Question

Ag/AgCl electrode, saturated KCl used as electrolyte.

In Hg2Cl2 (reference electrode), Hg/Hg2Cl2 used as electrode, 1M saturated KCl used as electrolyte.

Their standard half-cell potentials are E0S=+0.223V and E0R=+0.268V, respectively. They are combined to form a full cell for concentration sensing, i.e., as a concentration cell.

1) What type of electrode will it be (potenometry or amperometry)?

2) What is the anticipated output value from the sensor if the sampled solution has 1M concentration? (T=250C).

3) If the output value is -0.065V, use Nernst equation to calculate the concentration of the sampled solution (T=250C).

4) Warm up the test solution in Q3 to T=370C, what is the anticipated output of the sensor?

Please explain each step.

Answer 1

1) It will be potentiometry type of electrode.

2) According to this we use Nernst equation to indentify anticipated output value.

Hg ₂Cl₂ + 2e^- -------> 2Hg²⁺ + 2Cl^- Eo= + 0.268 V

Hg²⁺+ 2e^- ------------> 2Hg ; Eo= 0.223 V

E= E−RT/nF lnQ

R = 8.314 J/mol/K

F = 96500 C/mol

n = 2

E = 0.223 - 0.268 = -0.045 V

E = -0.045 - 8.314 X 523.15 / 2 X 96500 log 1 /2 X (1)

E = -0.045 - 8.314 X 523.15 / 2 X 96500 log 0.5

E = -0.045 - 0.022 log 0.5

E = -0.045 - 0.022 X -0.301

E = -0.045 + 0.0066

E = - 0.038 V

3) If output value is - 0.065 V

E= E−RT/nF lnQ

-0.065 = -0.045 - 8.314 X 523.15 / 2 X 96500 log Q

-0.065 + 0.045 = - 0.022 logQ

-0.02 = -0.022 logQ

0.02/0.022 = logQ

logQ = 0.909

Q = Antilog 0.909

Q = 1.87 M

4)

E= E−RT/nF lnQ

E = -0.045 - 8.314 X 643.15 X 0.909 / 2 X 96500

E = -0.045 - 0.025

E = -0.07 V

If the temperature changes from 250 degree celsius to 370 degree celsius then output value changes from -0.065 V to -0.07 V.