The sulfate ion (also sulphate ion), SO42-, is the end base of sulfuric acid, H2SO4, which is a diprotic strong acid with pKa values of -3 and 1.99, and thus fully dissociated in most aqueous solutions. Sulfate is doubly charged and carries a hydration layer comprised of 3.1 associated water molecules. The full dissociation, the number of charges, and high molar conductivity results in high sensitivity when the chloride ion is monitored by conductivity detection. Some properties of the sulfate ion are summarized in the table below.
Sulfate 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. However, the retention may vary significantly between different columns depending on the nature of the ion exchange groups and their proximity. Some HILIC columns may also retain sulfate, 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.
|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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