Potentiometric sensors selectivity coefficient

Here, the potentiometric selectivity coefficient is given with respect to the hydroxyl ion. Single-crystal lanthanum fluoride is a wide bandgap semiconductor in which the electrical conductivity is due only to the hopping mobility of fluoride ions through the defects in the crystal. It does not respond to the La3+ ion because of the slow ion exchange of that ion. Hydroxyl ion is the only other ion that has appreciable mobility, and is the only known interference. For this reason, the measurements with a fluoride electrode are always done below pH 7, which circumvents this interference. As shown later, the consideration of ionic and/or electronic conductivity of the membrane plays a critical role also in the design of the internal contact in nonsymmetric potentiometric sensors. 

S-enalapril assay can be done using the potentiometric electrode based on impregnation of 2-hydroxy-3-trimethylammoniopropyl-/i-cyclodextrin (as chloride salt) solution in a carbon paste, in the 3.6 x 10 5-6.4 x 10-2 mol/L (pH between 3.0 and 6.0) concentration range with a detection limit of 1.0 x 10 5 mol/L [25]. The slope is near-Nernstian 55.00 mV/decade of concentration. The average recovery of S-enalapril raw material is 99.96% (RSD — 0.098%). The potentiometric selectivity coefficient over D-proline (6.5 x 10 4) proved the sensor s enantioselectivity. S-enalapril was determined from pharmaceutical tablets with an average recovery of 99.59% (RSD — 0.20%). 

The EPME based on impregnation of 2-hydroxy-3-trimethylammoniop-ropyl-//-cyclodextrin (as chloride salt) solution in a carbon paste can be reliably used for S-trandolapril assay with an average recovery of 99.77% (RSD — 0.22%) [24]. The linear concentration range is 10 4-10 2 mol/L on the 2.5-5.5 pH range. The detection limit is of 10 5 mol/L magnitude order. The slope is near-Nernstian 52.45 mV/decade. The sensor enantioselectivity was determined over D-proline, when a 10 4 magnitude order was obtained for potentiometric selectivity coefficient. 

An investigation on the effects of the electrostatic force and the Mulliken charge distribution on the selectivity of MIPs was carried out based on DFT for hydroxyzine- and cetirizine-imprinted polymers [Azimi et al., 2014]. The results showed a correlation between the selectivity coefficients and the theoretical charge distributions and also showed that charge distribution based model was able to predict the selectivity coefficients of MIP based potentiometric sensors. 

Azimi,A., Javanbakht, M. (2014). Computational prediction and experimental selectivity coefficients for hydroxyzine and cetirizine molecularly imprinted polymer based potentiometric sensors, AnaL Qiinh 812,184-190. 

The inner filling solution for the sensors is usually 0.01 M NaCl, which is also used to condition the potentiometric sensors. Electrochemical potential is measured with the following galvanic cell Ag/AgCl/bridge electrolyte/sam-ple solution/ion-selective membrane/inner filling solution/ AgQ/Ag. A high impedance pH-mV meter is used to measure the electrochemical potential. Selectivity coefficients are evaluated by the matched potential method (also known as method of mixed solutions), or via the separate solution method. 

S electivity an expression of whether a sensor responds selectively to a group of analytes or even specifically to a single analyte. Quantitative expressions of selectivity exist for different types of sensors. For potentiometric sensors, e.g. (Chap. 7, Sect. 7.1), it is given by the selectivity coefficient. 

Graphite electrodes were coated electrochemically with 3-methylPT, then iodine was incorporated into the polymer phase by electrochemical oxidation of iodide at a potential of -f0.70 V. This modified electrode as a potentiometric iodide ion sensor was found to be suitable for the measurement of iodide concentrations down to 10 M its selectivity coefficients for most of the potential interferents (15 anions have been studied) have been estimated to be of the order of 10 [207]. 

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