PA I - Redox Titration Pharmaceutical analysis

Unit: IV
REDOX TITRATION
By
ELAVARASI.E
Assistant Professor,
Faculty of Pharmacy
Bharath Institute Of higher education and research

Introduction:
▶ Redox Titration is a laboratory method of determining the
concentration of a given analyte by causing a redox reaction
between the titrant and the analyte. These types of titrations
sometimes require the use of a potentiometer or a redox indicator.
▶ Redox titration is based on an oxidation-reduction reaction between
the titrant and the analyte.
▶ In order to evaluate redox titrations, the shape of the corresponding
titration curve must be obtained. In these types of titration, it
proves convenient to monitor the reaction potential instead of
monitoring the concentration of a reacting species.

Oxidation & Reduction
Oxidation: A substance can undergo oxidation can occur via:
▶ The addition of oxygen.
▶ Removal of hydrogen which was attached to the species.
▶ The donation/loss of electrons.
▶ An increase in the oxidation state exhibited by the substance.
C + O2 → CO2
(Here carbon has gained oxygen and it has been oxidized to carbon dioxide)
Fe+2 – e − → Fe+3
Sn+2 – 2e− → Sn+4
▶ In above reactions, there is loss of electrons, hence the concerned ions are said
to be oxidized.

Reduction: A substance can undergo reduction can occur via:
▶ The addition of hydrogen.
▶ The removal of oxygen.
▶ The acceptance of electrons.
▶ A reduction in the overall oxidation state.
CUO + H2 → CU + H2O
▶ In above reaction, copper oxide has been reduced to copper by loss of oxygen.
CU+3 + e− → CU+2
(Here gaining of 1 electron, cupric ion is said to be reduced to cuprous ion.)

What are oxidation-reduction reactions?
▶ Oxidation-reduction reactions are chemical reactions that involve
the transfer of electrons between the reacting species. These electron
transfers are accompanied by a change in the oxidation state of the
reactants.
▶ In a chemical reaction, oxidation and reduction occur
simultaneously. If one atom is oxidized, other is reduced.
Fe+2 + Cl2 → 2Fe+2 + 2Cl− [2(FeCl)]
▶ In the above reaction, Ferrous ion is oxidized to Ferric ion by loss of
1 electron and Chlorine has been reduced to chloride ion by gain of
1 electron. Therefore, these reactions are called as redox reactions.

Oxidizing & Reducing Agents
What are oxidizing agents?
▶ These are the agents which themselves get reduced i.e. they accept electrons
and oxidize other elements. They are also called oxidizer or oxidants.
▶ Examples: potassium dichromate, potassium permanganate, ceric
sulphate, iodine, nitric acid (HNO3) and hydrogen peroxide (H2O2) etc.
What are reducing agents?
▶ These are the agents which themselves get oxidized i.e. they donate electrons
and reduce other elements. They are also called as reductants or antioxidants.
▶ Examples: oxalic acid, ferrous sulphate, titanious sulphate,
sodium thiosulphate, zinc and lithium, etc.

KMnO4 + FeSO4 + H2SO4 → MnSO4 + Fe2 (SO4)3 + K2SO4 +H2O
(Here, electrons are transferred from FeSO4 to KMnO4)
▶ Mn (+7) in KMnO4 has been reduced to Mn (+2) in MnSO4.
▶ Fe(+2) in FeSO4 has been oxidized to Fe (+3) in Fe2(SO4)3.
▶ Thus, KMnO4 acts as oxidizing agent and FeSO4acts as
reducing agent in acidic medium.
Half Reaction:
▶ In redox titrations, two reactions take place simultaneously, oxidation and reduction.
Oxidation and reduction reactions, each are known as half reactions.
Consider the following reaction:
Ce+3 + Fe+3
Fe+3 + e −
Ce+4 + Fe+2
▶ Oxidation half reaction may be written as:
Fe+2
▶ Reduction half reaction may be written as:
Ce+4 + e−
Ce+3

Detection of End Point In Redox Titrations:
Following indicators can be used in redox titrations to determine the end
point.
1. Internal or redox indicators
2. Self indicators
3. External indicators
4. Specific indicators
5. Potentiometric methods

