Analyte: fluoride

The fluoride ion, F^–, is the base of hydrofluoric acid, HF, which is a rather weak, but highly corrosive acid, with a pKa of 3.2. The fluoride ion is singly charged and carries a hydration layer comprised of 2.7 associated water molecules, which is quite a lot considering its small size. Some properties of the chloride ion are summarized in the table below.

Fluoride analysis

Fluoride is frequently analysed by ion chromatography (IC) with suppressed conductivity detection along with the other common inorganic anions, using either carbonate-bicarbonate eluents or hydroxide eluents, but only columns with a sufficiently hydrophilic surface will provide sufficient retention of fluoride to separate it from non-retained solutes and the “water dip”. The water dip is the negative peak sometimes occurring in ion chromatography due to injection of aqueous solutions with lower conductivity than the suppressed eluent. Insufficient separation from the water dip may lead to poor peak integration which impacts calibration linearity and limits the lower quantification limits.

Table of some fluoride ion properties at 25 °C

Ion	Molecular weight, M	Molar conductivity, λ₀	Diffusion coefficient, D	Ionic radius, r	Hydration shell, Δr
	(g/mol)	(S·cm²/mol)	(mm²/ms)	(pm)	(pm)
Fluoride, F^–	19.00	55	1.48	133	79
M, was retrieved from Chemical Aid Molecular Mass Calculator. Values for λ₀ & D, are from CRC Handbook of Chemistry and Physics, 75th Ed., D.R. Lide (Ed.), CRC Press Inc. (1994), Boca Raton, pp 5-90 to 5-92. Data for r & Δr, plus the number of water molecules, n, mentioned in the text, are from Y. Marcus, J. Chem. Soc. Faraday Trans., 87 (1991) 2995-2999. The pKa values mentioned in the text are from Organic Chemistry Data pKa compilation.

Learn more

To learn more, browse the application examples below, return to the knowledge overview, or contact Diduco to discuss your specific analysis.

Applications

ion chromatography

Preserving eluent quality with dedicated storage protected from ambient carbon dioxide

To ensure that liquid chromatography analysis methods meet regulatory demands of retention repeatability as dictated by pharmacopoeia and the ICH guidelines, it is important to protect weakly buffered eluents from ambient carbon dioxide. The formation of carbonate species will otherwise change the pH and strength of the eluent, thus shifting retention.

ion chromatography

Screening of contaminants and forced degradation products in lithium ion battery electrolyte

Electrolytes in rechargeable batteries for cars, mobile phones, and other portable electric devices typically contain negatively charged counterions that together with lithium and a non-aqueous solvent form an ionic liquid. The bis(fluorosulfonyl)imide anion is a rather new ionic liquid constituent which thus need new quality control procedures.

ion chromatography

Comparing suppressed and non-suppressed ion chromatography

Suppressed ion chromatography with conductivity detection is a powerful and sensitive technique to analyse charged ionic species, especially anions of strong acids. The chemically regenerated membrane suppressor Xenoic® XAMS can enable analysis of such anions down to levels of a few microgram per litre (part per billion, ppb).

ion chromatography

Quantification of common anions in bottled drinking water according to EPA method 300.1A

Official procedure for quality control of drinking water according to the United States Environmental Protection Agency. This procedure allows quantification of the seven common inorganic anions fluoride, chloride, nitrite, bromide, nitrate, phosphate and sulfate in the ranges recommended by WHO and national authorities.

ion chromatography

Hydroxide gradient elution of organic acids and inorganic anions

Gradient elution is a powerful tool when aiming to separate many compounds having a wide range of different chemical characteristics. Hydroxide gradients are the most common approach in ion chromatography since they can be converted into pure water with nearly zero conductivity.

ion chromatography

Quantification of common anions in drinking water according to EPA method 300.0A

Older version of the official procedure for quality control of drinking water according to the United States Environmental Protection Agency.

ion chromatography