สารบัญ

Contents
MENU

6.4 POTENTIOMETRY

6.4 POTENTIOMETRY

      Potentiometry is an electrochemical method based on the measurement of the electrical potential between a pair of suitable electrodes immersed in the solution to be analyzed. The apparatus required consists of an indicator electrode, a reference electrode and a device for measuring potential.

6.4.1 POTENTIOMETRIC TITRATION

      In a potentiometric titration, the end-point of the titration is determined by following the variation of the potential difference between two electrodes (either one indicator electrode and one reference electrode or two indicator electrodes) immersed in the solution being examined as a function of the quantity of titrant added.

      The potential is usually measured at zero or practically zero current.

Apparatus

      The apparatus used (a simple potentiometer or electronic device) comprises a voltmeter allowing readings to the nearest millivolt. The choice of indicator electrode depends on the substance being examined and may be a glass or metal electrode (for example, platinum, gold, silver or mercury). The reference electrode is generally a calomel or a silver-silver chloride electrode. For acid-base titrations and unless otherwise prescribed, a glass-calomel or glass-silver-silver chloride electrode combination is used. Several acceptable electrode systems for potentiometric titrations are summarized in Table 1.

Method

      Plot a graph of the variation of potential difference as a function of the quantity of the titrant added, i.e. for an acid-base titration, pH versus ml of titrant added, for a precipitimetric, complexometric, or oxidation-reduction titration, mv versus ml of titrant added, continuing the addition of the titrant beyond the presumed equivalence point. The end-point corresponds to a sharp variation of potential difference. The equivalence point may also be determined mathematically without plotting a graph. Two types of automatic electrometric titrators are available. The first is one that carries out titrant addition automatically and records the electrode potential differences during the course of titration as a sigmoid curve. In the second type, titrant addition is performed automatically until a preset potential or pH, representing the end-point, is reached, at which point the titrant addition ceases.

Table 1 Potentiometric Titration Electrode System

6.4.2 POTENTIOMETRIC DETERMINATION OF IONIC CONCENTRATION USING ION-SELECTIVE ELECTRODES

      The potentiometric determination of the ion concentration is carried out by measuring the potential difference (E) between two suitable electrodes immersed in the solution to be examined; the indicator electrode is selective for the ion to be determined and the other is a reference electrode.

      The potential E of an ion-selective electrode varies linearly with the logarithm of the activity ai of a given ion, as expressed by the Nernst equation:

where E0 = a constant called the standard electrode potential, which is characteristic for each half-reaction,

           R = gas constant,

           T = absolute temperature,

           F = Faraday’s number, and

           z= charge number of the ion including its sign.

      At a constant ionic strength, the following holds:

Apparatus 

      Use a voltmeter allowing measurements to the nearest 0.1 millivolt and whose input impedance is at least 100 times greater than that of the electrodes used. Ion-selective electrodes may be primary electrodes with a crystal or non-crystal membrane or with a rigid matrix (for example, glass electrodes), or electrodes with charged (positive or negative) or uncharged mobile carriers, or sensitized electrodes (enzymatic substrate electrodes, gas-indicator electrodes). The reference electrode is generally a silver-silver chloride electrode or a caomel electrode, with suitable junction liquids, producing no interference.

Procedure

      Carry out each measurement at a temperature constant to ±0.5º, taking into account the variation of the slope of the electrode with temperature (Table 2). Adjust the ionic strength and possibly the pH of the solution to be analyzed using the buffer reagent described in the monograph and equilibrate the electrode by immersing it in the solution to be analyzed, under slow and uniform stirring, until a constant reading is obtained.

      If the electrode system is used frequently, check regularly the repeatability and the stability of responses, and the linearity of the calibration curve or the calculation algorithm in the range of concentrations of the test solution; if not, carry out the test before each set of measurements. The response of the electrode may be regarded as linear if the slope S of the calibration curve is approximately equal to k/zi per unit of pCi .

Table 2 Values of k at Different Temperatures

Method I (Direct Calibration)​

      Measure at least three times in succession the potential of at least three reference solutions spanning the expected concentration of the test solution. Calculate the calibration curve or plot on a chart the mean potential E obtained against the concentration of the ion to be determined expressed as –log Cor pCi .

      Prepare the test solution as prescribed in the monograph; measure the potential three times and, from the mean potential, calculate the concentration of the ion to be determined using the calibration curve.

Method II (Multiple Standard Additions)

      Prepare the test solution as prescribed in the monograph. Measure the potential at equilibrium ET of a volume VT of this solution of unknown concentration CT of the ion to be determined. Make at least three consecutive additions of a volume VS negligible compared to VT (VS ≤ 0.01VT) of a reference solution of a concentration CS known to be within the linear part of the calibration curve. After each addition, measure the potential and calculate the difference of potential ΔE between the measured potential and ET. ΔE is related to the concentration of the ion to be determined by the equation:

where VT = volume of the test solution,

          CT = concentration of the ion to be determined in the test solution,

          V= added volume of the reference solution,

          CS = concentration of the ion to be determined in the reference solution, and

           S = slope of the electrode determined experi mentally, at constant temperature, by measuring the difference between the potentials obtained from two references whose concentrations differ by a factor of 10 and are situated within the range where the calibration curve is linear.

      Plot on a graph 10ΔE/S (y-axis) against VS (x-axis) and extrapolate the line obtained until it intersects the xaxis. At the intersection, the concentration CT of the test solution in the ion to be determined is given by the equation:

Method III (Single Standard Addition)

      To a volume VT of the test solution prepared as prescribed in the monograph, add a volume VS of a reference solution containing an amount of the ion to be determined known to give a response situated in the linear part of the calibration curve. Prepare a blank solution in the same conditions. Measure at least three times the potentials of the test solution and the blank solution, before and after adding the reference solution. Calculate the concentration CT of the ion to be analyzed using the following equation and making the necessary corrections for the blank:

where VT = volume of the test solution or the blank,

          CT = concentration of the ion to be determined in the test solution,

          V= added volume of the reference solution,

          C= concentration of the ion to be determined in the reference solution,

         ΔE = difference between the average potentials measured before and after adding VS, and

           S = slope of the electrode determined experi mentally, at constant temperature, by measuring the difference between the potentials obtained from two reference solutions whose concentrations differ by a factor of 10 and are situated within the range where the calibration curve is linear.

APPENDICES • 6.4 POTENTIOMETRY
view 2,229 ผู้เข้าชม / View
หมายเหตุ / Note : TP II 2011 PAGE 491-494