Charge-transfer complexes of barbiturates and phenytoin.

Simple and sensitive spectrophotometric methods are described, for the first time, for the determination of sodium salts of phenobarbital (1), thiopental (2), methohexital (3) and phenytoin (4). The methods are based on the reaction of these drugs as n-electron donors with the sigma-acceptor iodine and various pi-acceptors: 7,7,8,8-tetracyanoquinodimethane; 2,3-dichloro-5,6-dicyano-1,4-benzoquinone; 2,3,5,6-tetrachloro-1,4-benzoquinone; 2,3,5,6-tetrafluoro-1,4-benzoquinone; 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone; tetracyanoethylene and 2,4,7-trinitro-9-fluorenon. Depending on the solvent polarity, different coloured charge-transfer complexes and radicals were developed. Different variables and parameters affecting the reactions were studied and optimized. The formed complexes were examined by UV/VIS, infrared and (1)H-NMR. Due to the rapid development of colours at ambient temperature, the obtained results were used on thin layer chromatograms for the detection of the investigated compounds. Beer's plots were obeyed in a general concentration range of 1-400 mug ml(-1) for the investigated compounds with different acceptors. Interference from some co-formulated drugs was also studied. No interference was observed due to additives commonly present in the pharmaceutical preparations. The proposed methods could be applied successfully to the determination of the investigated compounds in pure and pharmaceutical dosage forms with good accuracy and precision, the recoveries ranged from 98.7+/-0.5 to 101.1+/-0.5%. The results were compared favourably with the official methods.