Conductometric detection of anions of weak acids in chemically suppressed ion chromatography: Critical point concentrations

Previously we demonstrated that the conductometric determination of anions of weak acids in chemically suppressed ion chromatography could be enhanced, by conversion of the acid to a conjugate salt, if the anion was present above what we termed the critical point concentration (CPC), In this paper we have developed a simple theoretical model for the calculation of CPCs for weak acid/conjugate salt pairs. The CPC was found to be dependent on the acid ionization constant K-a and the molar ionic conductivities (MICs) of the ions present. For monovalent anions with a MIC in the common range 25-75 S cm(2) mol(-1), with sodium or potassium as cations, the CPCs could be estimated from the expression pC approximate to pK(a) - 1, where pC = -log CPC, For formate, benzoate,; and acetate, excellent agreement was found between the calculated and experimental CPCs, with a mean deviation of 0.05 mM. For fluoride, the calculated and experimental CPCs were 7.4 and 9.2 mM, respectively. Experimental CPCs could not be determined for other anions as their calculated CPCs were below the detection limits of the IC system, The simple theoretical model could also be used to estimate the expected conductivity enhancement for the conversion of weak acid to a particular conjugate salt. The effects of detector linearity and dispersion in the IC system on the use of CPCs are also discussed.