Domain: Analytical chemistry

A cation-sensitive electrode is used to determine the activity of calcium in the presence of sodium. The potential of the electrode in 0.0100 M CaCl2 measured against an SCE is 195.5 mV. In a solution containing 0.0100 M CaCl2 and 0.0100 M NaCl, the potential is 201.8 mV. What is the activity of calcium ion in an unknown solution if the potential of the electrode in this is 215.6 mV versus SCE and the sodium ion activity has been determined with a sodium ion-selective electrode to be 0.0120 M? Assume Nernstian response.

Grading rubric:
criterion: States the Nernstian slope for a divalent ion at 25 °C as approximately 29.58 mV per decade.
weight: 10
criterion: Calculates the ionic strength of the 0.0100 M CaCl2 calibration solution, explicitly using z^2 = 4 for Ca2+.
weight: 20
criterion: Calculates the ionic strength of the mixed 0.0100 M CaCl2 + 0.0100 M NaCl calibration solution.
weight: 10
criterion: Applies the Extended Debye–Hückel equation to estimate the activity coefficients of Ca2+ and Na+ in the calibration solutions.
weight: 10
criterion: Calculates the calcium ion activity in the 0.0100 M CaCl2 standard solution using the derived activity coefficient.
weight: 20
criterion: Rearranges the Nikolsky–Eisenman equation to isolate the calcium ion activity for the unknown solution.
weight: 10
criterion: Reports the final calcium ion activity as 0.0196 M.
weight: 20

Answer:
0.0196 M