CHE425L POTENTIOMETRY WITH K + ION-SELECTIVE ELECTRODE READING Skoog, Holler and Crouch: Chapter 23 and Appendix 3. A. INTRODUCTION Potentiometry is a static electroanalytical method in which the potential of an indicating electrode in contact with a solution at equilibrium is measured in the absence of current flow across the electrochemical cell. The potential measured depends on the activity of ions according to Nernst equation. The platinum electrode used in CHE322L last semester, is an example of an electrode that is commonly used for potentiometric measurements. The potential of the Pt electrode is dependent on the concentration of the oxidized and reduced species of a redox couple according to the Nernst Equation. The Pt electrode is a non-selective electrode because it responds to the potential developed by many redox couples. Electrodes that respond selectively to the activity of a specific ion are commonly referred to as ion-selective electrodes. The first electrode of this type was the glass electrode, which responds selectively to the hydronium ion. The success of the glass ph electrode was followed by the development of glass electrodes of slightly different composition responding selectively to Na + and to K +. Since then, several ion-selective electrodes have been developed that respond selectively to a wide variety of ions, including F -, Cl -, I -, S 2-, ClO 4 -, NO 3 -, SO 4 2-, PO 4 3 -, Ca 2+, Pb 2+, Mg 2+, and Fe 2+. These electrodes all consist of an internal reference electrode separated from the test solution by a membrane which acts reversibly to a specific ion or group of ions. Three types of membranes are used (see next page from C&E News for schematic representations): glass, solid-state and liquid membrane electrode. In a glass electrode, a partially hydrated glass membrane separates the test solution from an inner solution. The internal reference electrode solution contains, in addition to the ions which determine the potential of the reference electrode, a constant activity of the ion of interest. In solid-state electrodes, the internal reference solution is separated from the test solution by a single crystal, or polycrystalline, insoluble salt, e.g. a silver halide. The salt acts as an ionic conductor for specific ions, and the potential developed across it depends on the activity of the specific ion in the sample solution according to the Nernst Equation. In a liquid membrane electrode, the membrane consists of a polymer for structural support, impregnated with a solvent immiscible in water. An ion carrier dissolved in the immiscible solvent gives the electrode its specificity. The K + selective electrode examined in this experiment is composed of polyvinyl-chloride (PVC), with bis(2-ethylhexyl)adipate as solvent, and valinomycin as the ion carrier. 1
CHE425L POTENTIOMETRY WITH K + ION-SELECTIVE ELECTRODE 2
CHE425L POTENTIOMETRY WITH K + ION-SELECTIVE ELECTRODE One of the limiting features of ion selective electrodes is that they respond to a number of ions, termed interferents, in addition to the ion of interest. For instance, the K + selective glass electrode responds more strongly to H + and Ag + than to K +, and is only moderately more selective for K + over Na +. The major advantage of the liquid membrane K + electrode, based on polyvinylchloride, bis(2-ethylhexyl)adipate, and valinomycin, is the high selectivity for K + over Na +, Ag +, and H +. Ion-selective electrodes measure the activity, and not the concentration, of the specific ion. The activity of a specific ion, ai, is related to its concentration, Ci, by ai = γ ici where γ i is the activity coefficient of the specific ion. The activity coefficient depends on the ionic strength, µ, of the solution according to the Debye-Hückel equation: logγ i = 0.5z 2 i µ where z i is the charge on the specific ion. This equation is reasonably accurate up to ionic strengths of about 0.1 M for 1:1 electrolytes. The ionic strength depends on the concentrations and charges of all the ions in the solution according to: µ 1 2 = zi 2 Ci At low ionic strengths, activities approach concentrations. At high ionic strengths, activities can be substantially less than concentrations. In such cases, concentrations can only be determined from ion-selective electrode measurements by accounting