ANALYTICAL cHEMISTRY - V

Experiment 2

Principle

• Detection of peroxide gives the initial evidence of rancidity in unsaturated fats and oils. It gives a measure of the extent to which an oil sample has undergone primary oxidation the extent of secondary oxidation may be determined from p-anisidine test.

• The peroxide value is defined as the amount of peroxide oxygen per 1 kilogram of fat or oil. Traditionally, this was expressed in units of milliequivalents, although in SI units it is expressed in millimoles per kilogram.

• The peroxide value is determined by measuring the amount of iodine which is formed by reaction of peroxides with iodine ion.

2 I⁻ + H₂O + H₂O₂ → 2 OH⁻ + H₂O + I₂

• The base produced in this reaction is taken up by the excess of acetic acid present. The iodine liberated is titrated with sodium thiosulphate.

2 S₂O₃²⁻ + I₂ → S₄O₆²⁻ + 2 I⁻

• The acidic conditions (excess acetic acid) prevent formation of hypoiodite which would interfere with the reaction. Peroxide values of fresh oils are less than 10 milliequivalents/kg and when peroxide values are between 30 and 40 milliequivalents/kg, a rancid taste is noticeable.

Apparatus required

Iodine flask, Glass road, Burette, Pipette, Measuring cylinder, Funnel, etc.

Chemicals required

Acetic acid, Chloroform solution (3:1), KI solution (30%), 0.02 N Na₂S₂O₃.5H₂O, Starch (as indicator), etc.

Preparation of reagents

1. Acetic acid - Chloroform (3:1)

Take 375 ml acetic acid and 125 ml chloroform to form 3:1 mixture of the two of 500 ml.

2. 30 % KI solution

Weigh 30 g KI and dissolve it in 100 ml distilled water.

3. 0.02 N Na₂S₂O₃.5H₂O solution

1000 ml 1 N Na₂S₂O₃.5H₂O ≡ 248.20 g

250 ml 0.02 N Na₂S₂O₃.5H₂O ≡ 1.241 g

Procedure

Weigh 0.5 g oil in two iodine flasks. Add 30 ml acetic acid - chloroform solution and swirl to make it miscible. Add 0.5 ml 30% KI solution and let it stand with occasional shaking for 1 minute. Add 30 ml distilled water and titrate it with 0.02 M sodium thiosulphate from burette till solution turns almost yellow. Now add starch indicator and again titrate with 0.02 M Na₂S₂O₃.5H₂O from burette till it becomes colorless. Similarly take a reading for blank as well.

Result

Peroxide values,

1. For groundnut oil = ______________ milliequivalents/kg.

2. For corn oil = ________________ milliequivalents/kg.

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. Discuss how high peroxide values will affect the quality of oils.

2. What will happen if we use an excess of 30% KI solution?

3. Do vegetable ghee and saturated fats have peroxide values?