A simple and accurate method for the determination of chloroquine diphosphate (CQ), pyrimethamine (PYM) and trimethoprim
(TMP) in pure, pharmaceutical and urine samples is described. The method involved an ion pair extraction procedure with chloroform
using an acid base indicator, bromocresol purple (BCP), as a counter ion. The ion pair extracted in organic phase was yellow,
and the absorbance was measured at λmax = 420 nm. Beer’s law was obeyed over the concentration range of 1.25-8.75, 0.62-7.5,
and 1.25-10.71 μg/mL, respectively. The Sandell’s sensitivity was 0.01258, 0.01102 and 0.01464 μg/cm2. The Ringbom optimum concentration range, which was 2.5-7.5, 1.25-7.5 and 2.5-10.7 μg/mL for CQ, PYM, and TMP, respectively, exhibited reliability of the method. Stoichiometry and thermodynamic of the complexes were evaluated by Job’s method and Benesi-Hildebrand plot. The statistical results of intra- and inter-day estimation of drugs and comparison with the reported methods demonstrated high precision and accuracy of the method. The procedure was rapid, simple and suitable for quality control purpose.