Chapter 23 Potentiometry

1
Chapter 23 Potentiometry J.Willard Gibbs ?
Nernst(??? ??) ???? ??? pass?? E? ?? ?? ???
cell?? ??? ?? - Potentiometry 23A General
Principles
2
Fig. 23-1 A cell for potentiometric
determinations
3
23B Reference Electrodes ?? ??? ?? vs ??? ??? ??,
?? ??, ???
?
Known standard solution?? ?? 1.
Hydrogen Gas Electrode Ultimate standard elec.???
???? ???? ?? ??? ?? ??? ???, ??? ?? ?? ?? ?? ?? ?
???? (Fig 2.2 ??) ?? Pt foid(?? ? ??) ? 2H
2e- H2(??) H2gas ?? ? ??? ???. ???
???. ? ?? ?? ? ?? ?? ??, ?? ?? ??
? Pt surface? ?? ? ???
?As, CN-, Hg
? H2S
4
2. Calomel Electrodes HgHg2Cl2(stad), Cl-(xM)
Cl-? ??? ??
Stsn. ? S.C.E(???, ?? ?? ?, ??? ??)
1normal ? N.C.E ????? ???
Decinormal ? D.C.E ? ?? ?? ? 80?????? ?? ? ?
table 23-1 ??? ?? ??(2000-3000ohom) ? high
input impedence? ? ?? Half reaction Hg2Cl2
2e- 2Hg 2Cl- E 0.2676V E Eo -
(0.0591/2)log(1/Cl-2)  
5
(No Transcript)
6
3. Silver-silver chloride Electrodes
AgCl(salt),KCl(xF)Ag Half reaction
AgCl e- Ag Cl- E 0.2223V ??
Ag wire? AgCl?? plating ? KCl? immerse Internal
reference electrode? ?? ?? In vivo??? ??, 275???
??   4. Mercury/mercurous-sulfate
Electrodes Calomel, Ag-HCl ?? ?? ??
HgHg2SO4(salt),SO42-(xM)SO42- or
SO42-(salt) E 0.64V 0.5M
H2SO4 E 0.68V
7
5. Thallium - analgam/Thallous - chloride
Ele. T1(40analgram) T1Cl(sat), KCl(sat)
Internal elec.? ?? ?? ???? 4?? reference? ? ??.
8
(No Transcript)
9
???? ????? ?? ? ???, ??? ?? ?? ?? ?? ?? Nerst
equation ?? ? ??? ?? ??? ??? ? ?? ??? ?? ? ???
???? ????? ??. (No hysterisis) ? ?? ??? ??
(Ag-AgCl) ???? ??? ?? ??? ??. ? ??? ?? no
hysterisis
10
?? ??) pH -logan2 -logHfH ??? A,f? ??? ?
? ???? pH??? ??? ??? ? ??. Operational Definition
of pH in aqueous solution.
11
??, Pt/H2, buffer soln., Cl-, AgCl/Ag Primary 6?
(3.5-10.5) Secondary 2? (3.5gt, 10.5lt)
12
23C Metallic Indicator Electrodes  1. Mercury
indicator electrodes ?? ? EDTA? ?? ?? ??
??? potentiometric ? Redox
titrations 2. Solid indicator electrodes
Ag/Ag, Cu2/Cu, Pb2/Pb, Cd2/Cd ?? ?
metal/metal ion electrode ? Direct
potentiometric measurement of the activity of
metal ion? ???, Ag/Ag? ??
? Pt, Au electrode ? inert redox indicator
????? complexing agent? ??? ion??
potentiometric titration? ??
? pH - sensitive solid indicator elec.? ??
Sb2O3 6H 6e- ? 2Sb 3H2O 3.
Ion selective membrane electrodes ?
??? ?? ?? ??? ?? ?? ?
? ??
13
(No Transcript)
14
(No Transcript)
15
(No Transcript)
16
23C-4 Metallic Redox Indicators
17
(No Transcript)
18
23D Membrane Indicator Electrodes 1. pH electrode
Glass electrode (1919, ??),1930???? ?? ?? 1) ??
fig ?? ? H-ion-responsive glass membrane
(0.03-0.1mm? ??) 50-500Meggtohm ? Internal
reference electrode ex) Ag/AgCl in HCl
Hg/Hg2Cl2 in HCl ? Internal electrolyte
solution   Entire cell for operation Internal
reference electrode/internal electrode/H-responsi
ve, glass membrane/external solution//external
reference electrode
19
(No Transcript)
20
2) Glass membrane? ?? ? EMF? mechanism(M2O, MO,
MO2M2O3) ?? ??? ??(Li2O 28, CsO 3, La2O3 4,
SiO2 65) SI ? O ? O
?Si?ONa ? NaGel_? ?? ? O
M?? ??? non-bridging "-" charged oxygen
? - Si - ? O  
21
Fig. 23-4
22
Fig. 23-5
If immersed in aqueous soln. NaGl-H ?
HGl-Na(??) ?? ?? gel layer??.
23
(No Transcript)
24
Asymmetrical potential ? ??? ??? ?? ?? ??? ???
??
(??, ?? ?) ?? ??? ?? ?? ????
??? ?? ?? Alkaline error pHgt9?? H ? ??? ?? ???
?? (Skoog??) HGl- B BGl- H ?
?? ? ?? a'1 b'1 b'1 a1
25
(No Transcript)
26
Fig. 23-6
27
(No Transcript)
28
(No Transcript)
29
(No Transcript)
30

