Learning objectives Introduction Preparation of a standard solution used for redox titration Oxidizing and reducing agents used in volumetric analysis N/10 potassium permanganate preparation N/10 potassium dichromate preparation N/10 Iodine solution preparation Examples of redox titrations Conclusion References

1. REDOX TITRATIONS
Prepared by
G. Nikitha, M.Pharmacy
Assistant Professor
Department of Pharmaceutical Chemistry
Sree Dattha Institute Of Pharmacy
Hyderabad
Subject: Pharmaceutical Inorganic Chemistry
Year: Pharm-D 1st Year

2. CONTENTS
 Learning objectives
 Introduction
 Preparation of a standard solution used for redox titration
 Oxidizing and reducing agents used in volumetric analysis
 N/10 potassium permanganate preparation
 N/10 potassium dichromate preparation
 N/10 Iodine solution preparation
 Examples of redox titrations
 Conclusion
 References

3. LEARNING OBJECTIVES
In These topic we will discuss
 Brief introduction of Redox Titration
 Preparation of a standard solution used for redox titrations
 Oxidizing and reducing agents used in volumetric analysis
 N/10 potassium permanganate preparation
 N/10 potassium dichromate preparation
 N/10 Iodine solution preparation
 Examples of redox titrations

4. INTRODUCTION
Redox titration is also known as Oxidation-Reduction reaction electrons are
transferred from reducing agents to the oxidizing agents.
 Redox- Reduction + oxidation
 Both processes occur simultaneously
 Hence, one species is oxidised, another is reduced
 So, what is oxidation, and what is reduction?
 3 different versions of the definition:
Oxidation
Loss of electron
(Gain of Oxygen)
Reduction
Gain of electron
(Loss of Oxygen)

5. 1. In terms of Oxygen:
Oxidation: Gain of oxygen in a species
Reduction: Loss of oxygen in a species
2. In terms of Hydrogen:
Oxidation: Loss of hydrogen in a species
Reduction: Gain of hydrogen in a species
3. In terms of Electrons:
Oxidation: Loss of electrons in a species
Reduction: Gain of electrons in a species

6. OXIDATION REDUCTION
1. Gain in oxygen 1. Loss of oxygen
2. Loss of hydrogen 2. Gain of hydrogen
3. Loss of electrons 3. Gain of electrons

7. REDUCING AGENT AND OXIDIZING AGENT
 The substance that donates electrons in a redox reaction is the REDUCING
AGENT
 The substance that takes electrons in a redox reaction is the OXIDIZING
AGENT

8. ASSIGNING OXIDATION NUMBERS
 An oxidation number is a positive or negative number assigned to an atom to
indicate its degree of oxidation or reduction.
 As a general rule, a bonded atom’s oxidation # is the charge that it would have
if the electrons in the bond were assigned to the atom of the more
electronegative element.

9. RULES FOR ASSIGNING OXIDATION NUMBERS
Rule Example
1. The oxidation number of any uncombined element is
0
The oxidation of Na (s)is 0
2. The oxidation number of a monatomic ion equals
the charge on the ion.
The oxidation of Cl- is -1
3. The more electronegative element in a binary
compound is assigned the number equal to the charge
it would have if it were an ion.
The oxidation number of O in NO
is -2
4. The oxidation number of fluorine in a compound is
always -1.
The oxidation number of F in LiF
is -1.
5. Oxygen has an oxidation number of -2 unless it is
combined with F when it is +2 or it is in a peroxide
such as H2O2 when it is -1
The oxidation number of O in NO2
is -2

10. Rule Example
6. The oxidation state of hydrogen in most of its
compounds is +1 unless it is combined with a metal
in which case it is -1
The oxidation number of H in LiH is
-1.
7. In compounds, group 1 and 2 elements and
aluminum have oxidation numbers of +1, +2 and +3
respectively
The oxidation number of Ca in
CaCO3 is +2
8. The sum of the oxidation numbers of all atoms in a
neutral compound is 0
The oxidation number of C in
CaCO3 is +4
9. The sum of the oxidation numbers of all atoms in a
polyatomic ion equals the charge of the ion
The oxidation number of P in
H2PO4
- is +5

11. The sum of the oxidation numbers of all the atoms in a compound is zero.
1. CuO
Oxygen is -2
The oxidation number of copper must be calculated
X+-2 =0
X= +2
2. Na2SO4
Na is +1 because it is a group 1 metal
Oxygen is -2
The oxidation number of sulphur must be calculated
2(+1) + X +4(-2) =0
(2) +X +(-8) = 0
X= +6

12. The sum of the oxidation numbers of all the atoms in a polyatomic ion is the charge
of the ion.
1. NO3
-
Oxygen is -2
The oxidation number of nitrogen must be calculated
X+ 3(-2) = -1
X= +5
2. PO4
3-
Oxygen is -2
The oxidation number of phosphorous must be calculated
X+ 4(-2) = -3
X= (-8) = -3
X = +5

13. Preparation of a standard solution used for redox titration
 For oxidation of reducing agents a standard solution of oxidizing agent solution
has been used as titrant. Similarly for the reduction of oxidizing agents a
standard solution of reducing agent find use as titrant.
 Normal or molar solutions are obtained by dissolving an equivalent weight or
molecular weight of substance per liter respectively.
 The equivalent weight of an oxidant or a reductant refers to the moldecular
weight divided by the number of electrons which one mol of substance gains or
loses in the reaction.
 For example: in order to prepare 1M solution of KMnO4 158 g of KMnO4
should be dissolving in one liter of water while to prepare 1N KMnO4 solution
158/5 =31.6 g of KMnO4 must be dissolved in one liter.