for the ionic strength effect. This can be done by: 1) preparing a calibration curve of electrode potential vs ion concentration in a medium of ionic strength comparable to that of the test solution; or 2) calculating the concentration from the measured activity using the above equations if the concentrations of other ionic species are known. The ion-selective electrodes respond to the activity of the specific ion according to the Nernst equation, which for the K + selective electrode is: E 2.3RT F = Ec + log a K (1) where E is the measured potential of the K + electrode relative to a standard reference electrode and a K is the potassium ion activity in the sample solution. E C is the standard cell potential; that portion of the K + electrode potential due to the internal reference electrode, internal solutions, and the specific membrane. "2.3 RT/F" is the Nernst factor 3
CHE425L POTENTIOMETRY WITH K + ION-SELECTIVE ELECTRODE (59.16 mv at 25 C for a ±1 ion). R and F are the gas and Faraday constants respectively, and T is the temperature in Kelvin. Because E C is different for each electrode membrane, a calibration curve of electrode potential relative to the potential of a reference electrode vs ion concentration or activity must be prepared. The response of ion selective electrodes is linear in log a i over only a finite range. It is necessary to prepare a calibration curve to determine this range, the value of E C, and the slope of the linear response curve. The response is termed "Nernstian" if the slope is equal to 2.3 RT/nF, or 59.16 mv at 25 C for a ±1 ion. However, in many cases the slope is less than that predicted by the Nernst equation by 5 to 10 mv. If the response of a K + electrode is linear in log [K + ] at the concentration of a sample solution, it is possible to use Gran s method of multiple standard additions to determine the unknown concentration (1). This technique can be used to minimize matrix effects such as varying ionic strength, or the presence of interfering ions. To apply Gran s method, the observed parameter must be proportional to concentration. Rearranging equation 1 and taking the antilog gives equation 2, 10 E S E S + C = γ 10 [ K ] (2) added where S is the actual slope determined for the electrode in calibration solutions and γ is E S the activity coefficient in the unknown solution. A plot of 10 versus added [K + ] should give a straight line with a concentration axis intercept of -[K + ] unknown. B. EXPERIMENT SUMMARY In this experiment 1) potassium will be determined in diluted orange juice and tap water. 2) the effect of ionic strength on potentiometric measurements will be studied 4
POTENTIOMETRY WITH K + ION-SELECTIVE ELECTRODE C. EXPERIMENTAL PROCEDURE Solutions 1) 1.00 M NaCl (1L) 2) 0.100 M KCl (1L) 3) 0.010 M KCl for storing electrode 4) Orange juice (20 ml per group) C.1 Electrode care The electrode used in this experiment an Orion Model 93-19 K + selective electrode and a Ag/AgCl reference electrode. The potential developed between the electrodes is measured with an Orion Model 370 ph/mv meter. When not in use, the electrodes are stored in 0.01 M KCl. Prior to making any measurements, uncap the filling hole of the reference electrode and add the filling solution. Note that the filling solution for the Ag/AgCl electrode is 0.1 M NaCl. The K + selective electrode should not be exposed to organic solvents or to aqueous solutions containing organic solvents. C.2 Making measurements With the meter in STANDBY mode, raise the electrodes and rinse them with distilled water. Wipe down the sides and gently blot the ends of the electrodes with Kimwipes. Be careful. Vigorous blotting on the end can ruin the K + electrode. Place a beaker containing the solution to be measured along with a stirring bar on the magnetic stirrer. Set it to stir at a slow speed. Immerse the electrodes in the solution. Only the ends need to be immersed; avoid colliding with the stirring bar. Switch the meter to mv mode and record the reading once it has stabilized. (This should take less than one minute for [K + ] 10-5 M. At lower concentrations it may take longer) 5
