The selectivity ratio for a cation-selective electrode for B+ with respect to A+ is found from measurements of two solutions containing different activities of the two ions. The subsequent potential readings were obtained: (1) 2.00 × 10-4 M A+ + 1.00 × 10-3 M B+, +237.8 mV; and (2) 4.00 × 10-4 M A+ + 1.00 × 10-3 M B+, +248.2 mV. Calculate KA/B. The electrode response is 56.7 mV/decade.