14. Potassium Permanganate- MnO4
- + 8H+ + 5e Mn2+ + 4H2O
Iodine - I2 + 2e 2I- (Iodine)
Ceric salt solutions - Ce4+ + e Ce3+ (Cerous)
Thiosulphate - 2S2O3
2- S4O6
2- + 2e (Tetrathionate)
Oxalic acid - C2O4
2- 2CO2
2- + 2e

15. Oxidizing and reducing agents used in volumetric analysis
Most commonly used oxidizing agents in volumetric analysis are
 potassium permanganate,
 iodine,
 potassium iodate,
 ceric ammonium sulphate
reducing agents
 oxalic acid,
 sodium thiosulphate,
 ferrous ammonium sulphate,
 titanous sulphate or chloride etc.

16. 1. N/10 Potassium permanganate (KMnO4)
In presence of excess of dilute sulphuric acid and reducing agent two moles of
KMnO4 furnish five atoms of available oxygen:
2KMnO4 + 3H2SO4 K2SO4 + 2MnSO4 + 3H2O + 5(O)
Equivalent weight of KMnO4 = Mol. Wt
A solution of KMnO4 having 3.16g of KMnO4 in 1000ml is having approximately
the normality 0.1N. Potassium permanganate is not a primary standard because
it is reduced by atmospheric organic matter or organic matter present in water.
5
=
158
5
= 31.6

17. A. Standardization with oxalic acid:
In acidic medium Oxalic acid is oxidized by KMnO4 to water an carbon dioxide.
2MnO4
- + 5C2O4
2-+16H+ 2Mn2++10CO2+8H2O
This oxidation reaction occurs quantitatively only at a temperature of about 600
- 700 C.
A solution having 6.3g of oxalic acid in 1000ml of water gives the normality of
0.1N . A measured volume of this 0.1 N oxalic acid is treated with sulphuric
acid. It is then heated to about 700 C. While hot the solution is titrated with
approximately N/10 KMnO4 until the solution attains pink colour (end point).

18. b. Standardization with Sodium Oxalate:
Sodium Oxalate is regard as a better standardizing agent than oxalic acid as sodium
oxalate occurs in a pure state and has been free from water of crystallization.
2MnO4
- + 5C2O4
2-+16H+ 2Mn2++10CO2+8H2O
A solution having 6.3g of oxalic acid in 1000ml of water gives the normality of
0.1N . A measured volume of this 0.1 N oxalic acid is treated with sulphuric
acid. It is then heated to about 700 C. While hot the solution is titrated with
approximately N/10 KMnO4 until the solution attains pink colour (end point).

19. c. Standardization with ferrous ammonium sulphate:
It is a stable double salt . It gets oxidized in cold by KMnO4 in presence of H2SO4
and may be represented by the equation
2MnO4
- + 16H+ +10Fe2 + 2Mn2++10 10Fe3+ +8H2O
A known quantity of ferrous ammonium sulphate is dissolved in a measured
volume of distilled water. Before making up the volume a few drops of
sulphuric acid have to be added to avoid any oxidation by air. A measured
volume of the solution is acidified and titrated with N/10 KMnO4 solution until
the solution attains pink colour (end point).

20. 2. N/10 Potassium permanganate (K2Cr2O7 )
It is a salt of dichromic acid H2Cr2O7 . In acid solution each mole of K2Cr2O7 gives
3 atoms of oxygen which are used for oxidizing purposes.
K2Cr2O7
2- + 4H2SO4 K2SO4+Cr2(SO4)3+ 4H2O +3(O)
It is available in pure state and its N/10 solution is obtained by dissolving one-
sixtieth of the gram molecular weight and diluting the solution to 1000 ml. This
reagent finds use in standardizing approximately 0.1N iodine and also in ferrous
iron titrations.

