Inclusion of Secondary Equilibria in the Mass Balance

HPLC analytes involved in adsorption equilibrium processes could also participate in other types of processes, such as ionization, tautomerization, solvation, and so on. The analyte behavior in adsorption equilibrium is certainly affected by the presence of secondary equilibrium, and if it could be considered as an independent process based on the time and position of the analyte within the column, this equilibrium could be incorporated in the solution of mass-balance equation. 

Let us assume the presence of ionic equilibrium for an ionizible basic analyte  

Above equilibrium is dependent on the mobile-phase pH and the relationship between ionic and nonionic form of the analyte is described by Henderson-Hasselbalch equation 

The left-hand side of equation (2-33) expresses the presence of the gradient of the analyte in the column cross section. In the case of an ionizable analyte, there are two forms of the analyte present, and using expression (2-73) the left-hand side of equation (2-33) should be written in the form 

The equilibrium concentration of the ionic analyte form (cbh+) in the second term could be substituted with Cb from expression (2-73)  

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