Aminoglycoside antibiotics occupy leading positions in the treatment of serious infectious-inflammatory disorders. However, the relatively low therapeutic ratios of these drugs makes their successful administration possible only if the concentration of the active compound in the blood is thoroughly controlled. Slow elimination of aminoglycosides may result in oto-and nephrotoxicity manifestations [1,2].The practical need for reliable and convenient methods for monitoring the level of antibiotics in the blood stimulates investigations devoted to the development of such methods for determining aminoglycosides. Our analysis of the published data showed that the quantitative determination of aminoglycoside antibiotics is usually performed using microbiological [3], spectrophotometric [4], and chromatographic [5] techniques. Only a few papers have been devoted to the development of electrochemical methods for the determination of antibiotics of the aminoglycoside group (see, e.g., [6]).The aim of this study was to develop an ionometric technique for determining gentamicin and kanamycin in various medicinal forms and biological fluids. EXPERIMENTAL PARTThe experiments were performed with commercial gentamicin sulfate (4% injection solution in ampules) and kanamycin sulfate (lyophilized powder in flasks). The initial stock (1´10 -2 M) solutions were prepared using accurately weighed amounts of the initial drugs, and the less concentrated solutions (from 1´10 -3 to 1´10 -5 M) were obtained by serial dilutions. The names, structural formulas, and molar weights of the studied and auxiliary compounds are given in Table 1.The electrode-active compound (EAC) was gentamicin tetraphenylborate (Gen-TPB) obtained by mixing equimolar amounts of sodium tetraphenylborate (Na-TPB) and gentamicin sulfate (C TPB =10 -2 M, V = 20 ml; C Gen =10 -2 M, V = 20 ml) at room temperature. The mixture was allowed to stand for three days for precipitation, after which the precipitate was washed by decantation with distilled water and dried in a thermal box at 60°C.The electrochemical measurements were performed using ion-selective electrodes (ISEs) based on Gen-TPB ion associates with poly(vinyl chloride) PVC membranes plasticized by dibutylphthalate (DBP) in the ratio PVC : DBP =1 : 3 (C EAC = 0.01 mole/kg DBP). The liquid-filled film electrodes had the form of PVC tubes with membrane disks of the same diameter glued to thoroughly lapped edge faces. The glue was prepared by dissolving 0.5 g PVC in 5 ml of tetrahydrofuran or cyclohexane. After drying of the glue, the electrode tube was filled with the standard solution comprising a mixture of the corresponding antibiotic solution (C =10 -3 M) and sodium sulfate (C =10 -3 M) in a 1 :1 ratio. Prior to measurements, the ISE was conditioned in a 10 -3 M solution of the corresponding antibiotic.The potentiometric procedure was performed in a cell with a silver chloride reference electrode (EVL-1M3) using an I-130 potentiometer; the error of emf measurements was ± 1 mV. The pH measurements were perform...