Lecture 14

Redox Titrations

aAnalyte + bTitrant cC + dDproducts

Requirements:

-One reaction with integer coefficients a and b

- The reaction is fast enough to perform the whole procedure in 20-40 min

- Titrant solutions are stable and their concentrations can be easily established by the simple procedures

-There is an easy way to establish the end point of titration (that is, the convenient indicator is available)

Titrant solutions are stable and their concentrations can be easily established by the simple procedures

K2Cr2O7 - no change in 30 years

Primary standard -chemical compound that you can weighand disolve to prepare solution of known concentration without standartization

Requirements: - Purity of at least 99.9%- Stable and easy to purify if necessary- Convenient to weigh- Well soluble and stable in solution- non-toxic, non-expensive

Secondary standard -solution of known concentration which was established bytitration of the primary standard

Oxidants:

CeIV

MnO4-

Cr2O72-

BrO3-

IO3-

I2

strong

Weak, selective

][

][log06.05.1

3Ce

CeE

IV

][

][log06.077.0

2

3

Fe

FeE

][

][log06.077.0

2

3

Fe

FeE

][

][log06.05.1

3Ce

CeE

IV

Emin =0.77 + 0.063= 0.95

-E(SCE)

Emax =1.5-0.060.06=1.32

-E(SCE)

Any potential in this rangeis be satisfactory

99.9%

100.1%

][

][log06.077.0

2

3

Fe

FeE

Emin =0.77 + 0.063= 0.95

OHCreHOCr 232

72 72614

23

14272

][

]][[log

6

06.036.1

Cr

HOCrE0.01 M K2Cr2O7

[Cr3+]=10-2 M[Cr2O7

2-]=10-5M[H+]=1 M

E1.3V

1.0

1.3

0.899.9%

100.1%

][

][log03.077.0

3

Tl

TlE

Emin = 0.77+0.033=0.86-E(SCE)

OHICleHClIO 223 3462

][

][]][[log

4

06.024.1

2

623

ICl

HClIOE

Emax = 1.24-0.0153=1.19

-E(SCE)

@ 1M HCl

BrO3- + 5Br- + 6H+ 3Br2 + 3H2O

NH2

SO2NH2

NH2

SO2NH2

Br Br

+ 2Br2

Starch

I