Electrodes and Potentiometry

Reference electrodes – maintains a fixed (reference) potential Indicator electrodes – responds to analyte activity Solution: Measure Fe2+ and Fe3+

Right electrode: Fe3+ + e- ! Fe2+ oE+

= 0.771 V

Left electrode: AgCl(s) + e- Ag(s) + Cl oE! = 0.222 V

Standard Hydrogen Electrode (S.H.E.) – Chapter 14 -ultimate reference against which all other electrodes are measured, directly or indirectly.

!

"Ecell

o = EMo # E

SHEo

Since the potential of the SHE is defined as zero:

!

"Ecell

o = EM

o # 0 = EM

o

Silver-Silver Chloride Electrode

Ag (s)AgCl [sat’d], KCl [x M]

AgCl (s) + e- ! Ag (s) + Cl-

And according to the Nernst equation:

!

E = EAgCl/Ag+o -

0.0592

1 log[Cl-]

- Prepared using sat’d KCl solution, small in nature and used at higher temps.

Calomel Electrode (saturated calomel electrode (S.C.E.))

HgHg2Cl2 [sat’d], KCl [sat’d]

!

1

2Hg2Cl2(s) + e" #Hg(l)+ Cl"

!

E = EHg

2Cl

2/Hg

o -0.0592

2 log

(1)[Cl- ]2

1

-Concentration of Cl- is fixed (sat’d KCl is 4.2 M), thus electrode potential remains constant. -0.1 M or 1.0 M KCl -Temperature, but less of an affect in less concentrated solutions Metal Indicator Electrodes – Metal submersed in a solution of its ions, i.e. silver wire dipping into silver nitrate solution, can be used in conjunction with calomel.

For the reversible half-reaction: Ag+ + e- ! Ag (s) Eo= 0.800 V

The reference half-cell reaction is:

Hg2Cl2 (s) + 2e- ! 2 Hg (l) + 2Cl- E- = 0.241 V

!

E = E + " E " = 0.779 " 0.05916 log 1

[Ag+]" 0.241

= 0.558 + 0.05916 log [Ag+]

- a voltage change by 59.16 mV (at 25oC) for each factor of 10 change

in [Ag+]. Ex. A 100 mL solution containing 0.100) M NaCl was titrated with 0.100 M NaCl was titrated with 0.100 M AgNO3. Calculate the voltage after the addition of 65.0 mL of AgNO3. Titration reaction: Ag+ + Cl- ! AgCl(s)

For which the e.p. is 100.0 mL. At 65.0 mL, 65.0% of Cl- has precipitated and 35.0% remains in solution:

!

[Cl-] = (0.350)(0.1000M)

100.0

165.0= 0.0212 M

To find cell voltage, we need to know [Ag+]:

[Ag+] [Cl-] = Ksp !

!

[Ag+] = Ksp

[Cl-]= 1.8 x 10-10

0.0212= 8.5 x 10-9M

E = 0.558 + 0.05916 log (8.5 x 10-9) = 0.081 V

Ion-Selective Electrodes – how they work

- respond selectively to only one ion - key feature = thin membrane, e.g. liquid based ion-selective electrode

shown in Fig. 15-8.

- Glass, polymer, crystalline or glass/gas-permeable membrane

- contains “liquid” based hydrophobic organic polymer with ion exchanger for C+. pH Measurement with a Glass Electrode (15-6)

- most common ion selective electrode - Errors in pH measurement, p. 329

Selectivity Coefficient

- Glass electrode among the most selective - Read: selectivity coefficient, solid-state electrodes