Assay of Ascorbic acid by iodometry: Pharmaceutical Chemistry Practical
Limit test Chlorides, Sulphate, Iron, Heavy metals * Identification tests for Anions and Cations * Prep. & std Sodium Hydroxide, Potassium Permanganate * Assay Ferrous sulphate, Calcium gluconate, Sodium chloride, Ascorbic acid, Ibuprofen * Determination of Melting point & Boiling point * Preparation Benzoic acid from Benzamide, Picric acid from Phenol * Identification & purity test Aspirin, Caffeine
Assay of Ascorbic Acid by Iodometry
Volumetric flask, Burette, Pipette, Weighing bottle with clamp, Conical flasks, Funnel, Beakers
Vitamin C tablets, Potassium iodate, Potassium iodide, Sodium thiosulphate, Sulphuric acid, Sodium carbonate, Starch
0.03 M Sodium thiosulphate :
It is prepared by dissolving about 7.5g of sodium thiosulfate pentahydrate in about 200 cm3 of distilled water taken in a 1.0 dm3 volumetric flask and adding about 0.1 g of sodium carbonate to it. The solution is then diluted to the mark with distilled water.
0.5% Starch indicator solution:</strong>
It is prepared by mixing 0.25g of soluble starch with 50 cm3 of distilled water taken in a 100 cm3 conical flask or beaker heating it with stirring at about 800 C for about 5 minutes. The solution and is then allowed to cool to room temperature.
10% Potassium iodide solution:
It is prepared by dissolving 100g of KI in about 200 cm3 of distilled water taken in a one litre beaker or conical flask and stirring well to dissolve it. It is followed up by making up the volume to 1 litre by adding more distilled water.
0.3M Sulphuric acid:
It is prepared by diluting about 4.2 cm3 of conc. sulphuric acid to 250.0 cm3 with distilled water in a conical flask or beaker.
Preparation of the solution of the given ascorbic acid tablets:
Accurately weigh a vitamin tablet and grind to a powder in a clean mortar with the help of a pestle.
Dissolve the powder in about 20 ml of 0.3M H₂SO, solution and carefully transfer to a 500 cm3 volumetric flask. Dilute to the mark with distilled water.
Preparation of standard solution of potassium iodate
Mass of weighing bottle + potassium iodate = ml g………….g
Mass of weighing bottle (after transferring potassium iodate) = m2 g=………….g
Amount of potassium iodate transferred = m1 – m2 = mg =……………g
Molar mass (Mm) of potassium iodate = 214 g mol”1
Volume of potassium iodate prepared = 100 cm3
Molarity of potassium iodate solution =
=10m………M / 214
Preparation of potassium iodate primary standard
Accurately weigh about 0.6 g of KIO, (Mm, 214 g mol-1) and quantitatively transfer to about 30 ml of distilled water taken in a 100 ml volumetric flask. Stir to dissolve and dilute to the mark with distilled water.
Standardization of sodium thiosulphate solution
Pipet out 10 cm3 of KIO3 solution into a 100 cm3 conical flask and add about 8-10 cm3 of 10% KI solution and 10 cm3 of 0.3M H2SO4 solution.
Titrate the solution of potassium iodate by adding Na2S2O3 solution from the burette.Continue addition till the solution becomes pale yellow.
Add about 2 cm3 of starch solution and continue adding sodium thiosulphate dropwiseuntil the violet colour of the starch iodine complex just disappears.
Repeat the standardization procedure at least three times and record observations in Observation Table.
Determination of ascorbic acid in the solution prepared at ‘a’ above by back titration
Pipette out 10 cm3 of the ascorbic acid solution and transfer to a 100 cm3 conical flask.
Pipette out 10 cm3 of KIO3 solution to the conical flask containing ascorbic acid solution (you may use a burette to transfer potassium iodate solution)
Add about 8-10 cm³ of 10% KI solution and 10 cm3 of 0.3 M H2SO4 solution, and titrate the solution by adding Na2S2O3 solution from the burette. Continue addition till the solution becomes pale yellow.
Add about 2 cm3 of starch solution and continue adding sodium thiosulphate dropwise until the violet colour of the starch iodine complex just disappears.
Repeat the procedure at least three times and record your observations in Observation Table.
Substituting the values, of the molarity and the volume of thiosulphate used, the molarity of back titrated KIO3 M’iodate = lodate_________ M
We can calculate the amount of iodine used for the oxidation of ascorbic acid as follows:
a) The amount of iodine generated in the reaction mixture
Let the molarity of standard potassium iodate = Miodate M
This implies that the amount per cm3 of the standard solution
. = Miodate mM per ml
Volume of standard potassium iodate used = 10.0 ml
The amount of potassium iodate taken = 10.0 x M₁ mM
. As each mole of potassium iodate provides three moles of iodine (I2) therefore the amount of iodine produced = 3 × 10.0 × Miodate mM= 30.0 x Miodate mM
b) The amount of iodine remaining in the reaction mixture after the oxidation of ascorbic acid iodate
Let the molarity of potassium iodate back titrated = M 2 iodate M
This implies that the amount per cm3 of the iodate solution back titrated
= M2iodate mM
Volume of potassium iodate used = 10.0 ml
The amount of potassium iodate back titrated = 10.0 x M 2iodate mM
As each mole of potassium iodate provides three moles of iodine (1₂) therefore the amount of iodine left (or back titrated)
= 3 x 10.0 x M’iodate mM= 30.0 × M 2iodate mM
The amount of iodine used for the oxidation of 10.0 cm3 solution of ascorbic acid
= 30.0 (Miodate -M’ iodate) mM
The amount of iodine used for the oxidation of 1.0 cm3 solution of ascorbic acid
= [30.0 (Miodate – M’iodate )] / 10 mM
= [3.0 (Miodate – M’ iodate)] mM
Therefore, the molarity of the ascorbic acid solution
= [3.0 (Miodate – M’ iodate)] M
The amount of ascorbic acid per litre = Molarity x molar mass
[3.0 (Miodate – M’iodate )] M x 176.1g mol – 1
[3.0(Miodate – M,iodate)] × 176.1g dm – 3
As the vitamin C tablet was dissolved to make 500 cm3 of the solution, we need the amount of ascorbic acid in 500 ml.
The amount of ascorbic acid in 500 ml of the solution
= [1.5 (Miodate – M’ lodate)] x 176.1g
= [264.15 (Miodate – M’ lodate)] g
Substituting the values of the molarities of standard and back titrated potassium iodate respectively in the expression we get, the amount of ascorbic acid in the given tablet of Vitamin C =…g
Result: The amount of ascorbic acid in the given tablet of Vitamin C =….g
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