❖ IODINE TITRATIONS
These are redox titrations involving standard iodine solution either employed directly or an
equivalent amount of iodine is liberated indirectly from the reaction mixture and then
assayed.
1. Iodimetry: determination involves direct titration with standard iodine solution
2. Iodometry: determination in which equivalent amount of iodine is liberated by the
sample and the liberated iodine is titrated against standard sodium thiosulphate solution
IODIMETRY
Type of redox titration in which standard solution of Iodine used as the titrant which act as an
oxidizing agent.
Principle:
Aqueous solutions of iodine are weak oxidizing agents in aqueous solutions that are
used for the determination of strong reducing agents.
During the reaction iodine is reduced according to the half reaction,
I2 + 2e 2I-
Analysis of reducing agents in iodimetry is based on this reversible reaction, Hence, it
is a much weaker oxidizing agent than potassium permanganate
➢ Reducing agents such as arsenite, sulphite, thiosulphite and stannous chloride etc
can be quantitatively determined by direct titration with iodine
Eg:
I2 + 2Na2S2O3 2NaI + Na2S4O6
sodium thiosulphate sodium tetrathionate
➢ Since iodine is a mild oxidizing agent, it can’t reduce arsenic (III) or antimony (III).
➢ Iodine is highly volatile and slightly soluble in water. Hence, it is difficult to prepare
the solution of iodine.
But it forms a soluble triiodide ion in solution of iodide
I2 + I- I3-
, So, it should be dissolved in presence of excess of potassium iodide which improves
the solubility and also reduces the volatility
➢ Standard solutions of iodine may be prepared from accurately weighing the pure
reagent
PREPARATION AND STANDARDIZATION OF IODINE SOLUTION
➢ Preparation of 0.05M (0.1N) Iodine solution
Dissolve about 14g of iodine in a solution of 36g of KI in 100ml of water, add 3 drops of HCl
and dilute with water to 1000ml.
➢ Standardization of 0.05M iodine solution
Iodine solution may be standardized by using the following:
a) Primary standard – arsenic trioxide (As2O3)
b) Secondary standard – sodium thiosulphate solution
Standardization using arsenic trioxide
Arsenic is the best primary standard for standardizing iodine solutions. In aqueous solutiom,
it is present as arsenous acid (H3AsO3) is oxidized by iodine.
H3AsO3 + I2 + H2O H3AsO4 + 2HI (2H+ +2 I-)
Arsenous acid arsenic acid
Arsenic trioxide is readily soluble in sodium hydroxide with the formation of sodium arsenite
As2O3 + 6NaOH 2Na3AsO3 + 3H2O
If iodine is added to this alkaline solution, it will react with sodium hydroxide to form sodium
hypoiodite (NaIO) and sodium iodide
2NaOH + I2 NaIO + NaI + H2O
So, the excess of NaOH is neutralized with hydrochloric acid using methyl orange as
indicator until yellow colour is changed to pink
Sodium bicarbonate is added to neutralize the hydriodic acid,HI, formed in the reversible
reaction.
H3AsO3 + I2 + H2O H3AsO4 + 2HI (2H+ +2 I-)
Arsenous acid arsenic acid Hydriodic acid
These are redox titrations involving standard iodine solution either employed directly or an
equivalent amount of iodine is liberated indirectly from the reaction mixture and then
assayed.
1. Iodimetry: determination involves direct titration with standard iodine solution
2. Iodometry: determination in which equivalent amount of iodine is liberated by the
sample and the liberated iodine is titrated against standard sodium thiosulphate solution
IODIMETRY
Type of redox titration in which standard solution of Iodine used as the titrant which act as an
oxidizing agent.
Principle:
Aqueous solutions of iodine are weak oxidizing agents in aqueous solutions that are
used for the determination of strong reducing agents.
During the reaction iodine is reduced according to the half reaction,
I2 + 2e 2I-
Analysis of reducing agents in iodimetry is based on this reversible reaction, Hence, it
is a much weaker oxidizing agent than potassium permanganate
➢ Reducing agents such as arsenite, sulphite, thiosulphite and stannous chloride etc
can be quantitatively determined by direct titration with iodine
Eg:
I2 + 2Na2S2O3 2NaI + Na2S4O6
sodium thiosulphate sodium tetrathionate
➢ Since iodine is a mild oxidizing agent, it can’t reduce arsenic (III) or antimony (III).
➢ Iodine is highly volatile and slightly soluble in water. Hence, it is difficult to prepare
the solution of iodine.
But it forms a soluble triiodide ion in solution of iodide
I2 + I- I3-
, So, it should be dissolved in presence of excess of potassium iodide which improves
the solubility and also reduces the volatility
➢ Standard solutions of iodine may be prepared from accurately weighing the pure
reagent
PREPARATION AND STANDARDIZATION OF IODINE SOLUTION
➢ Preparation of 0.05M (0.1N) Iodine solution
Dissolve about 14g of iodine in a solution of 36g of KI in 100ml of water, add 3 drops of HCl
and dilute with water to 1000ml.
➢ Standardization of 0.05M iodine solution
Iodine solution may be standardized by using the following:
a) Primary standard – arsenic trioxide (As2O3)
b) Secondary standard – sodium thiosulphate solution
Standardization using arsenic trioxide
Arsenic is the best primary standard for standardizing iodine solutions. In aqueous solutiom,
it is present as arsenous acid (H3AsO3) is oxidized by iodine.
H3AsO3 + I2 + H2O H3AsO4 + 2HI (2H+ +2 I-)
Arsenous acid arsenic acid
Arsenic trioxide is readily soluble in sodium hydroxide with the formation of sodium arsenite
As2O3 + 6NaOH 2Na3AsO3 + 3H2O
If iodine is added to this alkaline solution, it will react with sodium hydroxide to form sodium
hypoiodite (NaIO) and sodium iodide
2NaOH + I2 NaIO + NaI + H2O
So, the excess of NaOH is neutralized with hydrochloric acid using methyl orange as
indicator until yellow colour is changed to pink
Sodium bicarbonate is added to neutralize the hydriodic acid,HI, formed in the reversible
reaction.
H3AsO3 + I2 + H2O H3AsO4 + 2HI (2H+ +2 I-)
Arsenous acid arsenic acid Hydriodic acid
