A cation-sensitive electrode is used to determine the activity of calcium in the presence of sodium

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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 (aq) measured against an SCE is +195.5 mV. In a solution containing 0.0100 M CaCl2 (aq) 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 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.

 

Hint 1: Although we often work in terms of concentration as an approximation, this question is specifically asking for the activity. You may use the Debye-Hückel limiting law to determine activity coefficients.

 

Hint 2: Any dissolved ions contribute to the ionic strength, even if the analyte ion.