REDOX TITRATION

REDOX Titrations

based upon titrations with reducing or oxidizing agents

REDOX Titrations

• Oxidizing Agent + Reducing Agent = Reduction Product + Oxidation Product

• Reducing Agent + Oxidizing Agent Reduction Product + Oxidation Product

REDOX Titrations may be performed __

using visual indicators or by measuring the potential with an appropriate indicating electrode to construct a potentiometric titration curve

REDOX Titrations

areas of applications

• food analyses

• industrial analyses

• pharmaceutical analyses

Oxidation Half -Reaction

• increase in oxidation state

• loss of electrons

• reducing agent is oxidized

• oxidation product

• h RA = # e- loss

Reduction Half-Reaction

• decrease in oxidation state

• gain of electrons

• oxidizing agent is reduced

• reduction product

• h OA = # e- gained

Self-Indication

if titrant is highly colored, this color may be used to detect the end point

Self-Indication

• the end point does not occur at the equivalence point, but at a fraction of a drop beyond

• error is small

Starch Indicator

used for titrations involving iodine

Starch Indicator

• iodo-starch complex which is a very dark-blue color

• color reaction is sensitive even to very small amounts of iodine

REDOX Indicators

• depends on the half-reaction potentials

• rxn must be rapid and reversible for a sharp end point to be detected

• the colors of the oxidized and reduced forms are difference

REDOX Indicators

highly colored dyes that are weak reducing or oxidizing agents that can be oxidized or reduced

Common REDOX Indicators

• ferroin

• diphenylaminesulfonic acid

Ferroin

tris (1,10-phenanthroline) iron (II) sulfate

Ferroin

• one of the best indicators

• useful for titrations with cerium (IV)

Ferroin

end point

red to pale blue

Diphenylaminesulfonic acid

• used for titrations with dichromate in acid solution

• care must be taken in disposing of Cr (VI) since it is an environmental pollutant that is carcinogenic

Diphenylaminesulfonic acid

end point

colorless to purple

Titrants : Oxidizing Agents

• iodine (iodimetric method)

• KMnO4

• K2Cr2O7

• Cerium (IV)

• KBrO3

Titrants : Reducing Agents

• Na2S2O3

• Iron (II)

• Chromium (II)

• Titanium (III)

Iodine

• moderately strong oxidizing agent

• more selective titrant than the strong oxidizing agents

Iodine

performed in neutral or mildly alkaline (pH 8) to weakly acid solutions

Iodine

high-purity I2 can be obtained by

sublimation

Iodine

usually standardized against __

a primary standard reducing agent such as As2O3

Iodine

has a low solubility in water but

the complex I3 - is very soluble

Iodimetric Method

titrant

iodine

Iodimetric Method

analyte

• H2S

• SO3 2- Sn2+

• As3+ N2H4

Iodimetric Method

indicator

starch

Iodimetric Method

titration method

direct and back titration

Iodometric Method

titrant

Na2S2O3

Iodometric Method

analyte

oxidizing agent treated with an excess of iodide

Iodometric Method

indicator

starch

Iodometric Method

titration method

indirect titration

KMnO4

• widely used strong oxidizing titrant

• acts as self-indicator for end-point detection

• can be standardized using primary standard Na2C2O4

K2Cr2O7

• slightly weaker oxidizing agent

• availability as a primary standard

• used in the titration of iron (II)

Cerium (IV)

• powerful oxidizing agent

• ferroin is the suitable indicator for cerate titrations

salt of cerium (IV), (NH4)2Ce(NO3)6 can be obtained

as a primary standard

ferroin is the suitable indicator for

cerate titrations

Cerium (IV)

§ can be prepared from

(NH4)Ce(SO4)4 . 2 H2O and CeO2 . 4 H2O

Cerium (IV)

standardized against primary standard

As2O3, Na2C2O4 or electrolytic Fe

KBrO3

• primary standard form is available from commercial sources

• a convenient and widely used stable source of bromine

1 mol KBrO3 =

3 mol Br2

KBrO3

used in the determination of organic compounds that react with bromine

bromine

is incorporated into an organic compound either by substitution or by addition

Na2S2O3

• only common reducing agent that is stable to air oxidation

• can be kept for long periods of time

• KIO3 is an excellent primary standard

Iron (II)

• only slowly oxidized by air in sulfuric acid solution

• prepared from Mohr’s salt and Oesper’s salt

Iron (II)

can be used to titrate strong oxidizing agents such as cerium (IV), chromium (VI) and vanadium (V)

Chromium (II)

• very powerful reducing agent but readily air-oxidized

• difficult to handle

Chromium (II)

used in the titration of

• iron (III)

• copper (II)

• tin (IV)

• chromate

• vanadate

• chlorate

Titanium (III)

• very powerful reducing agent but readily air-oxidized

• difficult to handle

Titanium (III)

used for the titration of the oxidized forms of copper, iron, silver, gold, bismuth, uranium and tungsten

Common Oxidants

• KMnO4

• KBrO3

• Ce4+

• K2Cr2O7

• I2

KMnO4

rp

Mn2+

KBrO3

rp

Br1-

Ce4+

rp

Ce3+

K2Cr2O7

rp

Cr3+

I2

rp

I 1-

KMnO4

h

5

KBrO3

h

6

Ce4+

h

1

K2Cr2O7

h

6

I2

h

2

Common Reductants

• Fe2+

• Na2S2O3

• Cr2+

• Ti3+

• As 3+

Fe2+

op

Fe3+

Na2S2O3

op

S4O6 2-

Cr2+

op

Cr3+

Ti3+

op

TiO2+

As 3+

op

As 5+

Fe2+

h

1

Na2S2O3

h

1

Cr2+

h

1

Ti3+

h

1

As 3+

h

4

analyte

• is usually in a mixed oxidation state or is in an oxidation state other than that required for titration

• auxiliary reagents should not interfere in the titration and must be removable

analyte

auxiliary reagents

should not interfere in the titration and must be removable

PRE-REDUCTION Auxiliary Reducing Agents (ARA)

• Na2SO3

• SO2

• SnCl2

• Metallic Reductors

PRE-REDUCTION Auxiliary Reducing Agents (ARA)

Na2SO3 & SO2

• good reducing agents in acid solutions

• the excess can be removed by bubbling with CO2 or in some cases boiling

PRE-REDUCTION Auxiliary Reducing Agents (ARA)

SnCl2

• used for the reduction of Fe3+

• excess can be removed by addition of HgCl2

PRE-REDUCTION Auxiliary Reducing Agents (ARA)

Metallic Reductors

widely used for preparing samples

PRE-REDUCTION Auxiliary Reducing Agents (ARA)

Metallic Reductors

Jones Reductor & Walden Reductor

titration of the analyte follows

pre-treatment using one or a combination of the titration methods

PRE-OXIDATION Auxiliary Oxidizing Agents (AOA)

• HClO4

• K2S2O8

• Br2

• MnO4 1-

• H2O2

HClO4

• hot anhydrous form

• the excess can be removed by boiling the dilute solution

K2S2O8

• carried out in hot acid solution and a small amount of Ag1+ as catalyst

• excess is destroyed by boiling Br2

• excess is removed by adding phenol, which is brominated

MnO4 1-

• excess is destroyed by adding hydrazine, the excess of which is destroyed by boiling

H2O2

excess removed by boiling