Spectrophotometric study of the reaction mechanism between DDQ Π- and iodine σ-acceptors and chloroquine and pyrimethamine drugs and their determination in pure and in dosage forms

Two simple and accurate spectrophotometric methods are presented for the determination of anti-malarial drugs, chloroquine phosphate (CQP) and pyrimethamine (PYM), in pure and in different pharmaceutical preparations. The charge transphere (CT) reactions between CQP and PYM as electron donors and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) π-acceptor and iodine σ-acceptor reagents to give highly coloured complex species have been spectrophotometrically studied. The optimum experimental conditions have been studied carefully. Beer' law is obeyed over the concentration range of 1.0-15 μg ml−1 for CQP and 1.0-40 μg ml−1 for PYM using I2 and at 5.0-53 μg ml−1 for CQP and 1.0-46 μg ml−1 for PYM using DDQ reagents, respectively. For more accurate results, Ringbom optimum concentration range is calculated and found to be 10-53 and 8-46 μg ml−1 for CQP and PYM using DDQ, respectively and 5-15 and 8-40 μg ml−1 for CQP and PYM using iodine, respectively. The Sandell sensitivity is found to be 0.038 and 0.046 g cm−2 for DDQ method and 0.0078 and 0.056 g cm−2 for I2 method for CQP and PYM, respectively which indicates the high sensitivity of both methods. Standard deviation (S.D. = 0.012-0.014 and 0.013-0.015) and relative standard deviation (R.S.D. = 0.09-1.4 and 1.3-1.5%) (n = 5) for DDQ and I2 methods respectively, refer to the high accuracy and precision of the proposed methods. These results are also confirmed by between day precision of percent recovery of 99-100.6%, and 98-101% for CQP and PYM by DDQ method and 99-102% and 99.2-101.4% for CQP and PYM by I2 method respectively. These data are comparable to those obtained by British and American pharmacopoeias assay for the determination of CQP and PYM in raw materials and in pharmaceutical preparations.