Calibration of a glass electrode gave a reading of 145.9 mV with saturated potas

Question

Calibration of a glass electrode gave a reading of 145.9 mV with saturated potassium hydrogen tartrate buffer standard (pH = 3.557) and a reading of -182.5 mV with 0.01 m borax buffer standard (pH = 9.139). both measured at 30degreeC. What is the observed slope (mV/pH unit) of the calibration curve? What is the theoretical slope at 30degreeC? 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 54.0 mV with this electrode at 30degreeC?

Explanation / Answer

a) With the following expression you can calculate the observed slope:

E = Const + B (0.05916) log H+

E = const - B (0.05916) pH

Given E1 = 145.9 mV; E2 = -182.5 mV; pH1 = 3.557; pH2 = 9.139

Slope observed = E2 - E1 / pH2 - pH1

Slope observed = -182.5 - 145.9 / 9.139 - 3.557

Slope observed = -58.83 mV/pH

b) Theorical slope = -0.1984 x T

Theorical slope = 0.1984 * 303 = -60.12 mV/pH

c) B = slope observed / theorical slope

B = -58.83 / -60.12 = 0.9785

d) slope = E2 - E1 / pH2 - pH1

-58.83 = -182.5 - 54 / 9.139 - pH1

-58.83(9.139 - pH1) = -236.5

-537.65 + 58.83pH1 = -236.5

58.83pH1 = 537.65 - 236.5

pH1 = 5.119

Hope this helps

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