Determination of the dissociation constants (pKa) of secondary and tertiary amines in organic media by capillary electrophoresis and their role in the electrophoretic mobility order inversion.
Non-aqueous capillary electrophoresis (NACE) may provide a selectivity enhancement in separations since the analyte dissociation constants (pKa) in organic media are different from those in aqueous solutions. In this work, we have studied the inversion in mobility order observed in the separation of tertiary (imipramine (IMI) and amitryptiline (AMI)) and secondary amines (desipramine (DES) and nortryptiline (NOR)) in water, methanol, and acetonitrile. We have determined the pKa values in those solvents and the variation of dissociation constants with the temperature. From these data, and applying the Van't Hoff equation, we have calculated the thermodynamic parameters deltaH and deltaS. The pKa values found in methanol for DES, NOR, IMI, and AMI were 10.80, 10.79, 10.38, and 10.33, respectively. On the other hand, in acetonitrile an opposite relation was found since the values were 20.60, 20.67, 20.74, and 20.81 for DES, NOR, IMI, and AMI. This is the reason why a migration order inversion is observed in NACE for these solvents. The thermodynamic parameters were evaluated and presented a tendency that can be correlated with that observed for pKa values.
Cantu
Marcelo Delmar
MD
Laboratório de Cromatografia, Instituto de Química de São Carlos, Universidade de São Paulo, Av. Trabalhador São-carlense, 400, Cx. Postal 780, CEP 13560-970 São Carlos, SP, Brazil.
Hillebranda
Sandro
S
Carrilho
Emanuel
E
D000588:Amines / Q000737:chemistry*; D019075:Electrophoresis, Capillary / Q000379:methods*; D012680:Sensitivity and Specificity; D013816:Thermodynamics
D000588:Amines