Chapter 7 Electrochemistry

§7.3 Applications of Conductivity Measurement

A

B

C

A’B’

C’

V / ml

/ S

·m-

1

endpoint

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-1: H2O2/NH3;

SC – standard cleaning, SC-2: H2O2/HCl.

2. Measure the ionizability and dissociation constant

m

( )c Z F U U

c

c+ = c

m ( )U U F

At infinite dilution

m ( )F U U

m

m

( )

( )

U U F

U U F

Arrhenius suggested that the degree of dissociation of an electrolyte can be calculated using molar conductivity.

+AB A + B

0 0

(1 )

c

c c c

2 2 2m

m m m(1 ) 1 ( )

cc cK

c

Ostwald dilution law

Ostwald showed how one can measure the dissociation constant of an acid.

The equation can be rearranged to

m2m m m

1 1 1( )

a

cK

mc

m

1

m

1

2m

1

( )a

SK

Linearization

3. Determine solubility and solubility products

-Ag Ag Cl Cl H H OH OHc c c c

w Ag Ag Cl Cl Ag Cl( )c c S

w w

mAg Cl

S

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.

5. Determine ion product of water

Water can be taken as a dilute solution of a strong electrolyte

CH+ = COH- = Kw1/2

m m,H m,OHc

The specific conductance of highly purified water is 5.478 10-6

S·m-1. Calculate the concentration of hydrogen ion in water.

-1 2m m,H m,OH

0.05481S mol m

6 14 3

1 2m

7 3

5.478 10 S m0.9995 10 mol m

0.05481S mol m

1 10 mol dm

c

6. Conductometric titration

The conductance variation of a solution during titration can serve as a useful method to follow the course of the reaction.

Na+ + OH- + (HCl) =

5.19 20.52

Na+ + Cl- +(H+ + Cl-)

5.19 7.91 36.30

Precipitation reactions, neutralization reactions, and

coordination reactions

Ionic mobility of ions:A

B

C

A’B’

C’

V / ml

/ S

·m-1

endpoint