When calibrating a glass electrode, 0.025 m potassium dihydrogen phosphate/0.025 m disodium hydrogen phosphate buffer (Table 14-3) gave a reading of - 18.3 mV at 20°C and 0.05 m potassium hydrogen phthalate buffer gave a reading of + 146.3 mV. What is the pH of an unknown giving a reading of + 50.0 mV? What is the slope of the calibration curve (mV/pH unit) and what is the theoretical slope at 20°C? Find the value of in Equation 14-6.