POTENTIOMETRY WITH K + ION-SELECTIVE ELECTRODE After the measurement has been made, switch the meter to STANDBY and rinse the electrodes. Immerse the electrodes in the next test solution or in the 0.01 M KCl storage solution. Do not leave the electrodes in air for longer than 2-3 minutes. C.3 Calibration of the K + Ion Selective Electrode The potassium selective electrode will be calibrated over its useful range by successive 10-fold dilutions of 0.1 M KCl. To maintain a constant ionic strength, 0.1 M NaCl is used to dilute. Begin by preparing 1 L of 0.1 M NaCl by a suitable dilution of the stock NaCl solution provided. Following the procedure outlined above, measure the potential of the electrode in a 20- to 30-mL portion of the 0.1 M KCl solution provided. While waiting for the reading to stabilize, prepare a 0.01 M KCl/0.09 M NaCl solution by pipetting 10 ml of 0.1 M KCI into a 100-mL volumetric flask, and making up to the mark with 0.1 M NaCl. Once the 0.01 M KCl solution is prepared, record the reading for the 0.1 M KCl solution in the table below. Rinse and transfer the electrodes to the 0.01 M KCl solution, and prepare a 10-3 M KCl solution by pipetting 10 ml of the 10-2 M KCl/0.09 M NaCl solution into a 100-mL volumetric flask. Dilute to volume with 0.1 M NaCl. When the 10-3 M K + solution is prepared, record the reading for the 10-2 M KC1 solution. Continue in this fashion until [KC1] = 1 x 10-7 M. Calibration Data for K + ISE [K + ], M E(mV) 1 x 10-1 1 x 10-2 1 x 10-3 1 x 10-4 1 x 10-5 1 x 10-6 1 x 10-7 6
POTENTIOMETRY WITH K + ION-SELECTIVE ELECTRODE C.4 Determination of Potassium in Orange Juice Prepare a 1:10 dilution of orange juice by diluting 10-mL of orange juice with 0.1 M NaCl. With a pipet measure a 25-mL aliquot of the 1:10 dilution and measure its potential. Record the value in the table below. If the electrode does not stabilize, record several values and use the average. C.5 Standard Addition Procedure While monitoring the electrode potential in the same 25-mL aliquot, use a pipet to deliver 2 ml of 0.1 M KCl into the solution. Record the new value of the potential. Repeat this procedure until the total volume is 31 ml. (Again record several values of E after each addition if stability is a problem.) Standard Addition Calibration for K + in Diluted Orange Juice Solution Diluted Orange Juice Std. Add. 1 Std. Add. 2 Std. Add. 3 E (mv) C.6 Determination of K + in Tap Water Deliver a 50 ml sample tap water into a beaker. Insert the rinsed electrodes, and record the stabilized potential. Deliver 1 ml of 0.1 M NaCl by pipet to increase the ionic strength and re-record the potential. Then deliver 10 ml of 0.1 M NaCl, and record the potential when stabilized. Effect of Ionic Strength on Potentiometric Response for K + Solution Tap Water 1 ml NaCl 10 ml NaCl E (mv) 7
POTENTIOMETRY WITH K + ION-SELECTIVE ELECTRODE D. DATA ANALYSIS AND QUESTIONS 1. For the data acquired in Part C.3.1, make a plot of electrode potential versus log [K + ]. Over what region of this plot is the response linear? Report your value for the slope, S. 2. Calculate the ionic strength for the 1 x 10-7 M KCl solution. Using Table a2-1 on page 996 in your text, estimate the activity coefficient of K +. What is the activity of K + for this solution? 3. Discuss why a plot of E vs log[k + ] conforms to equation 1, and a plot of E vs log a K is not necessary. 4. Using the calibration curve from question 1 and the potential measured for the diluted orange juice, determine [K + ] in undiluted orange juice. 5. Make a Gran s plot, as discussed in the Introduction, for the standard additions of KCl to orange juice. Evaluate the concentration in undiluted orange juice using this method. Discuss and compare the calibration curve and standard addition methods for evaluating ion concentrations. 6. Based on your data, and using the calibration curve method, make an estimate of [K + ] in tap water. Is your value meaningful? Why or why not? How did the apparent value of [K + ] change with the addition of NaCl? Is this what you would expect to happen? Explain. F. REFERENCES 1. G.D. Christian, J.E. O Reilly, Instrumental Analysis, 2nd Ed., Chapter 2. 2. K. Srinivasan, G.A. Rechnitz, Selective Studies on Liquid Membrane Ion- Selective Electrodes, Anal. Chem. 41, 1203 (1969). 3. G.J. Moody, J.D.R. Thomas, Selective Ion Sensitive Electrodes, Chapter 2, Merrow, London, 1971. 4. D.A. Skoog, F.J. Holler and S.R. Crouch, Principle of Instrumental Analysis, 6 th Ed., Chapter 23 (2007) 8