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There are two basic methodolgies for measuring fluoride concentrations in water. Following is a comparison of the ion chromatogaphy and selective ion probe methods in the determination of fluoride.

Ion chromatography relies on the separation of ions based on their affinities for low capacity, strongly basic anion exchangers. The separated anions are converted to a highly conductive acidic form, and are measured by conductivity in that form. They are identified on the basis of retention time as compared to standards. The quantification is based on peak area or height. In waters of unknown matrices, this method is not recommended because simple organic acids such as formic and carbonic elute close to fluoride and will interfere. Since small samples are used for analyses, contamination from glassware, reagent water, or sample processing apparatus are not uncommon. Also, high concentrations of one ion may interfere with the retention of other ions. Some of these issues can be handled through the use of known additions, sample dilution, or gradient elution. Sample particulates need to be removed before injection. Standard Methods for the Examination of Water and Wastewater cites bias of about 0 to 7% for a variety of prepared concentrations at drinking water levels.

The key to the selective ion probe method is the fluoride electrode, which is an ion specific sensor. The electrode contains a laser-type doped lanthanum fluoride crystal across which a potential is established by fluoride solutions of different concentrations. The crystal is an interface between the sample solution and an internal reference solution. Generally, a silver/silver chloride reference probe is utilized. The electrode measures the activity of the fluoride ion, which will vary with ionic strength of the solution, as well as with pH, fluoride concentration, and fluoride complexing species. A buffer will help assure a uniform ionic strength background. The pH maintained by the buffer helps resolve issues associated with both high and low pH. The buffer will also complex interfering ions and free fluoride ions. Normally, no major adjustments are needed with any instrument routinely used for fluoride analyses in the range normally found in drinking waters at 0.2 to 2.0 mg/l. Quantifications are based either on direct readings against known standards, or plotting potential readings against concentrations on two-cycle semi logarithmic graph paper. Standard Methods cites errors ranging from 0.2 to 4.9% on synthetic samples within the normal drinking water range.

For most drinking water applications, it appears the selective ion probe method, with a properly buffered sample, provides the greatest flexibility for waters of various concentrations for good accuracy and least influence by interferences.