5 �� 0.5 and 53.0 �� 0.5 mV/decade, and lower limit of detection being 6��10-6 M and 4��10-6 (~ 2.3 and 1.5 ��g/ml) for AC-TPB and AC-PM sensors respectively. The least squares equation obtained from the calibration data as follow:E(mV)=(51.5��0.5)log[AC]+(134.0��0.5)for sensor?I(1)E(mV)=(53.0��0.5)log[AC]+(164.0��0.5)sensor?II(2)Table. 1Response characteristics of the PVC membrane sensors.2.2. Response mechanism of the proposed sensorThe mechanism of potential response of most liquid and liquid polymeric membrane sensor is based on ion exchange equilibrium and analyte extraction process at the membrane interface. The membranes frequently contain hydrophobically trapped, mobile sites [19] in plasticized poly (vinyl chloride).
Such membranes with charged sites are named sited membranes.
Ions of opposite sign in the membrane are counter ions. Ions of the same sign as sites are not present in significant quantities are known as coions. Sited membranes are selective to counter ion i.e only counter ions exchange into the membrane and therefore have some mobility in the membrane bulk.2.3. Effect of pHThe effect of pH of the AC test solutions (1��10-3, 1��10-4 and 1��10-5 M) on the sensor potential was investigated by following the potential variation over the pH range 1-12. The electrode response for AC concentrations was tested by various pH values, each time being adjusted by using hydrochloric acid and sodium hydroxide solution. Potential-pH plots (Fig.
1) reveal that, within pH range 2-6, the potential did not vary by more than ��0.5 mV.
At pH < 2 potential displayed by the sensors increased due to increasing the acid nature of the Brefeldin_A drug or interferences by hydrogen ion. At pH > 7.0, the potential displayed by the sensor sharply decreases due to formation of non-protonated acebutolol, the pKa value of AC = 9.4 (secondary aliphatic amine group) [20]. On the other hand upon testing different types of buffer solution e.g. citrate, phthalate, phosphate and acetate in the suitable pH range of the membrane sensor, phosphate buffer (pH 4.0) proved to be a more suitable measuring solution. All subsequent potentiometric measurements were made in phosphate buffer of pH 4.
0.2.4. Response timeThe average response time is defined as the time required for the electrode to reach a steady potential values within ��1 mV of the final equilibrium value, after successive immersion Cilengitide of the electrode in AC solutions each having a 10-fold difference, or after rapid 10-fold increase in concentration by the addition of AC. This time was found to be short, ranging form 15 sec for concentration ��1��10-4M and 20 sec for concentration ��1��10-4M.