Chapter 7 Electrochemistry
§7.3 Applications of Conductivity
Measurement
Main contents:
1) Monitoring the purity of water
2) Measurement of ionizability
3) Determine solubility product and ionic product
4) Conductometric titration
1. Monitor the purity of water
Group works:
Calculate the conductivity of pure water.
at 25 oC, Kw = 10-14, [H+] = [OH] = 10-7 mol·dm-3
water
Tap
water
Distilled
water
Deionized
water
Pure
water
/S·m-1
1 10-2
~1 10-3
<1 10-4
5.478 10-6
Fabrication of VLSI
Silicon wafer
In-situ monitoring of the cleaning process :
H2O2/H2SO4  DW rinse  dilute HF  SC-1 cleaning 
DW rinse  hot DW rinse  DW rinse  SC-2 cleaning 
DW rinse  dilute HF  DW rinse.
DW - deionized water,
SC – standard cleaning,
SC-1: H2O2/NH3;
SC-2: H2O2/HCl.
2. Measure the ionizability and dissociation constant
+



AB 
A
+
B

c
0
0
c(1   )
c
c
c Z  F (U   U  )
m 
c
At infinite dilution
m  F (U   U  )
c+ = c
m   (U   U  ) F
m  (U   U  ) F





m (U   U  ) F
Arrhenius suggested that the degree of dissociation of an
electrolyte can be calculated using molar conductivity.
Ostwald showed how one can measure the dissociation
constant of an acid.
cm
c 2 2
c 2
K


(1   )c 1   m (m  m )
Ostwald dilution law
The equation can be rearranged to
1
m

1
K a m 2
(m c) 
1
m
1
m
Linearization
1
m
1
S
K a (m )2
mc
3. Determine solubility and solubility products
Example:
The conductivity of a saturated AgCl solution is 1.86 
10-4 S·m-1, while that for water is 6.0  10-6 S·m-1. Molar
conductivity of AgCl is 0.01372 S·mol-1·m2. Calculate
the solubility product for AgCl.
  cAg Ag  cCl Cl  cH H  cOH OH


-





   w  cAg Ag  cCl Cl  S (Ag  Cl )



  w
  w
S

Ag  Cl
m





5. Determine ion product of water
Water can be taken as a dilute solution of a strong
electrolyte
CH+ = COH- = Kw
1/2

c


 m  m,H



m,OH 
The specific conductance of highly purified water is 5.478 10-6
S·m-1. Calculate the concentration of hydrogen ion in water.


m  m,H
 m,OH
 0.05481S  mol-1  m2



5.478 106 S  m 1
4
3
c  

0.9995

10
mol

m
m 0.05481S  mol1  m 2
 1107 mol  dm 3
6. Conductometric titration
 / S·m-1
The conductance variation of a solution
during titration can serve as a useful method
to follow the course of the reaction.
Ionic mobility of ions:
A
C
C’
B
A’
B’
endpoint
V / ml
Na+ + OH- + (HCl) =
5.19 20.52
Na+ + Cl- +(H+ + Cl-)
5.19 7.91 36.30
Precipitation reactions,
neutralization reactions, and
coordination reactions