Challenge Problem: Ceresa, Pretsch, and Bakker13 investigated three ion-selective electrod...

Challenge Problem: Ceresa, Pretsch, and Bakker13 investigated three ion-selective electrodes 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 SE with an inner solution of 1.00 × 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 × 10-2 M EDTA adjusted to a pH of 9.0 with 6.0 × 10-2 M NaOH. Electrode 3 (high Ca21 activity) had an inner solution of 1.00 M Ca(NO3)2.

(a) Determine the Ca21 concentration in the inner solution of Electrode 2.

(b) Determine the ionic strength of the solution in Electrode 2.

(c) Use the Debye-Hückel equation and determine the activity of Ca21 in Electrode 2. Use 0.6 nm for the aX value for Ca21.

(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 3 1029 M. The following data were obtained:

Activity of Ca2+, M	Cell Potential, Mv
1. 0 × 10-3	93
1. 0 × 10-4	73
1. 0 × 10-5	37
1. 0 × 10-6	2
1. 0 × 10-7	-23
1. 0 × 10-8	-51
1. 0 × 10-9	-55

Plot the cell potential versus the pCa and determine the pCa value where the plot deviates significantly from linearity. For the linear portion, determine the slope and intercept of the plot.

Does the plot obey the expected Equation 21-23?

(e) For Electrode 2, the following results were obtained:

Activity of Ca2+	Cell Potential, V
1.0 × 10-3	228
1.0 × 10-4	190
1.0 × 10-5	165
1.0 × 10-6	139
5.6 × 10-7	105
3.2 × 10 7	63
1.8 × 10-7	36
1.0 × 10-7	23
1.0 × 10-8	18
1.0 × 10-9	17

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 21-23 for the higher Ca21 activities?

(f) Electrode 2 is said to be super-Nernstian for concentrations from 1027 M to 1026 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 Ca21 uptake. What does this mean and how might it explain the response?

(g) Electrode 3 gave the following results:

Activity of Ca21, M	Cell Potential, mV
1.0 × 10-3	175
1.0 × 10-4	150
1.0 × 10-5	123
1.0 × 10-6	88
1.0 × 10-7	75
1.0 × 10-8	72
1.0 × 10-9	71

Plot the cell potential versus pCa and determine the range of linearity. Again, determine

the slope and intercept. Does this electrode obey Equation?

(h) Electrode 3 is said to have Ca21 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.

Equation