Calibration of a glass electrode gave a reading of 139.7 mV with saturated potas
ID: 918750 • Letter: C
Question
Calibration of a glass electrode gave a reading of 139.7 mV with saturated potassium hydrogen tartrate buffer standard (pH = 3.557) and a reading of -92.2 mV with 0.08 m HEPES, 0.08 m NaHEPES, 0.08 m NaCl buffer standard (pH = 7.454), both measured at 30°C.
(a) What is the observed slope (mV/pH unit) of the calibration curve?
(b) What is the theoretical slope at 30°C?
(c) What is , 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 46.9 mV with this electrode at 30°C?
Explanation / Answer
a.
Slope S = dE/dpH = (139.7-(-92.2))/(3.557-7.454) = - 231.9/3.897= - 59.51 mV/pH unit
b. The theoretical slope is
-2.303RT/nF
The known value at 298 K is - 0.05916 V (see Nernst equation)
The value at 303 K is
-0.05916 V x303K/298K = -0.06015 V = -60.15 mV/pH unit
c.
59.51 mV/pH / 60.15 mV/pH = 0.9893
d.
pHx = pH + (Ex-E)/S
(pH and E, any known pair, look also at a., same equation)
= 7.454 - (46.9 + 92.2)/ 59.51
= 7.454 – 2.337 = 5.117
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