Calibration of a glass electrode gave a reading of 122.7 mV with 0.05 m potassiu

Question

Calibration of a glass electrode gave a reading of 122.7 mV with 0.05 m potassium dihydrogen citrate buffer standard (pH = 3.766) and a reading of -89.6 mV with 0.008695 potassium dihydrogen phosphate, 0.0343 m disodium hydrogen phosphate buffer standard (pH = 7.400), 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_theoretical = (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 13.8 mV with this electrode at 30 degree C? pH =

Explanation / Answer

a)

Slope observed --> (mV2-mV1) / (pH2-pH1)

Slope observed --> (-89.6-122.7) / (7.4-3.766)

Slope = -58.420

b)

Slope, theoretical -->

Theorical slope = -0.1984 x T

T must be K = 30+273 = 303

so

theoretical --> -0.1984*303 = -60.115

c)

Beta = Slope observed/ theoretical

Beta =58.420/60.115 = 0.97180

d)

get pH for

13.8 mV

so

Slope observed --> (mV2-mV1) / (pH2-pH1)

-58.420 = (-89.6-13.8 ) / (7.4-pH)

solve for pH

pH = -( (-89.6-13.8 ) / -58.420 - 7.4) = 5.63

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