21. Iodimetry and Iodometry
Estimation of reducing substance by using standard iodine is called iodimetry.
Iodine in aqueous solution behaves as a mild oxidizing agent. Strong reducing
agents such as sodium thiosulphate, stannous chloride etc react readily with
iodine even in acidic solution.
2Na2S2O3 + I2 Na2S4O6 + 2NaI
The above solution is indirectly used for estimating an oxidizing agent which reacts
with KI to set free an equivalent of iodine
Example:
2Cu2++ 4I- Cu2I2 + I2
I2 + 2 S2O3
2- S4O6
2- 2I
Sodium tetrathionate

22. The indirect estimations of oxidizing agents involving the liberation of iodine and
subsequent volumetric estimations of iodine are termed as iodometry. The
indicator used in titrating iodine solution with sodium thiosulphate has been 1%
solution. It produces blue coloured adsorption complex with free iodine.
Preparation of starch indicator:
Nearly 1g of starch is mixed with 5ml of water to a thin paste in a motor. Then this
paste is poured into 100ml of boiling water. The mixture is boiled for 2-3
minutes and it is cooled for use.

23. N/10 Iodine solution
An approximately N/10 solution of iodine is prepared by transferring 12.7g of
iodine 18g of KI and 50ml of water to a stoppered iodine flask shaking the
mixture until the solution gets completed.
It is standardized with standard N/10 sodium thiosulphate solution which in turn
gets standard KMnO4 solution or K2Cr2O7 solution or KIO3 solution as follows
A. By using KMnO4 solution:
It is based on the fact that KMnO4in acid solution when treated with excess of KI
liberates free iodine. The liberated iodine is titrated against the given sodium
thiosulphate using starch as until the solution attains blue colour.
2MnO4
- +10I+16H+ 2Mn2++8H2O+5I2
I2 + 2 S2O3
2- S4O6
2- 2I

24. B. By using K2Cr2O7 :
It is also based on he fact that dichromate like KMnO4 is able to liberate I2 from KI
in the presence of acid. This liberated iodine is titrated with N/10 Na2S2O3 using
starch as indicator until the solution attains blue colour.
Cr2O7
2-+ 14H++6I- 2Cr3+ +7H2O+3I2

25. Examples:
1. Potassium permanganate against arsenic trioxide:
Potassium permanganate has been a salt of permanganic acid HMnO4 . In the
present of reducing agent and in sulphuric acid medium 2 moles of KMnO4
gives 5 atoms of oxygen.
2KMnO4 +3H2SO4 K2SO4+2MnSO4+3H2O+5O
Hence the equivalent weight becomes 158/5 =31.6g . For a N/10 solution 3.16g
gets dissolved in 1000ml of water. The solution is filtered through glass wool
and stored in amber coloured bottles (for protection against light).
Nearly 0.15g of dried arsenic trioxide is dissolved in 10ml of 20% sodium
hydroxide solution and the solution diluted to 100ml. This is then treated with
10ml of cold Hcl and titrated with KMnO4 solution until it gets changed to
slightly pink colour.
AS2O3 + 2O AS2O5 (Pink colour)

26. 2. Potassium permanganate against sodium thiosulphate :
A measured quantity of about 40ml of N/10 KMnO4 solution is added to a cold
solution of 15g of potassium iodide in 50ml water having 7 to 8ml of dilute Hcl.
The liberated iodine is treated with N/10 sodium thiosulphate employing starch
mucilage as an indicator.
2KMnO4+ 10KI + 16Hcl 12KCl+2MnCl2+5I2 + 8H2O
3. Potassium permanganate against oxalic acid
A weighed quantity of oxalic acid is dissolved in water which is acidified with
dilute sulphuric acid. It is heated to 60- 700 C and titrated with N/10 sodium
Thiosulphate KMnO4 solution to faint pink colour.
COOH+ [O] 2CO2 + H2O

27. 4. Iodine against trioxide:
An aqueous solution of iodine acts as a mild oxidizing agent. Iodine solution is
obtained by dissolving iodine in the presence of an excess of potassium iodide
in water.
I2 + H2O 2HI+ O
In order to prepare N/10 iodine solution 12.7g of iodine 18g of potassium iodide
are dissolved in 1000ml of water.
Nearly 0.15g of arsenic trioxide is first of all dissolved in sodium hydroxide. Then
it is acidified with Hcl treated with an excess of sodium bicarbonate and titrated
with iodine solution using starch mucilage as an indicator until there appears a
faint permanent blue colour.
AS2O3 + 3H2O 2H3ASO3
H3ASO3 + I2 + H2O H3ASO4 2HI

28. CONCLUSION
In these topic we have discussed that
 Brief introduction of Redox Titration
 Preparation of a standard solution used for redox titrations
 Oxidizing and reducing agents used in volumetric analysis
 N/10 potassium permanganate preparation
 N/10 potassium dichromate preparation
 N/10 Iodine solution preparation
 Examples of redox titrations

29. REFERENCE
 Pharmaceutical Chemistry -Inorganic Volume-1 by G. R. Chatwal.
 Essentials of Inorganic Chemistry by Katja A. Strohfeldt.
 Indian Pharmacopoeia.
 M.L Schroff, Inorganic Pharmaceutical Chemistry.
 P. GunduRao, Inorganic Pharmaceutical Chemistry, 3rd Edition
 A.I. Vogel, Text Book of Quantitative Inorganic analysis.
 Bentley and Driver's Textbook of Pharmaceutical Chemistry.

30. THANK YOU