• Ion-selective Electrodes(1SEs)
• G.Eisenman(1959) Na, K, Ca2, NH4, Mg2, ??
  ??? ?? ?? ?? ???? ?? ?? ?
• (??)
• A. ?? ???? ???? logarithmically??
• ? If, liquid function? reference -gt const
• ? Relative concentration error ? ? 43/mV(?? ???
  limitation)
• ?? ???? ?? ???? 0.5 ?? ?? ??(vs mono valent
  ion)
• ? ?? ??? ?? 5mV-10mV? ??
• 50????
• ?? ?? endpoint indicator in potentiometric
  titration?? ?? (0.1)

31
(No Transcript)
32
?? ??? ??? B2O3 or Al2O3 ??? ?? ? Na, K, Li
?? ??   Type of ion-selective electrodes ??
mechanism? ?? Membrane sensor? composition   1.
Glass electrodes pH glass ?? Table 23-3 ?? ? ????
?? ??? ?? ?? ? Divalent
cation? ???? ??.  
33
2. Solid-stste crystalline and pressured-pellet
electrodes   ex) Sensor?? LaF3 single crystal?
europium?? ??? fluoride?? (??) Fig 2.6 ??
?) LaF3 crystal ?) Internal
solu. NaF and NaCl ?) Internal
reference electrode (??) ?)
LaF3 ???? pure F-? conductor
F-? specific 1-10-6F, pH5-8 ?)
pH? ?? ?? F-? HF? ?? ??
pHgt8? ?? OH-? LaF3? ??? ?? ?? ? ??? ??? ?? ??
?? Mechanism LaF3 molecularhole ? LaF2 F-
34
(No Transcript)
35
? ???? ??? ????? ????? ????. ???? ???? ? ??
????? ?? ?? ??? ?? Table 23-3 ?? . Solid-State
,Electrode? ?? ? Ag2S?? S ??? CN-?? Ag ???
Hg2?? ? 10-610-7 ??? ?? ?? ?? ? Membrane?
solubility?? Polycrystalline- type? ?? ??
36
(No Transcript)
37
4. Liquid - membrane ion - Exchange
Electrodes (??Fig ??) Mechanism sample
solution ?? ?? ?? ? ???? ?? ? ?? ??? ?? ?? ??
???? ????? ?? ???? ? ?? ??? ??? ???? ?? ?? ??
?? (??) Inner tube ?. Internal reference
electrode (Ag/ Agcl ??) ?.
Aqueous electrolyte solu. containing the ion of
interest Outer tube ?. Organic ion-exchanger
????? , ????, ?,?? ?? ???? ????
???. ?. Thin
organic - membrane ex) K electrode diphenyl
ether ? ?? valinomycin ? K? ?? ?? ?? ??? ??
??? ? V K V?KV ?? ?KVSr?? , K ???
??
38
(No Transcript)
39
??? 1) Particular organic ion-exchanger
2) Organic soluent used to dissolve the
exchanger 3) pH 5.511?? ??
?? pH?? Ca2??? ??
40
(No Transcript)
41

• (?? ??)
• 1) Ca2 ??? Zn2, Fe2??? ?? ?? ??? Ca2?? Zn2,
  Fe2????? ??.
• 2) ?? ?? ?? ??? decanol ???? ?? alkaline-earth
  ion?
• ?? ??
• 3) Exchanger?R-S-CH2COO- ?Pb2,Cu2? ??
• 4) NO3-, ClO4-, BF4- ortho-phenanthroline???
• ?M(O-ph)32,??
• 5) Exchanger? dimethyl-distearyl-ammonium ion
  ?Cl-??? ??
• R4N
• ?? ?? ???? ???? ion exchanger? ???? ??
• 1???? ??
• ?? ?? 050C
• ?? Table 23-4 ??
• ??? ? ? ? ??? ???? ???? ??? ??

