Redox Titration | Types, Principle & Applictaion

Redox Titration | Notes

REDOX TITRATION –

As per PCI Syllabus here we have covered all the concepts related to Redox Titration. Redox titration notes have provided you with all the concepts in an easy way. Read below the types, applications and principles of redox titration. if you wanted to know about Redox Titration in an easy way then you should read our article till the end.

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Points to be covered :REDOX TITRATION -TYPES, PRINCIPLE & APPLICATION


Oxidation

It may be defined as a loss of electrons to an oxidizing agent ( that undergoes reduction) to yield a more positive or higher oxidation state.

Example:

  1. Fe2+ (Ferrous ion) into Fe 3+(Ferric ion)
  2. Cu (Copper) into Cu2+(Cupric acid)

Oxidizing agent ability to cause them to lose electrons.


Reduction

It may be defined as the gain of electrons from a reducing agent (that undergoes oxidation) to gives a more negative or lower oxidation state.

Example:


Oxidation-Reduction Reaction:-

It is defined as the reaction which takes place rapidly between two typically specific entities, one being a reducing agent & another one is an oxidizing agent.

Example:-  H2 + F2 → 2HF

Here hydrogen is being oxidized & fluorine is being reduced.


Redox potential:-

♦Redox potential is also known as oxidation-reduction potential / ORP

♦It is the measure of the tendency of a chemical species to acquire electrons & thereby be reduced.

♦Reduction potential is measured in volts (v) or millivolts (mV)

♦The potential difference between these 2 electrodes is determined which gives Redox potential.


NERNST EQUATION

♦The Nernst equation enables the determination of cell potential under non-standard conditions.

♦It relates the measured cell potential to the reaction quotient and allows the accurate determination of the equilibrium constant.

♦The Nernst equation is derived from the Gibbs free energy under standard conditions.

Then equation (1) be rewritten as,

E0= Standard electrode (or reduction) potential

E  = Potential observed at absolute temperature

R  = Gas constant = 8.314 joules/deg/mol-1

F   = Faradays constant= 96500 coulomb

T  = Absolute temperature (T)=2980K  =250k

N= number of electrons gained by an oxidant in being converted to a reducing agent.


REDOX INDICATORS

It may be defined as a substance that can be reversibly oxidized or reduced, having different distinct colours in the individual oxidized and reduced forms.

TYPES OF INDICATORS

a)Self indicators:-

A substance is said to be self indicators if it itself acts as an indicator in titration.

Example: KMnO4 acts as a self indicator during the titration of  KMnO4 and oxalic acid.

b)Internal indicator:-

The indicator which is used within the solution is called an internal indicator.

These indictors take part in the reaction.

Example:- N-phenyl anthranilic acid, starch

c)External indicator:-

The indicator which is not added to the solution is called an external indicator.

These indicators do not take part in the reaction.

Example- potassium ferrocyanide used as an external indicator during titration of ammonium with potassium dichromate.


d)Potentiometric Method:-

This is a Physico-chemical method that may be applied not only to those cases where the suitable indicator is not available but also to those cases in which the visual indicator methods fail or limited accuracy.


REDOX TITRATION CURVE

Stage 1 →Initial Region

At this point titration starts

Stage 2 → Buffering point

In this stage, where the reducing agent takes a small amount of time to lose all the electrons and subsequently the electron is gained by an oxidising agent to reach the equivalence point.

Stage 3 → Equivalence point

In this region, where % of oxidised and reduced form are equivalent to each other.

Stage 4 → Other Titration

It is the region showing the constant curve or potential and the colour showing by the indicator should be deep in this significant region.

 

REDOX TITRATION -TYPES, PRINCIPLE & APPLICATION

Cerimetry

♦ This type of titration involving ceric sulphate (Ce+4) as an oxidising agent.

♦Ceric sulphate is a powerful oxidising agent.

♦Ceric sulphate passes bright yellow colour.

♦During titration ceric sulphate undergoes reduction to ceric sulphate (Ce3+) which is colourless in nature.

Ce4+  +  1e → Ce3+

Preparation and standardization of ceric ammonium sulphate

I)Preparation

REDOX TITRATION -TYPES, PRINCIPLE & APPLICATION

II) Standardization of 0.1N ceric ammonium sulphate

REDOX TITRATION -TYPES, PRINCIPLE & APPLICATION

Advantage of ceric sulphate as an oxidising agent

♦Ceric salts in 0.1N solutions are not too dark to obstruct the vision in reading

Hence Molecular weight = Equivalent weight

♦Solution of ceric sulphate is very stable for a long period

♦No effect of light on the stability of ceric sulphate even on heating

♦Ceric sulphate acts as a self indicator

♦Ceric sulphate is a versatile oxidising agent hence it is used in almost all determination


Iodimetry

♦Iodimetry is defined as the titration directly involving iodine solution

♦A various reducing agent such as arsenite, sulphite, thiosulphate & stannous chloride etc is oxidized by iodine.

Preparation & standardization of iodine (0.1 N)

I) Preparation

REDOX TITRATION -TYPES, PRINCIPLE & APPLICATION

II) Standardization

REDOX TITRATION -TYPES, PRINCIPLE & APPLICATION

Principle

condition for iodimetry determination

a)Effect of pH

i)Neutral pH/ Mild alkaline/ Weakly acidic pH →In all this environment the titration usually give fairly good results.

ii)Strong alkaline solution: This condition has too high pH. here iodine(I2) will be disproportionate to give hypo iodate (IO) & hypo iodide (I) as shown below

iii)Strongly acidic solution: due to decomposition hydrolysis reaction etc preventing the solution from rendering strongly acidic

b)Effect of reaction time

Despite the use of a large amount of KI & acid, the rate of reaction between oxidant and I ions are usually slow and sufficient time should be given before titration.

c)Effect of light

Light accelerates the side reaction in which I–  ions are oxidized to I2 by atmospheric oxygen hence reaction mixture kept in a dark place.

d)Titration is conducted in cold condition because iodine is volatile also the sensitivity of starch diminishes with rising in temperature.

