|MadSci Network: Chemistry|
There are 3 commonly used methods to analyse for fluoride ion. Specific ion fluoride electrode; the SPADNS method that involves the bleaching of a colored zirconium complex by fluoride ion and measuring the remaining dye colorimetrically; and ion chromatography. The following links will give some info. The Hach site is easiest to use.
What i recommend doing is to talk with an applications rep and describe your samples in detail and then make arrangements for them to analyse some of your samples. I recommend 3 samples  a freshly made bath with an accurately known F- concentration [just make up a small amount]  A sample of a working bath,well mixed, sampled just before you are about to add replenishing fluoride and  a sample of the same bath after adding a known amount of fluoride again well mixed. This should determine the accuracy, sensitivity, and possible interferences. Then consider equipment needed, costs etc.
The specific ion electrode might give the best results because it measures the fluoride activity rather than concentration. The dye bleaching method might remove fluoride ion from weaker complexes in the solution and give a drifting endpoint.
Ion chromatography requires expensive equipment and is probably not useful for routine work unless there are other applications. It might be useful in trouble shooting on a contract basis.
If you have any questions you may contact me by email.
colorimetric and specific ion electrode
http://www.thermo.com/com/cda/SearchHome?new=Y&query=fluoride%20analysis -- mostly specific ion electrode (Spectronic has a colorimetric method because I wrote it for them years ago, but who knows today?)
http://www.rowe.com.au/CatPDFs/259.pdf -- F test paper possibly useful with serial dilution
Try the links in the MadSci Library for more information on Chemistry.