42

• 5. Ion-selective microelectrodes
• Micro sample? ?? ion? activity??? ??
• ? Tissue, tubules, capillaries
• Sensor glass, liquid ion-exchanger, solid state
  crystals
• Tip diameter 1.5? / length 3-5??? ??
• ?? ?? ?? Ag/AgCl
• ex) ??? ?? ?? ?? ??(frog skeletal muscle)
• Fine pt wire? liquid ion-exchanger ? PVC? ????
  coating? ??? ???

43
Fig. 23-10
44
2D ISFET(Ion-Selective Field-Effect Transistors)
45
pH meter
46
6. Gas-sensing probes Function mechanism thin,
goo-permeable membrane? ???? sensing element???
?? ?? Dissolved gas ? membrane ? internal
filling solu. ? internal sensor? ??? ???
monitor (in equilibrium, g ex) CO2, NH3, SO2
as in test solu. ? internal electrolyte) , NO2
???? H??? ?? ?????? glass ?? ?? SO2
electrode SO2 ? membrane ? (SO2H2O?HSO3-H) 10-6
-10-2M ?? ??
47
(No Transcript)
48
Mechanism of Response CO2(aq) CO2(g)
analyte membrane solution pores
CO2(g) CO2(aq) membrane internal
pores solution CO2(aq) 2H2O HCO3-
H3O internal internal
solution solution
CO2(aq) 2H2O HCO3 H3O analyte
internal solution
solution
49
2NO2(aq) 2H2O NO2- NO3- 2H3O external
internal solution
50
(No Transcript)
51
7. Enzyme - Substrate Electrodes ??? ??? ?????
??? coating?? ??. ex) Urea electrode
Urease Urea H2O -------gtHCO3- NH4
?urea ???? ?? ??
NH4 glass electrode? ??? polyacrylamide matrix?
coating. ? Nylon netting (engyme - gelmatrix) or
cellophane sheet? suporting Mechanism ??
??? ?? ??? NH4? ?? ??? ????
???? ammonium -sensitive electrode? ??
monitor ??? ??? 23 weeks (usable life time)
10-2M-10-5M urea ?? ???? ?? ???
substrates Urea, L-aminoacid, D-amino acid,
asparagine, glutamine, amyglaline, creatinine,
penicillin, ??
52
(No Transcript)
53
(No Transcript)
54
?? High-impedance electronic voltmeter
Negligible current ? drow ????? ?? error? ??  
?? Utility(portable) 100-300???
??(0.1PH) General-purpose 300-700, 0.05pH or
3mV, recorder?? ? Expended-scale 400-900,
0.01PH Research 900-1000, 0.002PH, 0.1mV
55
(No Transcript)
56
(No Transcript)
57
(No Transcript)
58
(No Transcript)
59
Application of ion-selective Electrodes ?? ??
air, water pollution, fundamental biomedical
research, oceanography, geology, agriculture,
clinical analysis ? 1. Direct potentiometry ?
HCl gas??(Cl-??? ??) ?? ?? 200ppm HCl
Accury 5 from 20-600ppm HCl ? Nitrate ?? ?
Soils ?? Ca2 ?? ? Serum, biological fluid ??
Ca2?? ? Br-, Cl-, F- in rain
60
(No Transcript)
61
2. Ionic-strength buffering fluoride in natural
waters ?? F ?? ???? ???? ??? ?? ?? ?? ???
??. (100-10-6M) ?? ?? ???? ?? ??? ????? ???
??? ??? volt??(V vs C) ???
?? ?? ?? ? ?? ?? ?? ?? (??? ?? F-???)   1M
NaCl, citrate buffer, acetate buffer? ??? ???
11? ??? ??(TISAB)
62
(No Transcript)
63
ex) ? ?? ?? NH3 ??(NH3?? ??) (sample
100ml NaOH pellet) (NH4 -gt NH3) ?
E1?? ( ) NH4Cl ? E2??
?? ?? 1ppb NH3 ? Ca2 in beer Cu2 in tap
water
64
Inherent Error in the Electrode (calibration
process) Disadvantage calibration? K? constant
?? ?????? inherent
uncertainty
65
(No Transcript)
66
The Operational Definition of pH
67
(No Transcript)
68
(No Transcript)