ANALYTICAL cHEMISTRY - V

MSCM303

Experiment 1

Aim of the Experiment

Analysis of vitamin C content in the given juice/squash sample.

Principle

  • Vitamin C, also known as ascorbic acid and ascorbate, is a vitamin found in various foods and sold as a dietary supplement. The deficiency of vitamin C results in a disease named scurvy, scurvy is characterized by spots on and bleeding under the skin, poor wound healing, weakness, sore arms, etc.

Structures of ascorbic acid and dehydroascorbic acid

  • Vitamin C refers to the L-enantiomer of ascorbic acid and its oxidized form, dehydroascorbic acid. Ascorbic acid is a weak sugar acid, structurally related to the glucose. In biological systems, ascorbic acid can be found only at low pH.

  • In solutions above this pH, vitamin C is predominantly found in ionized form, ascorbate. All of these molecules have vitamin C activity and thus are used synonymously with vitamin C, unless otherwise specified.

  • There are various methods for determination of ascorbic acid i.e. vitamin C. For example, vitamin C content of a food sample such as fruit juice can be calculated by measuring the volume of the sample required to de-colorize a solution of dichlorophenolindophenol (DCPIP) and then calibrating the results by comparison with known concentration of vitamin C.

Determination of vitamin C using iodimetric titration:

  • On titration of ascorbic acid with iodine, it is converted to i.e. oxidized to dehydrous ascorbic acid while iodine is reduced to iodide ions.

Ascorbic acid + I2 → 2 I− + Dehydroascorbic acid

  • Iodine is immediately reduced to iodide as long as there is only ascorbic acid present. Once all ascorbic acid has been oxidized, excess iodine is free to react with starch indicator forming blue-black starch-iodine complex which is the end point of titration.

  • Titration using potassium iodate, KIO3− is more preferred compared to that with iodine as it is more stable compared to iodine solution. The reactions involved are,

IO3− + 6H+ + 5e− → 1/2 I2 + 3 H2O

  • Combining above equations,

IO3− +5I− + 6H+ → 3I2 +3H2O

Ascorbic acid + I2 → 2I− + Dehydroascorbic acid

Apparatus required

Conical flask, Burette, Pipette, Funnel, etc.

Chemicals required

0.025 N I2 solution, Glacial acetic acid, 0.025 N Na2S2O3.5H2O solution, Starch (as an indicator), Fruit juice/squash sample, etc.

Preparation of reagents

  • 0.025 N I2 solution:

1000 mL 1 N I2 solution ≡ 127 g

100 mL 0.025 N I2 solution ≡ ________ g.

Triturate 0.3175 g I2 and double the amount i.e. 0.6350 g KI in mortar and pestle and dissolve it in water. Dilute it to the mark in a 100 mL volumetric flask.

  • 0.025 N Na2S2O3.5H2O solution:

1000 mL 1 N Na2S2O3.5H2O ≡ 248.19 g

100 mL 0.025 N Na2S2O3.5H2O ≡ _________ g.

  • 0.025 N CuSO4.5H2O solution:

1000 mL 1N CuSO4.5H2O ≡ 249.68 g

100 mL 0.025 N CuSO4.5H2O ≡ __________ g.

  • 10% KI solution:

Dissolve 10 g KI in water and make the volume to 100 mL in a 100 mL volumetric flask.

Procedure

For standardization of Na2S2O3.5H2O solution:

Pipette out 10 mL 0.025 N CuSO4.5H2O solution in a conical flask. Add (1:1) liquor ammonia solution till turbidity is obtained. Add glacial acetic acid till turbidity is dissolves. Add 10 mL 10% KI solution and titrate it against Na2S2O3.5H2O solution in a conical flask and add 10 mL glacial acetic acid. Titrate it against I2 solution from burette using starch as indicator.

For standardization of I2 solution using Na2S2O3.5H2O solution:

Pipette out 10 mL standardized Na2S2O3.5H2O solution in a conical flask and add 10 mL glacial acetic acid. Titrate it against I2 solution from burette using starch as indicator.

Experimental titration:

Pipette out 25 mL of fruit juice in a conical flask and add 10 mL glacial acetic acid to it. Titrate it against standardized I2 solution from burette using starch as an indicator.

Amount of Vit. C in Fruit Juices_1.mp4

Observation

Calculation

Amount of Vit. C in Fruit Juices_2.mp4

Result

Amount of vitamin C in 100 mL fruit juice = _________ mg.

Reported value (per 100 mL) = _________ mg.

% Error = __________ .

Reference Material

  1. G H Jeffery, J Bassett, J Mendham and R C Denney, Vogel's Textbook of Quantitative Chemical Analysis, 5th Edition

  2. S. Suzanne Nielsen, Food analysis, 4th Edition

Questions

  1. What are the sources of vitamin C?

  2. What is the difference between iodimetry and iodometry?

  3. Enlist the reducing as well as oxidizing agents that are in play in this titration.

Developed by

Dr. Viraj Bhanvadia,

Assistant Professor, Chemistry,

viraj.bhanvadia@gsfcuniversity.ac.in