Chemistry 321L Manual Page 23 Ion Selective Electrode Determination of Fluoride Ion I Introduction A Ion Selective Electrodes The amount of a specific ion contained in an aqueous solution can be determined by direct potentiometric measurement of the voltage of a galvanic cell such as shown below reference electrode analyte solution ion selective electrode The most common example of this is a pH measurement using a glass membrane sensitive to H The ion selective electrode ISE typically consists of an inner reference electrode plus a membrane that provides the interface between the sample solution and the ISE A potential develops across the membrane that depends on the difference in the activity of a specific ion on each side of the membrane An internal solution with a fixed concentration activity of the analyte ion means that the potential developed across the membrane is related to the analyte activity in the sample solution The overall measured cell potential can be expressed as where Eouter ref is the potential of the outer reference electrode Einner ref is the potential of the inner reference electrode Ejunc represents the various junction potentials that develop at liquid junction in the cell EISE is the potential developed across the ion selective membrane If the measurements are made with very little current flowing in the cell the reference electrode potentials are fixed and if the sample solution is essentially the same matrix for all measurements the junction potentials are also unchanged Then the measured cell potential can be expressed as Where R is the gas constant T is the temperature K F is the Faraday constant n represents the charge on the analyte ion and a is the activity of the analyte ion Chemistry 321L Manual Page 24 The ISE filling solution contains a large concentration activity of the analyte ion and is essentially unchanged during operation of the electrode aion inner is fixed Thus at 25EC For fluoride ion solutions at 25EC and constant ionic strength Thus for an ideal fluoride ISE the cell potential is linearly related to the logarithm of the fluoride ion concentration and should increase 59 16 mV for every 10 fold decrease in the F When the ionic strength of all standards and samples is constant the response of a real fluoride ISE is described by a similar relationship where is the electromotive efficiency and typically has a value very close to unity 0 98 B Direct Potentiometric Measurement To check if the electrode is working properly you will measure the the cell potential of two fluoride standards prepared in a total ionic strength adjustment buffer TISAB The TISAB contains an acetic acid acetate buffer that fixes the pH of the solution at about 5 At this pH the formation of HF is negligible and the concentration of OH the only other anion that the electrode responds to is insignificant It also contains NaCl to establish a high and constant ionic strength and a complexing agent that removes cations that could interfere by forming complexes with fluoride From a linear least squares fit to a plot of Emeas versus F you can obtain the slope S 0 05916 Typically S equals 56 2 mV C Method of Standard Addition The method of variable volume standard addition will be used to determine the fluoride content of an unknown solution In this approach a solution containing fluoride will be mixed with the TISAB and the potential will be measured Then successive amounts of a fluoride standard solution will be added and the potential will be measured after each addition The following describes how the unknown fluoride concentration can be obtained from these measurements Chemistry 321L Manual The measured potential E can be represented by where K is a constant S is the slope of the calibration curve and equals 0 05916 C is the analyte ion F concentration This equation can be rearranged to give The analyte ion concentration after any addition of the standard is given by where C0 is the analyte concentration before any standard is added V0 is the volume of the solution before any standard is added Cstd is the concentration of the standard solution Vstd is the volume of standard solution that is added Substituting this expression for C in the previous equation gives This equation can be rearranged to give Page 25 Chemistry 321L Manual Page 26 A plot of 10E S V0 Vstd versus CstdVstd will give a linear plot with an x intercept y 0 equal to the amount g of analyte in the solution before addition of the standard The analyte concentration g mL in the original unknown solution Cunk can then be determined by dividing by the volume of the unknown fluoride solution Vunk II Procedure A Preparation of Fluoride Standard Solutions 1 By serial dilution of the 1000 g mL fluoride standard solution prepare 50 mL each of 200 20 and 2 g mL fluoride standards in 50 mL volumetric flasks After thorough mixing transfer each diluted standard solution to a labeled plastic reagent bottle for storage Calculate the concentration of each diluted standard using the exact concentration of the stock solution B Calibration Check of Electrode 1 Carefully pipet 25 00 mL of the most dilute fluoride standard into a 50 mL volumetric flask and dilute to the mark with the TISAB Stopper the flask and thoroughly mix the solution 2 Transfer this solution to a 100 mL plastic beaker Place the beaker on a stirring plate add a magnetic stirring bar and begin stirring at a constant rate 3 Connect the fluoride ISE to a pH meter and set the meter to the mV mode Rinse the electrode with dionized water and blot dry 4 Lower the electrode into the standard solution When the reading is stable record the mV value 5 Repeat steps II B 1 4 for each of the remaining fluoride standards 6 Estimate of the slope S from the difference in the mV readings for each factor of ten increase in the fluoride ion concentration If your value is outside the expected range consult with your lab instructor Chemistry 321L Manual Page 27 C Analysis of Unknown 1 Carefully pipet 50 00 mL of your fluoride unknown which contains the TISAB at the same concentration as used for the standard calibrations into a 100 mL plastic beaker Place the beaker on a stirring plate add a magnetic stirring bar and begin stirring at a constant rate 2 Rinse the ISE with deionized water and blot dry 3 Lower the electrode into the unknown solution When the reading is stable record the mV value 4 Pipet 1 000 mL of the 200 g mL F standard solution into the unknown solution