Analytical Sciences

A Unified Ion Chromatographic System for the Determination of Acidity and Alkalinity

Wenzhi HU,*  Kiyoshi HASEBE,** Alexander ILES,** and Kazuhiko TANAKA***

*Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
**Division of Material Science, Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan
***National Institute of Advanced Industrial Science and Technology at Seto, 110, Nishiibara-cho, Seto, Aichi 489-0884, Japan

A unified ion chromatographic (IC) system was developed for the determination of acidity or alkalinity. Separation column used was a reversed-phase ODS packed column, which had been modified by saturating it with lithium dodecylsulfate. A slightly acidified LiCl (50 mM LiCl and 0.05 mM H₂SO₄) aqueous solution was used as the eluent. By conditioning the separation column in this way, both H⁺ and Li⁺ ions became bound to the stationary phase. Dodecylsulfate groups with Li⁺ counterions acted as cation-exchange sites for the separation of hydrogen ions (free acidity determination). The remaining dodecylsulfate groups, with H⁺ counterions acted as a titrant, which reacted with basic species (total alkalinity determination). The acidity or alkalinity of each sample was measured according to the change in conductance from the eluent baseline level. A positive peak was observed from those samples with a free acidity greater than their total alkalinity, due to the separation/elution of free H⁺ ions. A negative peak was observed from those samples with a free acidity less than their total alkalinity. This was due to an equivalent amount of eluent H⁺ ions being re-supplied to the stationary phase while the ''solid titrant'' consumed by the acid-base reaction was regenerated. The retention time for the peak corresponding to the acidity or alkalinity was governed by the retention time for H⁺ ions in this IC system. Samples with a free acidity greater than 2.25 uM (tested by determination of H⁺ ions in pure water in equilibrium with atmospheric CO₂) could be analyzed by this method. A very similar detection level was obtained for alkalinity (tested by analyzing standard aqueous NaHCO₃ solutions). Aqueous solutions of some strong-acid/strong-base inorganic salts were found to be slightly alkaline. This was measured as a percentage, relative to an NaHCO₃ solution at the same concentration. Solutions of NaClO₄, Na₂SO₄, NaI, NaNO₃, and NaCl, gave comparative alkalinity values of 8.75%, 1.83%, 0.42%, 0.35%, and 0.33%, respectively.

J-STAGE: View this article in J-STAGE