Question

Calibration of a glass electrode gave a reading of 134.9 mV with a 0.05 m potassium hydrogen phthalate buffer standard (pH = 4.015), and a reading of -163.2 mV with a 0.01 m borax buffer standard (pH = 9.139), both measured at 30°C. What is the observed slope (mV/PH unit) of the calibration curve? slope observed = mV/PH What is the theoretical slope at 30°C? slope theoretical = mV/pH What is B, the electromotive efficiency, to three significant figures? B = What is the pH of an unknown that gives a reading of 32.7 mV with this electrode at 30°C? pH =

Answer 1

a)  

Observed slope K = ( E₁ - E₂ ) / (pH_buffer1 - pH_buffer2)

= (134.9 mV - (- 163.2 mV)) /(4.015 - 9.139)

= (298.1) / (- 5.124)

Observed slope K = -58.177 mV /pH unit

b)

Theoritical slope E = E^o - 0.1984 T

= 0 - (0.1984(mV /pH unit .K) X 303 K)

   Theoritical slope E = -60.11 mV /pH unit

c)

ᵦ (beta) =  Observed slope K /  Theoritical slope E

= -58.177 / -60.11

ᵦ (beta) = 0.966

(d)

-58.177 mV /pH unit  = ( E₁ - E₂ ) / (pH_buffer1 - pH_buffer2)

= (134.9 mV - ( 32.7 mV)) /(4.015 - pH_unknown)

(4.015 - pH_unknown) = 102.2 mV / -58.177 mV /pH unit

4.015 - pH_unknown = - 1.7567

pH_unknown = 5.7717

   pH_unknown = 5.77

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