1J.Willard Gibbs → Nernst( )
pass E ; cell - Potentiometry
23A General Principles
23B Reference Electrodes
↓
,
(Fig 2.2 ) : Pt foid( ) 2H+ + 2e- = H2()
H2gas
,
Pt surface
1normal N.C.E
* 80 table 23-1
(2000-3000ohom) high input impedence
Half reaction
E = Eo - (0.0591/2)log(1/[Cl-]2)
 
: Ag wire AgCl plating KCl immerse
Internal reference electrode
In vivo , 275
 
Hg|Hg2SO4(salt),SO42-(xM)||SO42-
T1(40%analgram) | T1Cl(sat), KCl(sat)||
Internal elec.

, Nerst equation

. (No hysterisis)
(Ag-AgCl) .
no hysterisis
) pH = -logan2 = -log[H+]fH+
A,f pH .
Operational Definition of pH in aqueous solution.
, Pt/H2, buffer soln., Cl-, AgCl/Ag
Primary 6 (3.5-10.5)
Redox titrations
: 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
Sb2O3 + 6H+ + 6e- 2Sb + 3H2O
3. Ion selective membrane electrodes


Direct response to ion or species being measured to ion
<-> metallic or soluble plase
Cu2+ + 2e- = Cu(s)
23C-2 2nd-kind electrode :
Active metal in combination with an insoluble compound or salt sparingly soluble metal <-> metal
AgCl(s) + e- = Ag(s) +Cl- Eo = 0.222 V
Eind = 0.222 – 0.0592logaCl-
23C-3 3rd-kind electrode :
Miscellous glass elect.
1) : fig
H+-ion-responsive glass membrane
(0.03-0.1mm ) 50-500Meg>ohm
2) Glass membrane EMF mechanism(M2O, MO, MO2M2O3)
(Li2O 28%, CsO 3%, La2O3 4%, SiO2 65%)
SI


Na+Gl-+H+ ⇔ H+Gl-+Na+()
gel layer.
Asymmetrical potential :
(, …)
:
Alkaline error : pH>9 H+ (Skoog)
H+Gl- + B+ = B+Gl- + H+

a'1 b'1 b'1 a1
pH > 8
Inert electrode : gold, platinum
Low impedance
3. Antimony Electrode
: Glass tube Sb .
Glass tube .
Epoxy cement glass Sb stick .
: -> , , , Simplicity
: , Sb2O3
.
Active elect :
Ion-selective Electrodes(1SEs)
G.Eisenman(1959) : Na+, K+, Ca2+, NH4+, Mg2+,
()
If, liquid function reference -> const
Relative concentration error ±43%/mV( limitation)
±0.5% (vs mono valent ion)
: ±5mV-±10mV
50%
B. Activities
Chemical interference : complexaton
Fig 2.5 (Ca2+ )
Pure Ca2+ 6X10-6M (B )
Na+
1 ion
: 0-103
∴ Nernst aq. obey range
*
 
Divalent cation .
  ex) Sensor LaF3 single crystal europium fluoride
() : Fig 2.6
) Internal reference electrode
F- specific 1-10-6F, pH5-8

Mechanism : LaF3 + molecularhole LaF2+ + F-
3. Ag2S pressed-pellet membranes
Ag ionic conductor
α-AgS(cubic) β-Ag2S(ionic) silye crystal
mechanism in Ag2S
Ag2s ⇔ 2Ag+ + S2-
Ag+ activity : X-
AgX Ag+ + X-
Ag+ ,S2-
.
|
| Table 23-3 . Solid-State ,Electrode
Ag2S S CN-
Ag Hg2+
Membrane solubility
(Fig )


()
Outer tube : . Organic ion-exchanger
, , ,
.
. Thin organic - membrane
ex) K electrode: diphenyl ether valinomycin K

| | V | K+ +V⇔KV |⇔KV:Sr , K+:
: 1) Particular organic ion-exchanger
2) Organic soluent used to dissolve the exchanger
3) pH 5.511
pH Ca2+
Ca2+ : ( ) (RO)2PO2-
: phospate diesters, R: didecyl
dioctyl phenyl phosphorate
[(RO)2POO]2Ca = 2(RO)2POO- + Ca2+
( )
1) Ca2+ Zn2+, Fe2+ Ca2+ Zn2+, Fe2+ .
2) : decanol alkaline-earth ion

4) NO3-, ClO4-, BF4- : ortho-phenanthroline
M(O-ph)32+,
R4N+
1
: Table 23-4
5. Ion-selective microelectrodes
Tissue, tubules, capillaries
: Ag/AgCl
ex) (frog skeletal muscle)
*Fine pt wire liquid ion-exchanger PVC coating
Fig. 23-10
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
10-6-10-2M
Mechanism of Response
external internal solution
ex) Urea electrode
Mechanism : NH4+
ammonium -sensitive electrode monitor
: 2~3 weeks (usable life time)
10-2M-10-5M urea
: High-impedance electronic voltmeter
 
General-purpose : 300-700$, ±0.05pH or ±3mV, recorder
Expended-scale : 400-900$, ±0.01PH

Nitrate
Serum, biological fluid Ca2+
Br-, Cl-, F- in rain
2. Ionic-strength buffering : fluoride in natural waters
F .
(100-10-6M)
volt(V vs C)
( F-)
 
1M NaCl, citrate buffer, acetate buffer 1:1 (TISAB)
@ Method of standard Additions : NH3 in Aquaria,
 
(E2 : + test solution)
-> volume
E1, E2 combine
Antilog C
(sample 100ml + NaOH pellet) (NH4+ -> NH3)
E1
( ) + NH4Cl
E2
Inherent Error in the Electrode (calibration process)
Disadvantage : calibration K constant inherent
uncertainty
2)