Detecting Fluoride

[ecoli]

Hey, for a chem lab I have to design an experiment to detect the concentration of flouride in a mouth wash, or toothpaste...

 

I've been thinking, perhaps I can do a titration to detect the presence of flouride and maybe use spectographic analysis to determine the concentration. Or perhaps I can set up an electrochemical cell and measure the voltage, like in this page: http://chemweb.chem.uconn.edu/teaching/chem-232/Laboratory_Manual/GA8_ISE_toothpaste_rev7_99.pdf (except I doubt I'll have the necesary equipment for this exact procedure)

 

Do any of these ideas sound feasible?

[insane_alien]

spectographic analysis would be the best probably. low detection limits, doesn't actually alter the sample and its more accurate than a titration. if this method is open to you i'd go for it.

[woelen]

Another method may be to precipitate all fluoride with a calcium salt. This forms the very insoluble CaF2. With a centrifuge the precipitate can be collected and rinsed with distilled water. Again centrifuging and rinsing with distilled water and then a final centrifuge. Finally, the precipitate has to be heated in an oven at 200C for an hour or so. This removes all traces of water. The precipitate can be weighed.

 

This method of course requires a centrifuge and a very accurate scale.

[ecoli]

woelen... I think I have a centrifuge and we use analytical scales that measure to the .0001 But, I'd probably have to ask for calcium salt special... or is there a place I could buy it?

[jdurg]

If I remember right, toothpastes don't use a standard fluoride salt such as KF, or NaF. I think they use a sodium fluorophosphate salt which provides the fluoride needed but in a not-so-toxic form. (As plain fluorides are actually kind of toxic).

[ecoli]

If I remember right, toothpastes don't use a standard fluoride salt such as KF, or NaF. I think they use a sodium fluorophosphate salt which provides the fluoride needed but in a not-so-toxic form. (As plain fluorides are actually kind of toxic).

 

That's half true, I have toothpastes that have sodium fluorophosphate and a toothpaste that has NaF. They use both.

[JustStuit]

All 8 of my toothpaste tubes have NaF as primary ingredient except for a baking soda one which has monofluorophosphate. Every single Crest toothpaste (six) had NaF. I only had two brands, Colgate and Crest, but all of the Crests used fluorine.

[woelen]

woelen... I think I have a centrifuge and we use analytical scales that measure to the .0001 But, I'd probably have to ask for calcium salt special... or is there a place I could buy it?

Calcium salts are very easy to get your hands on. E.g. for drying purposes you can buy CaCl2. Purity is not even an issue. The CaCl2 usually also contains some NaCl and MgCl2, but none of these interferes with the test.

 

Another alternative is calcium carbonate. Very easy to obtain in pure form. Simply dissolve some of this in dilute HCl and filter the solution to get rid of solid particles. Use a small excess of CaCO3 in order to make the solution as little acidic as possible.

 

With this method of precipitatation it is very important that all solutions you use are perfectly clear. Any solid particles should be filtered out, before you make the precipitate of CaF2. Having solid particles in your liquids affects the outcome. They will also be centrifuged together with the precipitate.

[Skye]

Another method is by forming lead fluorochloride and back titrating, see Vogel's Quantitative Inorganic Analysis (though there's probably a method on the net ). It is volumetric so you don't need any special equipment, and if you are careful it should be fairly accurate.