ORGANIC cHEMISTRY - III

Experiment 3

Aim of the Experiment

Determination of Iodine number of sample Oil by Wij’s Solution.

Principle

Iodine value or iodine number is defined as the number of grams of iodine equivalent to the amount of Iodine Monochloride (ICl) consumed by 100 g of the oil.

Iodine value is sued to determine the degree of unsaturation of oils and fats.Like acid value and saponification value,each oil has its own characteristics.I.V. for a good lubricating oil,its I.V. should be as low as possible so as to avoid the oxidation and polymerization during its use.

Iodine value is determined by dissolving known weight of oil sample in CCl4 and treating with a known excess of Wij’s solution (solution of iodine monochloride(ICl) in glacial acetic acid) One molecule of ICl adds one each double bond present in oil.

The unreacted ICl oxidises the KI solution to I₂

ICl + I^- ------> I₂ + Cl^-

ICl + KI -------> I₂ + KCl

The liberated I₂ is titrated with Hypo solution using freshly prepared starch indicator added near the end point

S₂O₃^2- + I₂-------> S₄O₆^2- + 2 I^-

^{Thiosulphate ion Tetrathionate ion}

2Na₂S₂O₃ + I₂ -------> Na₂S₄O₆ + NaI

^{Sodium thiosulphate Sodium tetrathionate}

From the amount of Hypo consumed, the unreacted I₂ is determined.Thus iodine value can be determined as shown under calculations.

Materials required

(i) Standard sodium thiosulphate solution(Na₂S₂O₃) (N/20)

(ii) Potassium iodide solution (KI) 10%

(iii) Carbon tetra chloride as a solvent

(iv) Wij’s solution(solution of iodine mono chloride (ICl) in glacial acetic acid.

Indicator:

Freshly prepared starch solution added near the end point.

Procedure

Weigh accurately 0.5-1 g of given sample oil into a weighing bottle.Transfer it to the iodine titration flask.Now weigh the empty weighing bottle.Difference in the two weights, gives the exact weight of oil sample taken in idone titration flask.Add 25 ml of CCl₄ with the help of pipette and 25 ml of Wij’s solution with the help of burette.Take the same amount of CCl₄ and Wij’s solution to the another iodine titration flask.Close the flasks with glass stoppers moistened with 10%KI solution and shake vigrously to mix the contents.Keep both the flasks in dark for about 1 hour.Now add 20 ml of KI solution to both the flasks and then titrate with standard Hypo solution till the colour of solution changed to pale yellow.Add 10-12 drops of freshly prepared starch solution as indicator.Again titrate with the same Hypo solution until the disappearance of blue colour.Note down the volume of standard Hypo solution used in sample titration and in the blank titration i.e. without oil.

Observation

Weight of oil sample + weighing bottle = W₁ g

Weight of weighing bottle alone = W₂ g

Weight of oil sample =W₁ - W₂ = Wg

Volume of N/20 Hpyo used in the sample titration = A ml

Volume of N/20 Hypo used in the blank titration = B ml

Volume of N/20 Hypo equivalent to iodine (or ICl) consumed by W g of the oil = (B-A) ml

Calculation

Iodine present in (B-A) ml of N/20 iodine solution = (B-A) x Eq. Wt. X Normality /1000 g

Iodine value : Vol.of I₂ (ICl) used X Eq.wt. Of I₂ X Normality X 100 / Weight of oil X 1000

Result

Iodine number of sample Oil =

Reference

vlab.amrita.edu/?sub=3&brch=63&sim=1111&cnt=1

vlab.amrita.edu/?sub=3&brch=63&sim=1111&cnt=2

Questions

1. What is iodine value?

2. Theory of determining iodine value using Wij solution?

3. Why reaction mixture needs to kept in dark?