Calibration of a glass electrode gave a reading of 140.1 mV with 0.05 m potassiu

Question

Calibration of a glass electrode gave a reading of 140.1 mV with 0.05 m potassium dihydrogen citrate buffer standard (pH = 3.766) and a reading of -176.0 mV with 0.01 m borax buffer standard (pH = 9.139), both measured at 30 degree C. (a) What is the observed slope (mV/pH unit) of the calibration curve? Slope_observed = (b) What is the theoretical slope at 30 degree C? Slope_observed = (c) What is beta, the electromotive efficiency to three significant figures ?? Open the hint panel below for the definition of electromotive efficiency. beta = (d) What is the pH of an unknown that gives a reading of 22.7 mV with this electrode at 30 degree C? pH =

Explanation / Answer

a)

Observed slope = E1 - E2 / pHbuffer1 - pHbuffer2

                                   = 140.1 - (-176) / 3.766 - 9.139

Observed slope = - 58.83 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 = - 58.83 / 60.11

beta = 0.979

d)

- 58.83 = 140.1 - 22.7 / 3.766 - pHunknown

pH = 5.76

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