Download - 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
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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
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aE
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a
a
njE
a
a
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asyrefind
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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
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aqCOgCOaqCO
CO
CO
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eranl
solutionernal
probemembraneanalyte
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]log[05920
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log0592.0
pH! sense toelectrode membrane glass usecan
][
constant assuming
2
2
c
2
2
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HCO
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3
-3
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ECO.L'
EE
CO.L'
COa
KL
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Ka
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ref
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Summary
cell
cell
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