A new electrode sensitive and reasonably selective for strychnine Is developed. It is based on the use of strychnine-picrolonate ion-pair complex In nitrobenzene as a liquid membrane. The electrode shows Nernstlan response down to M strychnine solutions over a pH range of 2-7 with a cationic slope of 57 mV/concentration decade. Direct potentiometric determination of samples down to 5 ppm shows an average recovery of 101.3% and a mean standard deviation of 2.4 YO. Potentiometric tiratlon wlth either picrolonic acid or sodium tetraphenylboron shows an average recovery of 99.1 YO and a mean standard deviation of 1.2%. No interferences are caused by some of the other alkaloids, and many of the substances normally occuring as natural contaminants with strychnine.Strychnine is one of the most important alkaloids related to the indole group. It is used, in small doses, as a tonic and stimulant of the central nervous system as well as the respiratory, muscular, and cardiovascular centers, but large doses are fatal. Thus, accurate determination of strychnine is greatly needed in veterinary, pharmaceutical, medicinal, and forensic sciences. Direct spectrophotometry by measuring absorbance at 262 nm has been recommended by the British Pharmacopoeia ( I ) . However, many organic impurities and other alkaloids produce substantial extraneous absorption in the same region and require prior separation ( I , 2).T h e need for a rapid specific method applicable to the determination of strychnine on the micro-and ultramicroscales, in the presence of basic organic impurities, is evidenced by the fact that most of the reported methods are susceptible to environmental reaction conditions and suffer from lack of specificity, sensitivity and stoichiometry. Moreover, a time-consuming extraction of strychnine from all other interferents is a common step in many of the analytical procedures dealing with impure samples (2).Recently, several sensitive potentiometric sensors selective for some organic compounds have been reported (3). Electrodes have been prepared that are selective for the following alkaloids: nicotinate ( 4 ) , methadone (5), methacholine (6), neostigmine (6), diphenhydramine (7), novocaine (8) and ephedrine (9). Tetraphenylboron (6,8,9), dipicrylamine (6, 8), or quaternary ammonium ( 4 , 5 ) derivatives of such compounds were used as electroactive materials in polymeric (4, 5 , 9) or liquid ( 4 , 6,8,9) membranes. However, the complexes of strychnine have not yet been investigated.T h e present work describes a new potentiometric sensor sensitive and reasonably selective for the determination of strychnine down to the ppm scale, with acceptable accuracy and free from many of the difficulties usually encountered in the previously published methods.
EXPERIMENTALApparatus. Potentiometric measurements were made with an Orion Microprocessor Ionalyzer (model 901) using the strychnine electrode in conjunction with an Orion double junction reference electrode (model 90-02) with 10% potassium nitrate in the outer compartment. Argen...
