Effect of pH
The pH is evaluated as a critical factor to increase the partition
coefficient of the analyte between aqueous and surfactant-rich
phases which lead to increase the extraction efficiency. Therefore
the effect of pH on extraction efficiency of carmine was investigated
in the pH range of 2–8. The absorbance of surfactant-rich
phase containing carmine was measured at 513 nm. As the results
shown in Fig. 1, increase of pH up to 5 lead to an increase in absorbance
and then decreased. Decrease in extraction efficiency at higher pHs (pH > 5) can be attributed to dissociation of carboxyl
group of carmine to carboxylate ion. Therefore, pH 5 was selected
as the optimum pH.