Map Sapling Learning macmillan learning Calibration of a glass electrode gave a reading of 147.9 mV with 0.05 m potassium dihydrogen citrate buffer standard (pH 3.766) and a reading of -67.5 mV with 0.008695 potassium dihydrogen phosphate, 0.0343 nm disodium hydrogen phosphate buffer standard (pH 7.400), both measured at 30°C (a) What is the observed slope (mV/pH unit) of the calibration curve? Number Slopeed mV/pH (b) What is the theoretical slope at 30°C? Number Slopetheoretical mV/ pH (c) What is B, the electromotive efficiency to three significant figures?? Open the hint panel below for the definition of electromotive efficiency Number (d) What is the pH of an unknown that gives a reading of 57.7 mV with this electrode at 30°C? Number
Solution
a)
Observed slope = E1 - E2 / pH buffer1 - pH buffer2
= 147.9 - (-67.5) / 3.766 - 7.400
Observed slope = - 59.27 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 = - 59.27 / 60.11
beta = 0.986
d)
- 59.27 = 147.9 - 57.7 / 3.766 - pHunknown
pH = 5.29
.
Map Sapling Learning macmillan learning Calibration of a glass electro.docx
1. Map Sapling Learning macmillan learning Calibration of a glass electrode gave a reading of
147.9 mV with 0.05 m potassium dihydrogen citrate buffer standard (pH 3.766) and a reading of
-67.5 mV with 0.008695 potassium dihydrogen phosphate, 0.0343 nm disodium hydrogen
phosphate buffer standard (pH 7.400), both measured at 30°C (a) What is the observed slope
(mV/pH unit) of the calibration curve? Number Slopeed mV/pH (b) What is the theoretical slope
at 30°C? Number Slopetheoretical mV/ pH (c) What is B, the electromotive efficiency to three
significant figures?? Open the hint panel below for the definition of electromotive efficiency
Number (d) What is the pH of an unknown that gives a reading of 57.7 mV with this electrode at
30°C? Number
Solution
a)
Observed slope = E1 - E2 / pH buffer1 - pH buffer2
= 147.9 - (-67.5) / 3.766 - 7.400
Observed slope = - 59.27 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 = - 59.27 / 60.11
beta = 0.986
d)
