Summary of Potentiometry:pH and Ion Selective ElectrodesPotentiometric Sensors

I = controlled at 0 AmpsEeq is measured In generalEeq = 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)

• Ecell = Ecathode - Eanode

Cell reaction is the sum of the two above

2 AgCl (s) + H2 = 2 Ag (s) + 2 Cl- + 2 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

Measurements in Potentiometry; I = 0 Amps; equilibrium

Cell: working electrode + reference electrode (E half cell = const)

Working or indicator

Ecell = EWE – Eref - Ejunction

Variable resistor

Simple potentiometric measuring circuit

galvanometer

Move slidewire (arrow) until G shows I = 0, then V = Ecell = EeqIn practice this is all automatic in modern potentiometers or pH meters

Vvoltmeter

Reference electrodes: critical to both potentiometry and voltammetryThey keep a nearly constant half cell potential during experiment

Normal Hydrogen, Pt| H2 (1 atm), HCL (0.01 M), NHETHE STANDARD, E = 0 V, but not practical

Standard Calomel ElectrodeHg| Hg2Cl2 (s), KCl (sat’d.)SCE

Set up as self-containedHalf 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 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-]2 orE = 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 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-]2 orE = 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 AmpsIn pH electrode, the membrane is a very thin glass layer

Stand alone glass pH electrodemust be used with reference

Glass pH electrode combined with internal reference electrode

0.1 M HCl

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 ISE measure activity, not conc.

if the ion Is H+, E = const - 0.0592 pH; pH = -log aH+

Fast response is important, < 1 s in buffer for most pH electrodes

E, mV

Log aion

Nernstian region, slope = 0.0591/z

ISE

Most pH meters read pH directly,But must be calibrated daily

How a glass pH electrode responds to H+ ions

Ag/AgCl0.1 N HClaH+ = const

E2 E1

Inner hydratedlayer

outer hydratedlayer

Dry glass

0.1 mm 50 mm

Test solutionaH+ , soln

Li+ in glass can exchange with H+ (both small ions), giving rise to E1 and E2 H+ DOES NOT cross the membrane

(must store in in water or buffer to maintain hydrated layer)

EM = E1 – E2 + Eint ref; or EB = E1 – E2 (boundary E) Ecell = const + EB

a2 a1

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 pHe.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 ionsKj = selectivity coefficient

Acid error pH < 1, origin unknownpH 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

Fluoride ISE – Solid State

Detection limit 10-9 M

Impregnated with ion exchanger;Ca++ ISE, Ca(dodecylphosphate) +Polymer like PVC;Phosphate groups bind Ca++

Crystals or solid state have the analyteIon present, e.g. LaF3

Detection limit ~10-8 to 10-6 M

Urea Enzyme ISE

(NH)2CO + 2 H2O + H+

Urease enzyme

2 NH4+ + HCO3-

Detection limit ~10-6 M