume standard addition method, and adding increasing volumes of [F-] to the sample, you could probably use half a
tablet.
I'd still go with one full tablet though. You could always check out the Standard Methods.
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DieForelle
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Thanks for the advice francis!
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