1. Internal or redox indicators : These are the redox indicators (also called an oxidation-reduction
indicators) which undergo oxidation-reduction reaction at the equivalence point.
▶ The indicator has different colour in oxidized and reduced form. Thus, end point is marked by a
colour change. There are two common types of redox indicators:
 Metal-organic complexes (Eg. phenanthroline)
 True organic redox systems (Eg. Methylene blue)
e.g. Diphenylamine Ferroin
2. Self indicators: A coloured substance may act as its own indicator.
▶ For example: potassium permanganate, ceric ammonium sulphate, iodine.
▶ KMnO4 gets reduced in redox titration as follows:
MnO4
− + 4H+

3.External indicators: These are also called as spot test indicators. These were
used earlier when no indicators were available.
▶ For example: Ferricyanide ion was used to detect iron (II) ion by formation of iron
(II) ferricyanide (Turnbull’s blue) on a spot plate outside titration vessel.
4.Specific Indicators: It is as a substance which reacts in a specific manner with one
of the reagents in a titration to produce colour.
▶ For example: Starch gives deep blue colour with iodine (1% solution of starch is
used); Thiocyanate gives red colour with iron (III).
5.Potentiometric Method: It is a physicochemical method where the equivalence
point is detected by large changes in potential. This method can be used when
suitable indicators are not available, visual method is of limited accuracy (coloured
solutions or very dilute solutions). Potential is measured by a potentiometer. At end
point, there is sudden rise in potential.

TYPES OF REDOX TITRATIONS
Redox titration Titrant
Iodometry Iodine (I2)
Bromatometry Bromine (Br2)
Cerimetry Cerium(IV) salts
Permanganometry Potassium permanganate
Dichrometry Potassium dichromate

1. Iodometry & Iodimetry (Iodine Titration)
Iodometry: The oxidizing agent oxidizes iodide to
iodine. Iodine dissolves in the iodide-containing
solution to give triiodide ions, which have a dark
brown color. The triiodide ion solution is then
titrated against standard thiosulfate solution to give
iodide using starch indicator till blue colour changes
to colourless.
2I– + 2e → I2
I2 + I– → I3
–
Na2S2O3 + I3
– → NaI + Na2S4O6
I.Preparation of 0.1 M Sodium Thiosulphate Solution: Dissolve 25 g of sodium thiosulphate and
0.2 g of sodium carbonate in carbon dioxide free water and dilute to 1 litre with water.
II.Standardization of Sodium Thiosulphate: Dissolve accurately weighed 0.200 gm of
potassium bromate in water to produce 250 ml to 50 ml of this solution. Add 2 gm of potassium iodide
and 3 ml of 2M hydrochloric acid and titrate with sodium thiosulphate solution using starch
indicator until blue colour disappears.
Factor: Each ml of 0.1 M sodium thiosulphate is equivalent to 0.002784 gm of KBrO3

Conditions for Iodometric Titrations:
• Titration must be carried out in acidic conditions.
•In highly alkaline conditions, NaOI (Sodium hypo-iodide) is formed which is a
strong oxidizing agent as compared to iodine.
2NaOH + I2 → NaOI + NaI + H2O
• Reaction mixture must be kept in dark to decrease volatilization of iodine.
•Titration must be carried out in closed flask and in cold conditions to
prevent volatilization of iodine.

Iodimetry: In iodimetry, standard solution
of iodine is used as a titrant. It is based on
following reaction:
2I − I2 + 2
e−
• It is used for estimation of reducing
agents like arsenates (H3AsO3)
and thiosulphates (Na2S2O3).
I. Preparation of Iodine Solution: Dissolve 14 gm of iodine in solution of 36 gm of
potassium iodide in 100 ml of water, add 3 drops of hydrochloric acid and dilute with water to
1000 ml. Iodine is slightly soluble in water (0.00134 mol/lit). Therefore, potassium iodide is
added to increase the solubility and to decrease the volatility of iodine.
I2 + I − → I

