Chapter 23: Q. 23.9 (page 631)
What is an ionophore? What is the purpose of ionophores in membrane electrodes?
Charges are carried by ionophores. They are transported across the membrane by them. It improves the ion transporation in the membrane electrodes.
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Ceresa, Pretsch, and Bakker41 investigated three ISEs for determining calcium concentrations. All three electrodes used the same membrane, but differed in the composition of the inner solution. Electrode 1 was a conventional ISE with an inner solution of 1.00 x 10-3 M CaCl2 and 0.10 M NaCl. Electrode 2 (low activity of Ca2+) had an inner solution containing the same analytical concentration of CaCl2, but with 5.0 x 10-2 M EDTA adjusted to a pH of 9.0 with 6.0 x 10-2 M NaOH. Electrode 3 (high Ca21 activity) had an inner solution of 1.00 M Ca(NO3)2.
(a) Determine the Ca2+ concentration in the inner solution of Electrode 2.
(b) Determine the ionic strength of the solution in Electrode 2.
(c) Use the Debye-Hรผckel equation and determine the activity of Ca2+ in Electrode 2. Use 0.6 nm for the ฮฑX value for Ca2+ (see Appendix 2).
(d) Electrode 1 was used in a cell with a calomel reference electrode to measure standard calcium solutions with activities ranging from 0.001 M to 1.00 x 10-9 M. The following data were obtained.
Plot the cell potential versus the pCa and determine the pCa value where the plot deviates more than 5% from linearity (the limit of linearity; see Section 1E-2). For the linear portion, determine the slope
and intercept of the plot. Does the plot obey Equation 23-29 as expected?
(e) For Electrode 2, the following results were obtained.
Again plot cell potential versus pCa and determine the range of linearity for Electrode 2. Determine the slope and intercept for the linear portion. Does this electrode obey Equation 21-24 for the higher Ca2+ activities?
(f) Electrode 2 is said to be super-nernstian for concentrations from 10-7 M to 10-6 M. Why is this term used? If you have access to a library that subscribes to Analytical Chemistry or has web access to the journal, read the article. This electrode is said to have Ca2+ uptake. What does this mean and how might it explain the response?
(g) Electrode 3 gave the following results.
Plot the cell potential versus pCa and determine the range of linearity. Again determine the slope and intercept. Does this electrode obey Equation 23-29?
(h) Electrode 3 is said to have Ca2+ release. Explain this term from the article and describe how it might explain the response.
(i) Does the article give any alternative explanations for the experimental results? If so, describe these alternatives.
List the advantages and disadvantages of a potentiometric titration relative to a direct potentiometric measurement.
What are the advantages of microfabricated ISEs? Describe typical applications of this type of sensor.
The following cell was used for the determination of
Calculate if the cell potential is -0.313 V.
(a) Calculate the standard potential for the reaction
(b) Give a schematic representation of a cell with a silver indicator electrode and an SCE as reference that could be used for determining localid="1648042702202" .
(c) Derive an equation that relates the measured potential of the cell in (b) to (assume that the junction potential is zero).
(d) Calculate the of a solution that is saturated with and contained in the cell described in (b) if the resulting potential is 0.193 V.
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