The chloride ion, Cl–, is the base of hydrochloric acid, HCl, which is a strong acid with a pKa of -6.3, and thus fully dissociated in all aqueous solutions regardless of pH. Chloride is singly charged and carries a hydration layer comprised of 2.0 associated water molecules. The full dissociation and rather high molar conductivity results in good sensitivity when the chloride ion is monitored by conductivity detection.
Chloride is typically analysed by ion chromatography (IC) with suppressed conductivity detection and quite many different IC columns can accomplish the separation using either carbonate-bicarbonate eluents or hydroxide eluents. Many HILIC columns will also retain chloride, and this may thus be an alternative analysis technique, whereas reversed-phase (RP) and ion-exclusion chromatography (IEC, ICE) tend to not give sufficient retention.
Analysis of chloride is important in several industries, including food & beverage, pharmaceutical, energy & electronics, and environmental monitoring. As one of the seven common anions outlined in the WHO Guidelines for drinking-water quality, analysis of chloride is described in several standardized IC methods including US EPA 300.1, DIN/EN/ISO 10304-1, and ASTM D4327-17. Since chloride ion is the major component by weight in table salt (which is >97% sodium chloride, NaCl), its concentration is monitored in many processed food and beverages. Chloride is a frequent constituent of pharmaceutical formulations, both as counterion excipient in many tablets together with basic active ingredients, and as an active ingredient in several types of injection solutions, and its analysis is therefore described in several pharmacopoeia monographs. Trace levels of Cl– ions are monitored in battery electrolyte raw materials and circuit board manufacturing since its presence may disturb various electric processes. Because chloride is a corrosive agent, it is important to control its level in several industrial settings including heat exchangers, and also in building materials such as concrete.
|Ion||Molecular weight, M||Molar conductivity, λ0||Diffusion coefficient, D||Ionic radius, r||Hydration shell, Δr|
|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.|
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