II. a. Standardization of Iodine Solution with Arsenic Trioxide: Weigh 0.15 gm of arsenic
trioxide previously dried at 105°C for 1 hour and dissolve in 20 ml of 1 M sodium hydroxide
solution by warming if necessary. Dilute with 40 ml of water, add 2 drops of methyl orange
solution and add dilute HCl solution until the yellow colour changes to pink. Then add 2 gm
of sodium bicarbonate, dilute with 50 ml of water and add 3 ml of starch mucilage as
indicator. Slowly titrate with iodine solution until permanent blue colour is obtained.
Conditions for Iodimetric Titrations:
1. As iodine is volatile, titration
must be
conducted in cold conditions.
2. As solubility of iodine is low in
water,
excess of KI must be used to dissolve it.
3. Sufficient time should be given
before
titration, as iodine ionizes too slowly.
4. Thereaction mixture should be
kept in
dark, as
reactions.
light accelerates the
side
Iodide ions are oxidized to
iodine by atmospheric oxygen.
4I− + 4H+ O2 → 2I2+ 2H2O

2. Bromatometry
• When potassium bromates (KBrO3) is used as titrant, the titration is known
as
Bromatometry.
• Potassium bromate is a strong oxidizing agent and is used for assay of number
of
pharmaceutical substances like, mephensin, phenol, sodium salicylate, etc.
• Reagent can be employed in 2 ways:
1. Direct titration
2. Bromination
3.Direct titration: Reducing substances like As (III), Sb (II), Fe (II) and certain
organic sulphides and disulphides can be titrated directly with potassium bromate.
BrO3
− + HAsO2 → Br − + HAsO3
BrO3
− + Br− + H+ → Br2 + H2O
• Appearance of bromine is sometimes suitable for detection of end point. Organic indicators

2.Bromination of Organic Compounds: Potassium bromate-potassium bromide (KBr-
KBrO3) mixture is used for liberation of bromine.
KBrO3 + 5KBr + 3H2SO4 → 3K2SO4 + 3Br2 + 3H2O
• Bromine is then used for bromination of organic compounds. Excess of Bromine is then determined
by addition of KI.
Br2 + KI → I2 + Br-
I2 + S2O3
2− → I− + S4O6
2−
I.Preparation of 0.1 N Potassium Bromate: Weigh accurately about 2.784 gm of potassium
bromate and 12 gm of potassium bromide in water and dilute to 1000 ml with water.
II.Standardization of Potassium Bromate: Transfer 30 ml of 0.1 N potassium bromate solution
in 250 ml iodine flask. Add 3 gm potassium iodide and 3 ml of hydrochloric acid. Mix the
contents thoroughly and allow to stand for 5 minutes. Titrate the liberated iodine with previously
standardized 0.1 N sodium thiosulphate using 3 ml of freshly prepared starch solution as indicator
added towards the end.
Factor: Each ml of 0.1 N sodium thiosulphate is equivalent to 0.002784 gm of KBrO3.

3. Cerimetry
• Titrations involving determinations with cerium compounds are called Cerimetry.
• Atomic weight of Cerium is 140 gm/mol.
• It exists in 2 oxidation states: +4 and +3.
• In tetravalent (Quadrivalent) state it acts as a powerful oxidizing agent.
• It is of dark yellow colour in concentrated solution.
• On reduction trivalent cerium ion is obtained, which is colourless.
• Some of the compounds of Cerium such as Ceric ammonium nitrate (NH4)2Ce(NO3)6,
Ceric ammonium sulphate (NH4)4Ce(SO4)4, Ceric hydroxide Ce(OH)4 can be used
for determination.
• Indicators:

I. Preparation of 0.1 M Ceric Ammonium Nitrate Solution: Dissolve 65 gm of ceric
ammonium sulphate with help of gentle heat, in a mixture of 30 ml sulphuric acid
and 500 ml water. Cool it. Filter if turbid and dilute with water to 1000 ml.
II. Standardization of 0.1 M Ceric Ammonium Nitrate Solution: Weigh accurately about
0.2 gm of arsenic trioxide, previously dried at 105°C for 1 hour and transfer to 500 ml
conical flask. Wash with 25 ml of 8% w/v solution of sodium hyrdroxide, swirl to
dissolve, add 100 ml of water and mix. Add 30 ml of dilute sulphuric acid, 0.15 ml of
osmic acid solution, 0.1 ml of ferroin solution and titrate with ceric ammonium nitrate,
until pink colour is changed to very pale blue.
Factor: Each ml of 0.1 M ceric ammonium nitrate is equivalent to 0.004946 gm
of As2O3.

