Summary of Potentiometry:
pH and Ion Selective Electrodes
Potentiometric Sensors
I = controlled at 0 Amps
Eeq is measured
In general
Eeq = const – 0.0592 log aX
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+ + 2 e- = H2 (gas)]
(Pt, NHE reference, anode)
Cell reaction is the sum of the two above
2 AgCl (s) + H2 = 2 Ag (s) + 2 Cl- + 2 H+
• Ecell = Ecathode - Eanode
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
Measurements in Potentiometry; I = 0 Amps; equilibrium
Cell: working electrode + reference electrode (E half cell = const)
Working or indicator
Ecell = EWE – Eref - Ejunction
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
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
Half cell potential of the SCE – serves as a reference
against which other E’s are measured
Hg2Cl2 (s) + 2 e- = 2 Hg (l) + 2 ClUse 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-]2 or
E = Eo’ - 0.0592 log [Cl-] ; sat’d KCl is ~3.5 M at 25 oC
So ESCE = 0.244 V vs. NHE at 25 oC
Alternative reference: Ag|AgCl (s), KCl (sat’d.)
EAg/AgCl = 0.199 V vs. NHE at 25 oC
Half cell potential of the SCE – serves as a reference
against which other E’s are measured
Hg2Cl2 (s) + 2 e- = 2 Hg (l) + 2 ClUse 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-]2 or
E = Eo’ - 0.0592 log [Cl-] ; sat’d KCl is ~3.5 M at 25 oC
So ESCE = 0.2415 V vs. NHE
Alternative reference: Ag|AgCl (s), KCl (sat’d.)
EAg/AgCl = 0.2415 V vs. NHE
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)
ISE’s obey Nernst-like equations (25 oC)
E = const + [0.0592/z] log aion
if the ion Is H+,
E = const - 0.0592 pH;
ISE measure activity, not conc.
pH = -log aH+
Fast response is important, < 1 s in buffer for most pH electrodes
ISE
Nernstian region, slope = 0.0591/z
E, mV
Log aion
Most pH meters read pH directly,
But must be calibrated daily
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
a2
0.1 mm
E2
Dry glass
outer hydrated
layer
Test solution
aH+ , soln
a1
50 mm
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
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 = 0.0592 log (a1/a2)
Ecell = const + 0.0592 log (a1)
Ecell = const - 0.0592 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
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 + 0.0592 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
ISE’s for ions other than H+
• glass membranes, Na+, K+, NH4+ - 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
Fluoride ISE – Solid State
FLUORIDE ISE
Detection limit
10-9 M
Impregnated with ion exchanger;
Ca++ ISE, Ca(dodecylphosphate) +
Polymer like PVC;
Phosphate groups bind Ca++
Detection limit
Crystals or solid state have the analyte
Ion present, e.g. LaF3
~10-8 to 10-6 M
Urea Enzyme ISE
(NH)2CO + 2 H2O + H+
Urease
enzyme
2 NH4+ + HCO3-
Detection limit
~10-6 M