Experimental Parameters Affecting Retention when Applying Direct Conductivity Detection

3 Experimental Parameters Affecting Retention when Applying Direct Conductivity Detection 

The concept of employing free acids as eluents dates back to the investigations of Fritz et al. [67]. In 1981, they already demonstrated that free benzoic acid may be employed as eluent instead of the sodium or potassium salts that were utilized until then. This is understandable, taking into account that benzoic acid is partially dissociated in aqueous solution. The degree of dissociation may be calculated from the dissociation constant Ka according to the Ostwald dilution law [54]  

The degree of dissociation decreases with increasing electrolyte concentration. With the dissociation constant for benzoic acid, Ka = 6.25 10-5, the degree of dissociation of a benzoic acid solution is calculated to be a = 0.2 for the concentration of c = 1.25 10 3 mol/L used as an example. Therefore, such a solution contains 2.5 10-4 mol/L of oxonium ions and benzoate ions, respectively. Comparing such a benzoic acid solution with a sodium benzoate solution of the concentration c = 2.5 10-4 mol/L, an almost identical elution behavior is observed. 

With benzoic acid as the eluent, generally higher sensitivities are obtained for the analyte anions in comparison to sodium or potassium benzoate, respectively. This is illustrated in the following reaction scheme  

According to this scheme, analyte ions bound transiently to the fixed stationary charge are exchanged against an equivalent amount of benzoate ions, which leads to a corre- 

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