Eluant concentration and pH value

In addition to the type of eluant, its concentration is one of the most important parameters affecting retention. The pH value of the mobile phase is closely connected to the eluant concentration. When a carbonate/bicarbonate mixture is selected as an eluant, parameters such as ionic strength and pH value cannot be considered separately, because the selectivity changes as the concentration ratio of the two components is varied. 

In general, the retention of monovalent and divalent ions shifts forward as the eluant concentration increases. On the other hand, a change in pH primarily affects the retention behavior of multivalent ions, because the valency of such 

An ion chromatographic separation of the three orthophosphate species is impossible because of these pfC equihbria. When using the relatively weak sodium hydroxide eluant, the pH value increases as the concentration increases. With a 1 mmol/L NaOH (pH 11) eluant, orthophosphate and sulfate do not elute at all, primarily because the concentration of the monovalent eluant ions is too low for divalent analyte ions to elute. Moreover, at pH 11 about 10% of the orthophosphate exist as P04 ions, which have a much longer retention time than HP04 ions. When the sodium hydroxide concentration is increased, the monovalent and divalent ions are shifted forward in retention however, orthophosphate elutes later, because the pH value and, thus, the percentage of trivalent orthophosphate ions increases with increasing sodium hydroxide concentration. 

A more complex situation is the carbonate/bicarbonate buffer mixture. Due to the dissociation equihbritun. 

Clearly, the retention times of the anions investigated decrease with an increasing amount of carbonate at a constant bicarbonate content (pH 8.35 to 10.23). However, when bicarbonate is added at constant carbonate concentrations (pH 

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