Advantages of Cerimetry over Pemanganometry:
• Cerium can be reduced to only one oxidation state.
• It is very strong oxidizing agent and its intensity can be varied using different acids.
• Sulfuric acid solutions are extremely stable.
• No effect of light or heating for short time.
•Cerium nitrate is available in sufficiently pure form to directly prepare standard
solution.
Applications of Cerimetry:
• Titration of hydrogen peroxide.
• Ferrous compounds can be analyzed.
• Oxalates can be analyzed using ceric sulphate.
• Analysis of tartaric acid, phthalic acid, saliculic acid.
• Nitrogen compounds like hydroxylamines, nitrites can also be analyzed.

4.Permagnometry
• Potassium permanganate is widely used as oxidizing agent in redox titrations.
• It is readily available and inexpensive. It requires no indicators except in very
dilute
solutions.
• Permanganate undergoes a variety of chemical reactions, since manganese can exist
in
oxidation states of +2, +3, +4, +6, +7.
• It reacts rapidly with reducing agents as follows:
MnO4
− + 8H + + 5e − = Mn +2 + 4H2O
I. Preparation of 1 M KMnO4: Dissolve 158 gm of KMnO4 in water to make up volume to
1000 ml. Heat on water bath for 1 hour, allow standing for 2 days. Filter through glass wool.
II.a) Standardization of KMnO4: Primary standards like Arsenious oxide,
Sodium oxalate/Oxalic acid, Sodium thiosulphate, Iron can be used for standardization of

II. b) Standardization with Sodium Oxalate: Dissolve 6.7 gm of sodium oxalate
previously dried at 110°C in water and then make up volume to 1 litre. Pipette out 20 ml of
this solution to conical flask, add 5 ml of concentrated sulphuric acid and then warm to
70°C. Titrate against KMnO4 until pink colour persists for 30 s.
5Na2C2O4 + 2KMnO4+ 8H2SO4 K2SO4 + 2MnSO4+ 10CO2 + 5Na2SO4 + 8H2O

5.Dichrometry
• Titration involving potassium dichromate (K2Cr2O7) as titrant is
known as dichrometry. Dichromate titrations are based
on conversion of dichromate ion containing hexavalent
chromium into trivalent chromium ions.
7 2
Cr2O2− + 14 H++ 6e → 2Cr+3+ 7H O
• Indicators:
External indicators: Potassium
ferricyanide. Diphenylbenzidine,
Internal indicators: Diphenylamine,
Diphenylamine sulfonate.
Colour change: green to violet

I.Preparation of 0.1 N K2Cr2O7: Dissolve 4.9036 gm equivalent of K2Cr2O7
previously powdered and dried in a dessicator for 4 hours, and dissolve in water to
produce 1000 ml. Potassium dichromate can be obtained as a primary standard reagent
and standard solution may be prepared determinately and stored for long periods of time.
II. Standardization of Potassium Dichromate: To 20 ml of solution, add 1
gm of
potassium iodide and 7 ml of 2 M hydrochloric acid. Add 250 ml water and titrate against
0.1 m sodium thiosulphate using 3 ml starch solution until the colour changes from blue to
light green.
Factor: Each ml of sodium thiosulphate is equivalent to 0.0049 gm of K2Cr2O7.

2. Enlist the various titrants used in
redox
titrations.
3. Name redox indicators.
4. What is iodimetry?
5. What is equivalent weight?
6. Explain equivalent weights concept.
7. Explain the Nernst equation.
8. What are conditions for iodometric titrations.
9. Give difference between iodometry
and
iodimetry.
10. How will you prepare and standardize 0.1 N
sodium thiosulphate.
11. How will you prepare and standardize 0.1 M
ceric ammonium nitrate.
12. How will you prepare and standardize 0.1 N
Short Answer Questions: Long Answer
Questions:
1. What are redox titrations?
oxidizing
1. Explain the concepts of
agent and reducing agent
in
redox
titrations.
2. Explain the methods to balance redox
reactions.
3. Explain the methods of end point
detection in redox titrations.
4. Write a note on permanganometry.
5. Write a note ion cerimetry.
6. Give an account of dichrometry.
7. Write a note on bromatometry.
8. Write a note on preparations of
potassium permanganate. What
precautions are to be kept while
preparing it.
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PA I - Redox Titration Pharmaceutical analysis

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