Chapter 23 Potentiometer 1Principles

Measuring concentration using electrodes

Indicator electrodes used with reference electrode to measure potential of unknown solution

Ecell = Eindicator – Ereference+ Ej (potential arising from salt bridge)

Eindicator

- responds to ion activity

- specific (one ion) or selective (several ions)

Two general types of indication electrodes

- metallic

- membrane

Fig. 23-1 (p.660) A cell for potentiometric determination

2.1 Electrodes of the first kind- respond directly to activity of electrode ion

copper indicator electrode

Cu2+ + 2e- Cu(s)

Problems: simple but not very selectivesome metal electrode can not be use in acidic solutionssome easily oxidized (deaerated solutions)

pCuE

aEE

Cu

CuCuind

2

0592.0

1log

2

0592.0

0

0

2

2

2

2.2 Electrodes of the second kind

- respond to anion activity through formation of complex

silver electrode works as halide or halide-like anions

AgCl(s) + e- Ag(s) + Cl- E0 = +0.222 V

mercury electrode works for EDTA (ethylene-diamine-tetra-acetic acid)

HgY2- + 2e- Hg (l) + Y4- E0 = +0.21 V Y4-: EDTA anion

pCl

aEClind

0592.0222.0

log2

0592.0222.0

pY

aK

a

aE

Y

HgY

Yind

2

0.0592K

log2

0592.0

log2

0592.021.0

4

2

4

2.3 Electrodes of the third kind

- respond to different ion than metal electrode

mercury electrode works for EDTA

HgY2- + 2e- Hg (l) + Y4- E0 = +0.21 V

CaY2-Ca2+ + Y4- Kf = Ca2+Y4-/caY2-

CaK'-0.0592p

1log

2

0.0592log

2

0.0592-K

log2

0.0592-K

log2

0592.0

log2

0592.021.0

2

2

2

2

4

2

4

CaCaYf

Ca

CaYf

Y

HgY

Yind

aaK

a

aK

aK

a

aE

Membrane- Minimal solubility – solids, semi-solids and polymer- Some electrical conductivity - Selective reactivity with the analyte

Types (see Table 23-2 for examples)Crystalline - Single crystal {LaF3 for F-}

- Polycrystalline or mixed crystal: {Ag2S for S2- and Ag}

Noncrystalline- Glass:– {silicate glasses for H+, Na+}- Liquid: {liquid ion exchange for Ca2+ }

3.1 Glass pH electrodeContains two reference electrodes

Eind = Eb+Eref2

Ecell = Eind - Eref1

Fig. 23-4 (p.666) Glass-calomel cell for pH measurement

Fig. 23-3 (p.666) Glass pH electrode

Combination pH electrode (ref + ind)

Membrane structure

SiO4- frame work with charge balancing cations

In aqueous, ion exchange reaction at surface

H+ + Na+Glass- H+Glass- + Na+

H+ carries current near the surface

Na+ carries charge in interior

Fig. 23-4 (p.666) Silicate glass structure for a glass pH electrode

Boundary Potential Eb

pHLaL

a

aE

tcons,aa, ajj

a

a

njE

a

a

njE

EEE

b

''

b

0592.0log0592.0'

log0592.0

tan

log0592.0

log0592.0

'1

2

1

22121

2

'2

22

1

'1

11

21

Difference compared with metallic electrode: the boundary potential depends only on the proton activity

Asymmetry potential

Fig. 23-6 (p.669) Potential profile across a glass membrane

Boundary Potential Eb

pHtconsSCEEEE

E

EAgClAgEpHLE

pHLaLa

aE

tcons,aa, ajj

a

a

njE

a

a

njE

EEE

refindcell

asy

asyrefind

b

''

b

0592.0tan)(

solutions standardagianst n calibratio :

)/(0592.0

0592.0log0592.0'log0592.0

tan

log0592.0

log0592.0

1

2'

'1

2

1

22121

2

'2

22

1

'1

11

21

Sources of uncertainty in pH measurement with glass-electrode

1. Alkaline error

2. Others {Problems, #23-8)

Glass electrodes for other ions (Na+, K+, Cs+,…):

- Minimize aH+

- Maximize kH/NaNa+ for other ions

- modifying the glass surface (incorporation of Al2O3 or B2O3)

tcoefficieny selectivit

)log(0592.0tan

/

/

HNa

NaHNaHind

k

akatconsE

Fig. 23-7 (p.670) Acid and alkaline error of selected glass electrode

3.2 Crystalline membrane electrode (optional)

- Usually ionic compound- Single crystal- Crushed powder, melted and formed- Sometimes doped with Li+ to increase conductivity- Operation similar to glass membrane

Fluoride electrode

At the two interfaces, ionization creates a charge on the membrane surface as shown by

The magnitude of charge depend on fluoride ion concentration of the solution.

pFLaLE

FLaFLaF

Find 0592.0log0592.023

4.1 Gas sensing probes

simple electrochemical cell with two reference electrodes and gas-permeable PTFE membrane

- allows small gas molecules to pass and dissolve into internal solution

- analyte not in direct contact with electrode – dissolved

Fig. 23-12 (p.677) Schematic of a gas-sensing probe for CO2

activity gasfor ][

H (aq)CO

equation Overall

HCO3HOH (aq)CO

solution internal

)()()(

2

solution int

32

analyte external

2

-22

int

222

2

2

3

COa

a

aaK

HCOHO

in

aqCOgCOaqCO

CO

CO

HCOHeq

eranl

solutionernal

probemembraneanalyte

]0.0592log[

]log[05920

E

]log[05920

][log0592.0

log0592.0

pH! sense toelectrode membrane glass usecan

][

constant assuming

2

2

c

2

2

2

HCO

3

3

-3

COL"

ECO.L'

EE

CO.L'

COa

KL

aLE

COa

Ka

a

ref

refindell

HCO

eq

Hind

HCO

eq

H

cellM

M

cellM

cell

cell

cellM

EE

R

RRI

IR

E

EEerrorrel

cellE measuringfor device impedancehigh Need

)(

0592.0

)(

0592.0E

anions

0592.0

)(

0592.0E

cell

cell

KEnpA

pAn

K

for

KEnpX

pXn

K

cationsfor

EEE

Summary

cell

cell

refindcell