On-line extraction-spectrophotometric determination of neostigmine in pharmaceuticals using double membrane phase separator and monovalent dyestuff

A simple, sensitive and rapid spectrophotometric method for the determination of neostigmine by flow injection analysis (FIA) coupled with an ion associate extraction has been developed. The three-line manifold was assembled. Neostigmine(200 μl) was injected into a distilled water stream and the pH was adjusted to 10 with a borate–phosphate buffer solution. Then, the stream was mixed with the ion-pairing tetrabromophenolphthalein ethylester (TBPEH)-1,2-dichloroethane solution. After phase separation with a double membrane phase separator, absorbance was measured at 610 nm. A linear calibration graph was obtained between 1×10^?7 mol l^?1 and 5×10^?7 mol l^?1 of neostigmine. Up to 48 samples h^?1 could be processed with a relative standard deviation (R.S.D.) of 0.5% (n=5) for 4×10^?7 mol l^?1 neostigmine. The proposed system was applied to the simple, reproducible and rapid determination of neostigmine in commercial pharmaceuticals.