Calibration of a glass electrode gave a reading of 14b 5 mV with 0 05 m potassiu

Question

Calibration of a glass electrode gave a reading of 14b 5 mV with 0 05 m potassium hydrogen phthalate buffer standard (pH = 4.015) and a reading of -54 8 mV with 0.08 m HEPES. 0 08 m NaHEPES. 0 08 m NaCI buffer standard (pH = 7 454). both measured at 30 degree C What is the observed slope (mV/pH unit) of the calibration curve? What is the theoretical slope at 30 degree C? What is beta, the electromotive efficiency to three significant figures?? Open the hint panel below for the definition of electromotive efficiency. What is the pH of an unknown that gives a reading of 59 0 mV with this electrode at 30 degree C?

Explanation / Answer

a) observed slope = E2-E1 / pH2-pH1 = 145.5 - (-54.8) / 4.015 - 7.454 = 199.8 / -03.439 = -58.09 mV /p H

b) Theoretical slope = -RT ln10 / F = 2494.2 X 2.303 / 96485 = 0.0595

c) Panel not given

d) pH unkown = (Eunkown - ES2 / observed slope ) + 7.454 = [59 - (-54.8) / -58.09 ] + 7.454 = 5.49

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