PHA6116 LEC: Redox Titration

OXIDATION-REDUCTION TITRATION

a sample is made to react with a standard solution of an OA or RA until endpoint is reached

Oxidizing Agent

accepts electrons

Reducing Agents

donates electrons

OXIDATION

Add O

OXIDATION

Increase bond order of O

OXIDATION

Remove H

OXIDATION

Fe²⁺ → Fe³⁺

REDUCTION

I⁰ → I^- (iodine to iodide)

REDUCTION

Remove O

REDUCTION

Reduce bond order of O

REDUCTION

Add H

mixture

Many samples have analytes with a _ of oxidation states and reduces accuracy of titration

AUXILIARY REAGENTS

Reagents that convert most analytes into a single oxidation state

AUXILIARY RA

Jones reductor

Walden reductor

Jones reductor

amalgamated zinc

Walden reductor

granular metallic silver

AUXILIARY OA

Sodium bismuthate

AUXILIARY OA

Ammonium peroxydisulfate

AUXILIARY OA

Sodium and hydrogen peroxides

GENERAL/ TRUE REDOX INDICATORS

Change color upon being oxidized or reduced

GENERAL/ TRUE REDOX INDICATORS

Color changes independent on nature of analyte/ titrant

GENERAL/ TRUE REDOX INDICATORS

ORTHOPHENANTHROLINE

GENERAL/ TRUE REDOX INDICATORS

DIPHENYLAMINE

GENERAL/ TRUE REDOX & SPECIFIC INDICATORS

STARCH

SPECIFIC INDICATORS

POTASSIUM THIOCYANATE (KSCN)

ORTHOPHENANTHROLINE

Form stable complexes with ferrous

• (phen)3Fe3++e(phen)3Fe2+(ferroin)

• Reduction

DIPHENYLAMINE

Colorizes upon oxidation

diphenylamine (colorless)diphenylbenzidine (violet)

STARCH

Forms blue complexes with iodine (as triiodide)

SPECIFIC INDICATORS

Changes color depends on the substance

Reaction ratio

is dependent on oxidation change (NOT the charge of the cation)

5

MnO₄^-

6

Cr₂O₇^-2

6

BrO₃^-

1

Ce⁺⁴

2

I₂

2

C₂O₄^-2

1

S₂O₃^-2

4

AsO₂^-

1

Ti⁺³

1

Fe⁺³

PERMANGANATE METHOD

• Employed in the assay of: 

  • (1) H₂O₂ (direct)

PERMANGANATE METHOD

• Employed in the assay of: 

  • (2) Malic acid in cherry juice (indirect)

PERMANGANATE METHOD

• Employed in the assay of: 

  • Sodium nitrite (residual)

PERMANGANATE METHOD

STANDARD SOLUTIONS
1.	KMnO4 - standardized using Na2C2O4
2.	H2C2O4

PERMANGANATE METHOD

INDICATOR
1.	KMnO4 itself

CERIC SULFATE METHOD

• Employed in the assay of: 

  • (1) Ferrous sulfate and

CERIC SULFATE METHOD

• Employed in the assay of: 

  • (2) Menadione sodium (Vitamin K)

As₂O₃

used to standardize Ce(SO₄)₂

CERIC SULFATE METHOD

STANDARD SOLUTIONS

Ce(SO₄)₂

CERIC SULFATE METHOD

STANDARD SOLUTIONS

Mohr’s salt
(NH4)₂Fe(SO₄)₂ 4H₂O

CERIC SULFATE METHOD

STANDARD SOLUTIONS

Oesper’s salt
(C₂N₂H₁₀)Fe(SO₄)₂ 4H₂O

CERIC SULFATE METHOD

INDICATOR
1.	Orthophenanthroline

IODIMETRY

Sample is titrated with standard I₂ solution

IODIMETRY

• Employed in the assay of: 

  • (1) KSbOC₄H₄O_{6  +} H₂O,

IODIMETRY

• Employed in the assay of:

  • (2) HgO

IODIMETRY

• Employed in the assay of:

  • (3) sodium bisulfate

IODIMETRY

• Employed in the assay of:

  • (4) calomel

IODIMETRY

STANDARD SOLUTION
1.	I2 solution

IODIMETRY

	INDICATOR
1.	Starch TS

IODIMETRY

	REACTION
Reducing agent + I2 oxidized product + I-

IODOMETRY

Liberated I₂ is being titrated with standard Na₂S₂O₃ solution

IODOMETRY

Employed in the assay of oxidizing agents

IODOMETRY

STANDARD SOLUTION
1.	Na2S2O3

IODOMETRY

INDICATOR
1.	Starch TS

IODOMETRY

REACTIONS

Oxidizing agent + KI I2  I2 + 2Na2S2O3 2NaI + Na2S4O6

DIAZOTIZATION WITH NITRITE

Involves formation of diazonium salt

DIAZOTIZATION WITH NITRITE

Employed in the assay of sulfa- and aminoaryl- containing compounds

DIAZOTIZATION WITH NITRITE

	STANDARD SOLUTION
1.	Na2S2O3

DIAZOTIZATION WITH NITRITE

	INDICATOR
1.	Starch TS

BROMINATION

Employed in the assay of phenol, aniline, and resorcinol

BROMINATION

STANDARD SOLUTION

KBrO₃ (very stable)

BROMINATION

STANDARD SOLUTION

Na₂S₂O₃

BROMINATION

	INDICATOR
1.	Starch TS

BROMINATION

	REACTIONS
KBrO3 + KBr Br2  Analyte + Br2 Product  Br2 + 2KI I2 I2 + 2S2O3-2 Product

DICHROMATE

Employed in the assay of ferrous-containing compounds

DICHROMATE

Also used to assay for ethanol

DICHROMATE

STANDARD SOLUTIONS

K₂Cr₂O₇

Ferrous salt (Mohr’s or Oesper’s)

DICHROMATE

INDICATOR
1.	Diphenylamine