In 2000, Ceresa, Pretsch, and Bakker ¹ published a paper titled “Direct Potentiometric Information on Total Ionic Concentrations” . In this paper they investigated the use of three ion-selective electrodes to determine the concentration of calcium in solutions.

Retrieve the paper and answer the questions below:

1. Describe the main similarities and differences between the three electrodes.
2. Determine (using appropriate equations), the concentration, ionic strength and the activity for Ca ²⁺ in the electrode labeled “ ISE with Ca ²⁺ uptake ”.
3. Table 1, 2 and 3 report the data collected with electrode 1,2 and 3 respectively.
   1. Plot the cell potential vs -log(a _Ca 2+ ) for the three electrodes.
   2. Determine the value at which the plot deviates more than 5% from linearity.
   3. Determine the slope and intercept of the linear part of each plot.
   4. We can model a cation-selective electrode with the equation

E _cell = K − [(0.0592/n) * pX]

Where K is a constant, n is the number of moles of electron involved and pX =

−log ( a _X ) with X being a cation.

Do the plots follow this equation? Compare and discuss.

4. The researchers use the term “super-nerstian” in the article. Explain why.
5. Critically discuss the significance of the results obtained in this study.

Activity of Ca 2+ (M)	Cell Potential (mV)
1.0E-03	93
1.0E-04	73
1.0E-05	37
1.0E-06	2
1.0E-07	-23
1.0E-08	-51
1.0E-09	-55

Table 1: Data for electrode 1

Activity of Ca 2+ (M)	Cell Potential (mV)
1.0E-03	228
1.0E-04	190
1.0E-05	165
1.0E-06	139
5.6E-07	105
3.2E-07	63
1.8E-07	36
1.0E-07	23
1.0E-08	18
1.0E-09	17

Table 2: Data for electrode 2

Activity of Ca 2+ (M)	Cell Potential (mV)
1.0E-03	175
1.0E-04	150
1.0E-05	123
1.0E-06	88
1.0E-07	75
1.0E-08	72
1.0E-09	71