A simple spectrophotometric method has been established for the determination of chlorhexidine (CH) with the ohydroxyhydroquinonephthalein(QP)-manganese (II) complex. The apparent molar absorptivity and relative standard deviation for 50.6 µg of CH were 5.88X1041 mol-' cm-' and 0.47% (n=8), respectively. This method was applied to the determination of CH in a wide variety of pharmaceutical preparations with satisfactory analytical results. From the thermodynamic parameters (AG=-5.49 kcal mol-', AH=0.04 kcal mol-' and AS=18.60 cal mol-' K-') obtained from a van't Hoff plot, the binding between CH and the QP-manganese(II) complex was presumed to be a hydrophobic interaction. Chlorhexidine (CH), 1,1'-hexamethylene-bis[5-(4-chlorophenyl)biquanide], has been widely used as a general skin-and surface-antiseptic agent in various pharmaceutical preparations, and is usually formulated as a salt, either the hydrochloride (CH•HCl) or the gluconate (CH•Glu).Various spectrophotometric methods2-' 1 for the determination of CH have been reported. Materials used include Bromothymol Blue (BTB)2, Bromocresol Green (BCG)3'4, Methyl Orange (MO)5'6, tetrabromophenolphthalein ethylester (TBPE)', sodium hypochloriteg-'0 and Bratton-Marshall reagent.11 Since most of these methods require solvent extraction, they have such disadvantages as complexity of procedure and lack of reproducibility. In addition, they lose their effectiveness because of interference from other existing ingredients is pharmaceutical preparations.We noticed that an o-hydroxyhydroquinonephthalein (QP)-manganese(II) complex exhibited a considerable red shift in the presence of CH, and that this color reaction was sufficiently fast, sensitive and selective. In this work, a simple spectrophotometric method for the determination of CH by using the QP-manganese(II) complex was developed and applied to the assay of CH in a wide variety of pharmaceutical preparations. In addition, a binding process of the colored complex formed in the reaction between the QP-manganese(II) complex and CH was studied by using dual doublereciprocal plots and a van't Hoff plot.
ExperimentalReagents and solutions CH•HC1(supplied from Maruishi Pharmaceutical Co., Ltd.) was titrated potentiometrically with perchloric acid in anhydrous acetic acid medium;12 its purity was found to be 98.60%. A standard solution (1.0X103 M) of CH was prepared by dissolving proper quantities of the CH•HC1 in small amounts of 1 M hydrochloric acid and was diluted to 100 ml with water. A QP solution (5.0X104 M) and a manganese(II) solution (1.0X103 M) were prepared in methanol and deionized water, respectively, as described in a previous report.13 A 1.0% solution of poly(N-vinylpyrrolidone) (PVP, K-30) was prepared in methanol. A solution (1.0X10' M) of sodium tetraborate (borax, pH 9.3) was prepared as a buffer solution.All other reagents and materials were of analytical grade and were used without further purification. Water used as a solvent was deionized.
Apparatus
A ShimadzuModel UV-160 recording spectrophotom...
