Calibration of a glass electrode gave a reading of 143.7 mV with 0.05 m potassiu

Question

Calibration of a glass electrode gave a reading of 143.7 mV with 0.05 m potassium dihydrogen citrate buffer standard (pH = 3.766) and a reading of -172.1 mV with 0.01 m borax buffer standard (pH = 9.139), both measured at 30 degree C. What is the observed slope (mV/pH unit) of the calibration curve? Slope_observed = Number _____ mV/pH What is the theoretical slope at 30 degree C? Slope_theoretical = Number _______ mV/pH What is beta, the electromotive efficiency to three significant figures?? Open the hint panel below for the definition of electromotive efficiency. Beta = Number _________ What is the pH of an unknown that gives a reading of 42.0 mV with this electrode at 30 degree C? pH = Number ________

Explanation / Answer

a)

Observed slope = E1 - E2 / pHbuffer1 - pHbuffer2

                           = 143.7 - (-172.1) / 3.766 - 9.139

Observed slope = - 58.78 mV / pH

b)

theoretical slope E = Eo - 0.1984 T

                              = 0 - (0.1984 x 303)

theoretical slope E = - 60.11 mV/pH

c)

beta = observed / theoretical = - 58.78 / 60.11

beta = 0.978

d)

- 58.78 = 143.7 - 42.0 / 3.766 - pHunknown

pH = 5.50

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