ISE-activity

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University of Minnesota-Twin Cities *

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2121

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Chemistry

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Dec 6, 2023

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docx

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Ion-Selective Electrodes Students should be able to:  Write the equation for an ion-selective electrode  Use the equation to solve for potential or concentration  Use a calibration curve to find unknown concentrations  Understand how selectivity can effect potential measurements Ion selective electrodes detect a difference in the potential across a membrane. The ions on the inside of the electrode are held constant and therefor the potential can be measured as the ion concentration outside the electrode changes. A pH electrode is an example of an ion selective electrode, where the amount of H+ is constant on the inside of the glass membrane, and the H+ binding to the outside causes a potential difference that can be measured. Nernst Equation for ISE The electric potential difference for an ion-selective electrode is below: E = constant + 0.05916 V z log A o Where the constant is the combined constant potential from the inner electrode, z is the charge on the ion of interest, and A o is the activity of the ion if interest in the solution we are measuring. If we were monitoring H + the equation would be H + ¿ E = constant + 0.05916 V log A ¿ If the amount of H + ions in the solution increases by 10 fold, we would expect the potential to increase by +59.16mV. 1. Write the electric potential equation for an ion-selective electrode for: a. F - b. Ca 2+ 2. If there was a factor of 10 increase in the F - ion concentration, what magnitude and direction would you expect in the potential? 3. If there was a factor of 10 increase in the Ca 2+ ion concentration, what magnitude and direction would you expect in the potential

Ion-Selective Electrode Calibration Curves All ion-selective electrodes need to be calibrated before they can be used. This requires us to prepare a set of standards with known amounts of the ions in solutions with the same ionic strength. We usually do this by making up a high ionic strength buffer, 1.0M NaCl, and using it to dilute all of the standard solutions. 4. Why is it important that the ionic strength is the same in each solution? A calibration curve for F- has been established below -5.50 -5.00 -4.50 -4.00 -3.50 -3.00 -2.50 -40.0 -20.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 f(x) = − 58.5 x − 192.5 Fluoride Calibration Curve log[F-] potential (mV) 5. Based on your answer to question #3, identify which parts of the electric potential equation can be found using the linear regression equation. 6. If the potential of a solution had a potential of 0.0mV, what would be the fluoride ion concentration? Selectivity A ion-selective membrane is never 100% effective at detecting only the ions it is designed for. Errors in pH electrodes can occur when [Na + ] is detected instead of [H + ]. The ability to be selective for one ion in the presence of a competing ion is the selectivity coefficient of an ion. E = constant ± 0.05916 z A log [ A A + ∑ x K x , A pot A x ] [F-] (M) E (mV) 1.00E-05 100.0 1.00E-04 41.5 1.00E-03 -17.0

7. A fluoride ion-selective electrode has a selectivity coefficient F − ,OH − ¿ Pot = 0.1 K ¿ . The ion selective electrode has constant value of 100.0mV a. What will be the electrode potential when 1.0x10 -4 M F - is in a solution of pH 5.5? b. What will be the electrode potential when 1.0x10 -4 M F - is in a solution of 10.5?

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