Venlafaxine, 1–[2–(dimethylamino)–1–(4–methoxyphenyl) ethyl]cyclohexanol hydrochloride is a novel non-tricyclic antidepressant. Venlafaxine is a second generation antidepressant drug, has a neuropharmacologic profile distinct from that of existing antidepressants including tricyclic compounds, selective serotonin reuptake inhibitors and monoamine oxidase inhibitors. Venlafaxine imparts its antidepressant effects by inhibiting the neuronal uptake of norepinephrine, serotonin and to a lesser extent dopamine. It lacks monoamine oxidase activity and, more importantly, lacks the adverse effect profile of tricyclic antidepressants. Potentiometric detection based on ion-selective electrodes (ISEs) offers several advantages such as speed, ease of preparation and procedures, simple instrumentation, relatively fast response, wide dynamic range, reasonable selectivity, and low cost. In first part of this work,the construction and electrochemical response characteristics of polyvinyl chloride (PVC) membrane sensor for the determination of venlafaxine is described. The sensor having venlafaxinesodiumtetraphenyl borate (ion-pair) as an electroactive material and dibutyl phthalate (DBP) as a plasticizer in a PVC matrix in the percentage ratio of 5.1:66.0:28.9 (ion-pair:DBP:PVC, w/w).The sensor exhibits a linear response to venlafaxine in a concentration range of 1.0×10⁻¹ to 8.0×10⁻⁶ M with a limit of detection of 4.9×10⁻⁶ mol L−1 and with a slope of 29.4±0.1 mV decade−1 over the pH range 3.0–9.0. Selectivity coefficients of the sensor for venlafaxine relative to a numbers of potential interfering substances were investigated. The sensor is highly selective for venlafaxine over a large number of organic and inorganic compounds and some drugs. The sensor showing a fast response time of 10 s and was used over period of 2 months (for two determinations every day) with a good reproducibility. The sensor was successfully applied to determine venlafaxine in pharmaceutical,urine and blood serum samples with satisfactory results.                Morphine, 7,8-Didehydro-4,5-epoxy-17-methyl morphinan-3,6-diol sulfate (2:1)(salt), pentahydrate exhibits well known analgesic properties  which render the drug useful for the  treatment of chronic pain such as cancer pain. Since the clinical use of morphine has become increasingly common, it is important to find and use a selective and rapid method for the determination of morphine in human plasma or serum. In second part of this work, the construction and electrochemical response characteristics of polyvinyl chloride (PVC) membrane sensor for the determination of morphine is described.The electrode exhibits a Nernstian response over the morphine concentration range 5.0×10^-6 to 1.0×10⁻² M with a slope of 58.6±1.12 mV per decade of concentration. The other performances of this electrode are low detection limit of 3.3×10^-6 M, short response time (≤ 12 s), good repeatability and stability in response for at least 45 dayes and extend working pH range of 4.4-9.3. The proposed electrode is more selective for determination of morphine over a wide variety of common ions and some drugs. The electrode was used to determine morphine in human urine and plasma samples with satisfactory results. Good recoveries were obtained in all samples.

Key words: venlafaxine, morphine, potentiometric sensors, detection limit