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? (b) What is the theoretical slope at 30 degree C? (c) What is beta, the electromotive efficiency to three significant figures?? Open the hint panel below for the definition of electromotive efficiency. (d) What is the pH of an unknown that gives a reading of 13.8 mV with this electrode at 30 degree C?

Explanation / Answer

a)

slope = dY/dx = d(mV) / d(pH)

slope = (-89.6-122.7)/(7.4-3.766) = -58.420

b)

The theoretical slope:

E(T) = E°(T) - 0.1984(T) * pH

e(T) = 0 - 0.1984(T) = -0.1984*(30+273) = -60.1152

C)

for beta:

B = oblserved slope / theoretical slope

B = 58.420/60.1152

B = 0.9718

d)

finally

slope = E2-E1 / (pH2-pH1)

-59.44 =(122.7 - 13.8 )/(3.766- pH)

pH =-( (122.7 - 13.8 )/ (-59.44 ) -3.766)

pH = 5.5980

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