Redox titration

Iodometry

♦Indirect determination of oxidizing agents by titrations of liberated iodine from iodide like KI with standard Na2S2O3 is called Iodometry titration.

♦Oxidizing agents such as K2Cr2O, KMnO4,CuSO4 etc are reduced quantitatively when treated with a large excess of KI in an acidic/neutral medium & liberate an equivalent amount of iodine (I2).

♦Here, the formation of iodine takes place as a result of hydrogen iodide (HI), with an oxidising agent. (HI is obtained by the action of dilute HCl/H2SO4 on a solution of KI).

♦Free iodine is liberated as a result of the oxidation of KI in an acidic solution. the liberated iodine is treated with a standard solution of sodium thiosulphate.

Preparation and standardization of 0.1 M Na2S2O3

Titration involving iodine or dealing with iodine liberated in a chemical reaction is called Iodimetric & Iodometric titration respectively.

I)Preparation

REDOX TITRATION -TYPES, PRINCIPLE & APPLICATION

II)Standardization

REDOX TITRATION -TYPES, PRINCIPLE & APPLICATION


Bromatometry

The specific titration with potassium bromate is referred to as bromatometry. it is an effective and useful oxidizing agent in the qualitative determination of substances & organoarsenicals like carbasone (C7H9AsN2O4)

Principle

Titration is dependent upon the formation of Iodine monobromide [IBr] in a relatively higher actual strength of HCl solution.

Theory

Potassium Bromide (KBrO3) may be assayed by the addition of potassium iodide (KI) and dilute HCl and the chemical reaction involved may be expressed as given below

KBr03 + HI→ HIO3 + KBr

IO3+ 5I+ 6H+ → 3I2+ 3H2O

Preparation & standardization

I) Preparation

REDOX TITRATION -TYPES, PRINCIPLE & APPLICATION

II)Standardization

REDOX TITRATION -TYPES, PRINCIPLE & APPLICATION

Dichrometry

♦This titration used potassium dichromate as oxidising agent hence it is known as dichrometry

♦Potassium dichromate is used in an acidic medium and is maintained by the use of dilute sulphuric acid.

♦Solution of Potassium dichromate can be used for estimation of ferrous salts & iodides.

Cr2O72 + 14H+ + 6e → 2Cr3+ + 7H2O

♦The half-reaction for the dichromate system is

Cr2O72 + 14H+ + 6e → 2Cr3+ + 7H2O

♦The relevant half reaction is

Fe2+ → Fe3+ + e

♦The total reaction is

Cr02-7 + 6Fe2+ + 14H+ →2Cr3+ + 6Fe3+ + 7H2O


Advantages of Potassium dichromate (K2CrO7) as a powerful oxidizing agent over KMnO4

♦It is available in pure & stable form

♦An aqueous solution is sufficiently stable on proper storage

♦It is stable towards the light

♦It is used in acid as well as an alkaline solution

Preparation and standardization of 0.1 N K2CrO7

I) Preparation of 0.1N K2CrO7

REDOX TITRATION -TYPES, PRINCIPLE & APPLICATION

II) Standardization Procedure

REDOX TITRATION -TYPES, PRINCIPLE & APPLICATION

Principle

♦Ferrous ammonium sulphate (Mohr’s salt) is a stable double salt FeSO4 with an active constituent.

♦Acidic potassium dichromate solution is a strong oxidizing agent and is rapidly reduced by ferrous ion at the ordinary temperature to green chromic acid when added to Mohr’s salt solution containing dilute H2SO4

♦In this reaction ferrous sulphate is oxidized to ferric sulphate while ammonium sulphate remains unreacted. N-Phenyl anthranilic acid is used as an indicator. The indicator is not oxidised as long as Fe2+ ions are there in the solution.

Reaction

K2Cr2O7 + 4H2SO4 →  K2SO4 + Cr2(SO4)3 + 4H2O + 3[o]

6FeSO4 + 3H2SO4 + 3[O] → 3Fe2(SO4)3 + 3H2O

K2Cr2O7 + 6FeSO4 + 7H2SO4 → 3Fe2(SO4)3 + K2SO4 + Cr2 (SO4)3 + 7H2O

Titration with Potassium iodate

♦Potassium iodate (KIO3) is regarded to be strong oxidizing agent which may be employed in the assay of number of pharmaceutical substance like benzalonium chloride, Potassium iodide etc.

Principle

♦Iodate reacts quantitatively with both iodides (I) and iodine(I2)

♦The titration is carried out in the presence of saturated organic acids, alcohols and several other organic substances.

Molecular formula : KIO3

Molecular weight: 214


Preparation and standardization of Potassium iodate (0.05M)

I) Preparation

REDOX TITRATION -TYPES, PRINCIPLE & APPLICATION

II)Standardization

REDOX TITRATION -TYPES, PRINCIPLE & APPLICATION

KIO3 + 5KI + 3H2SO4 → 3I2 + 3K2SO4 + 3H2O

3 [2Na2S2O3] + 3[I3] → 3[ Na2S4O6] + 3[2NAI]

REDOX TITRATION -TYPES, PRINCIPLE & APPLICATION

Written By: Ms Mayuri Lendave (Asst. Professor)

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