1. Summary of Potentiometry:
pH and Ion Selective Electrodes
Potentiometric Sensors
I = controlled at 0 Amps
Eeq is measured
In general
Eeq = const – log aX

2. Electrode Potentials (review)
Electrochemical cell – two half cells
Convention, write both as reductions
2 AgCl (s) + 2 e- = 2 Ag (s) + 2 Cl- (cathode)
- [2 H e- = H2 (gas)] (Pt, NHE reference, anode)
Ecell = Ecathode - Eanode
Cell reaction is the sum of the two above
2 AgCl (s) + H2 = 2 Ag (s) + 2 Cl H+
Ecell = EAg/AgCl – EH+/H2 by convention EH+/H2 = 0
Ecell = EAg/AgCl = 0.46 V
DG = - n F Ecell ; DG = positive, non-spontaneous, electrolytic cell

3. Ecell = EWE – Eref - Ejunction
Measurements in Potentiometry; I = 0 Amps; equilibrium
Cell: working electrode + reference electrode (E half cell = const)
Working or indicator
Ecell = EWE – Eref - Ejunction

4. Move slidewire (arrow) until G shows I = 0, then V = Ecell = Eeq
Simple potentiometric measuring circuit
Variable resistor V
voltmeter
galvanometer
Move slidewire (arrow) until G shows I = 0, then V = Ecell = Eeq
In practice this is all automatic in modern potentiometers or pH meters

5. Reference electrodes: critical to both potentiometry and voltammetry
They keep a nearly constant half cell potential during experiment
Normal Hydrogen, Pt| H2 (1 atm), HCL (0.01 M), NHE
THE STANDARD, E = 0 V, but not practical
Standard Calomel Electrode
Hg| Hg2Cl2 (s), KCl (sat’d.)
SCE
Set up as self-contained
Half cell
Contact to test solution

6. Half cell potential of the SCE – serves as a reference
against which other E’s are measured
Hg2Cl2 (s) + 2 e- = 2 Hg (l) Cl-
Use Nernst equation:
E = Eo - [RT/nF] ln (aCl2 aHg2/acalomel) ; but a of pure solids =1
only aCl remains in the log term, and
E = Eo’ - [0.0592/2] log [Cl-] or
E = Eo’ log [Cl-] ; sat’d KCl is ~3.5 M at 25 oC
So ESCE = V vs. NHE at 25 oC
Alternative reference: Ag|AgCl (s), KCl (sat’d.)
EAg/AgCl = V vs. NHE at 25 oC

7. Half cell potential of the SCE – serves as a reference
against which other E’s are measured
Hg2Cl2 (s) + 2 e- = 2 Hg (l) Cl-
Use Nernst equation:
E = Eo - [RT/nF] ln (aCl2 aHg2/acalomel) ; but a of pure solids =1
only aCl remains in the log term, and
E = Eo’ - [0.0592/2] log [Cl-] or
E = Eo’ log [Cl-] ; sat’d KCl is ~3.5 M at 25 oC
So ESCE = V vs. NHE
Alternative reference: Ag|AgCl (s), KCl (sat’d.)
EAg/AgCl = V vs. NHE

8. Ion Selective Electrodes (ISE) - sensor surface usually a membrane
That adsorbs the ions, Eeq measured at I = 0 Amps
In pH electrode, the membrane is a very thin glass layer
0.1 M HCl
Stand alone glass pH electrode
must be used with reference
Glass pH electrode combined
with internal reference electrode
Glass membranes are made of SiO2, Li2O (or Na2O) and BaO (or CaO)

9. ISE’s obey Nernst-like equations (25 oC)
E = const + [0.0592/z] log aion ISE measure activity, not conc.
if the ion Is H+,
E = const pH; pH = -log aH+
Fast response is important, < 1 s in buffer for most pH electrodes
ISE
Nernstian region, slope = /z
E, mV
Most pH meters read pH directly,
But must be calibrated daily
Log aion

10. How a glass pH electrode responds to H+ ions
(must store in in water or buffer to maintain hydrated layer)
Ag/AgCl
0.1 N HCl
aH+ = const
Inner hydrated
layer
outer hydrated
layer
Dry glass
Test solution
aH+ , soln a1 a2
0.1 mm
50 mm E2 E1
EM = E1 – E2 + Eint ref; or EB = E1 – E2 (boundary E)
Ecell = const + EB
Li+ in glass can exchange with H+ (both small ions), giving rise to E1 and E2
H+ DOES NOT cross the membrane

11. EB is related to a1 and a2, but a2 is constant (0.1 M HCl)
These ion activities control the membrane potentials,
E1 and E2 and so control EB, at 25 oC
EB = E1 – E2 = log (a1/a2)
Ecell = const log (a1)
Ecell = const pH
In practice, pH meter incorporates these equations
And relates them to measurements with standard buffer,
And the output is a direct measurement of pH:
pH = pHstd + F (Ecell - Estd)/2.303 RT, R = gas const, T = abs.
temperature

12. Errors and Interferences in pH electrode measurements
Alkaline error: In basic solutions or high salt conc. NaCl, KCl,
Na+ and K+ interfere by adsorbing to the glass membrane
then pHobs < true pH
e.g. 0.1 M NaOH, pH 13, [Na+] is 0.1 M, [H+]= 1 x 10-13
Large error due to high [Na+], low [H+]
In general fpr ISEs, Nicolsky equation
Ecell = const log [a1 + Σ Kjaj], j = 1….n interfering ions
Kj = selectivity coefficient
Acid error pH < 1, origin unknown
pH electrodes reliable between pH 1 and 13 only

13. ISE’s for ions other than H+
• glass membranes, Na+, K+, NH different composition than pH
• solid state membranes, F-, S-
• liquid membranes, Ca+
• gas sensitive electrodes CO2, H2S, NH3
• enzyme electrodes – biological molecules
• can all be used with pH meter in mV-mode

14. Fluoride ISE – Solid State
Detection limit M

15. Impregnated with ion exchanger;
Ca++ ISE, Ca(dodecylphosphate) +
Polymer like PVC;
Phosphate groups bind Ca++
Crystals or solid state have the analyte
Ion present, e.g. LaF3
Detection limit ~10-8 to 10-6 M

16. Urea Enzyme ISE (NH)2CO + 2 H2O + H+ 2 NH4+ + HCO3- Urease enzyme
Detection limit ~10-6 M