Quantitativ fluoride determination using conductivity det.

[daggio]

Hello,

I am using a 0.0015 M H2SO4 mobile phase with a Rzex-ROH 7.8*300 mm column coupled to a conductivity detector, seperation of fluoride is excellent from water dip. Will quatitative determination fluoride work with this method?
I was just thinking that HF is not a strong acid and the dissosiation of fluoride ion will vary with concentration and pH, thus not giving a linear calibration curve. What will approximatly be the linera range for this set-up. Must I switch to an alkaline mobile phase.

Thanks for your answer.

[tom jupille]

With that dilute a mobile phase, the potential for problems definitely is there. The only way to tell for sure is to run a calibration curve and see how linear it actually is.

[vainsummer]

I think that you can change the moble phase to 0.0015 M Na2SO4, and the problem maybe sovled.

[vainsummer]

But the sulfate will be difficultly eluted if you use ion-exchange column. So my suggestion is that you'd better use Sodium hydroxide or potassium hydroxide.
If your column is ion-exclusion column, both separation and linearity will be better with 0.0015 M sulfuric acid.
The two modes have different separation mechanism.

[Markus Laeubli, Metrohm]

Dear Daggio

I do not know that column. But eluent and column diameter make me assume that you are running an ion exclusion application.
Ion exclusion is mainly used for weak acid like organic acids. As HF is a relatively weak acid it is retained and quantification is possible.
I assume a working range from the e.g. 50 µg/L up to the mg/L range.

Changing the eluent pH is not required unless the column is an anion exchange column. In that case an alkaline eluent combined with suppression might increase the sensitivity of the method.

[daggio]

Hello,

Thanks for your input.

Yes, its an ion-exclusion column and application.

I was just assuming that if you inject eg. 0.1 mg/mL or 0.2 mg/ml fluoride you will end up with pretty much the same responce on the conductivity detector as the conductivity is dependent on concentration of F-.
The detection limit is not an concern. I am actually running a limit test and I dont want to see more than approx. 0,45 mg/ml fluoride in my product. I was wondering if this concentration is actually too high using my described method.
Was that clear?

Thanks.

[Markus Laeubli, Metrohm]

Yes, I assume that you need to dilute your sample e.g. 1 : 100.
Just give it a try.

[Assembler]

Side comment: For quantitation of just F-, you could use a flouride ion selective electrode