DEFINATION: The Iodine Value
is a measure of the amount of unsaturation of fats & oils and is expressed
in terms of the number of centigrams of iodine absorbed per gram of sample (%
iodine absorbed).
SCOPE: Applicable to all normal oils and fats which
do not contain conjugated double bonds.
PRINCIPLE:
The
oil/fat sample taken in carbon-tetrachloride is treated with a known excess of
iodine monochloride solution in glacial acetic (Wijs solution). The excess of
iodine monochloride is treated with potassium iodide and the liberated iodine
estimated by titration with sodium thiosulphate solution.
APPARATUS:
1. Glass stoppered Erlenmeyer flask 500 ml
2. Measuring Cylinder ( 50 ml)
3. Bulb pipette ( 20 ml capacity)
REAGENTS:
1. Glacial Acetic Acid, free from
ethanol: AR grade
2. Carbon Tetrachloride or Chloroform: AR grade
3. Potassium dichromate: AR grade
4. Concentrated hydrochloric acid: AR
grade
5. Iodine Monochloride
6. Preparation of Wij’s solution when
using ampoules of Iodine monochloride:
Wij’s
Iodine monochloride solution - Carefully break open 1 X 50-g ampoule of Iodine
Monochloride (Use Iodine Monochloride of E. Merck or of similar quality) and
pour into 155ml of Glacial acetic acid.
Mix well and store in a stoppered amber colored bottle. This is the
stock solution. Dilute 50 ml of the
stock to 1000 ml using Glacial acetic acid. Mix well and store in a well
stoppered clean and amber colored glass bottle. This is the solution to be used
for determination of Iodine value.
OR
Preparation
of Wij’s solution when ampoules are not used.
Wij’s
Iodine monochloride solution - Dissolve 10 ml of iodine monochloride in about
1800 ml of glacial acetic acid and shake vigorously. Pipette 5 ml of Wij's
solution, add 10 ml of potassium iodide solution and titrate with 0.1N standard
sodium thiosulphate solution using starch as indicator. Adjust the volume of
the solution till it is approximately 0.2 N
or
prepare Wij’s iodine solution by dissolving 13 gm resublimed Iodine in 1 litre
acetic acid and pass in dried chlorine ( dried through H2SO4.) until original
Sod thiosulphate titre of the solution is not quite doubled. (characteristic
colour change at the end point indicates proper amount of Chlorine. Convenient
method is to reserve some amount of original I solution, add slight excess of
Cl to bulk of solution and bring to desired titre by re additions of reserved
portion). Store in an amber bottle sealed with paraffin until ready for use.
Wij’s solutions are sensitive to temp, moisture and light. Store in dark at
less than 30 0 C . Determine I / Cl ratio as follows:
Iodine Content –
Pipette 5 ml Wij Solution into 500 ml Erlenmeyer flask containing 150 ml
saturated Cl – water and some glass beads. Shake , heat to boiling point and
boil briskly 10 minutes. Cool, add 30 ml H2SO4 ( 1 + 49) and 15 ml 15 % KI
solution and titrate immediately with 0.1 N Na2 S2O3.
Total Halogen content
– Pipette 20 ml Wij’s solution into 500 erlenmeyer flask containing 150 ml
recently boiled and cooled water and 15 ml 15 % KI solution. Titrate
immediately with 0.1 N Na2S2O3.
I
/ Cl = 2 X / (3B – 2 X) where X = ml of 0.1 Na2 S2O3 required for I content and
B = ml required for total halogen content.
I
/ Cl ratio must be 1.10 ± 0.1
7. Potassium Iodide (free
from potassium iodate) solution (10 %)
:
Dissolve 10 g of Potassium Iodide AR Grade
in some quantity of distilled water and make up the volume to 100ml. with
distilled water. (prepared freshly)
8. Starch solution: Mix 5 g
of starch and 0.01 g of the mercuric iodide with 30 ml of cold water and slowly
pour it with stirring into one litre of boiling water. Boil for three minutes.
Allow to cool and decant off the supernatant clear liquid.
9. Standard Sodium
Thiosulphate solution: (0.1N)
Dissolve approximately 24.8 g of sodium
Thiosulphate crystals (Na2S2O3.5H2O) solution in distilled water and make up to
1 liter.
STANDARDISATION
OF 0.1 N SODIUM THIOSULPHATE
1. For 0.1 N sodium thiosulphate
accurately weigh about 0.10 g to 0.12 g of potassium dichromate previously
dried at 120° C for 2 hours, in a 500 ml flask.
2. Dissolve in 25 ml of distilled water
& swirl.
3. Add about 5 ml of conc. HCl followed by
20 ml of saturated potassium iodide solution.
4. Allow to stand for 5 minutes & then
add 100 ml of distilled water.
5. Titrate against sodium thiosulphate
solution from the burette until yellow colour has almost disappeared.
6. Add 1 to 2 ml starch indicator &
continue titration, until the blue colour just disappears.
Calculation: N = 20.394 x
wt. of K2Cr2O7
B.R.
Where,
N = Normality of the sodium thiosulphate
W
= Weight in g of the potassium dichromate, and
V
= Volume in ml of sodium thiosulphate solution required for titration
Theory:
When
oil is treated with excess of Wij’s solution i.e. ICl, some ICl gets
absorbed. This ICl adds on to the double
bonds present in the oil. Unused ICl reacts with KI solution and liberates an
equivalent amount of Iodine, which is titrated against standard Sodium
thiosulphate solution. Iodine value is
a measure of unsaturation of oils and fats.
Reaction:
Cl
I
ICl +
KI ----Ã
KCl + I2
I2 + 2Na2S2O3 -----Ã
2NaI + Na2S4O6
PROCEDURE:
Oil
/ fat may be weighed accurately following the Table given below:
|
Expected
Iodine Value
|
Weight in g of Sample
|
|
|
|
Max.
|
Min.
|
|
5
|
6.3460
|
5.0770
|
|
10
|
3.1730
|
2.5384
|
|
50
|
0.6612
|
0.5288
|
|
100
|
0.3173
|
0.2538
|
1. Weigh accurately the quantity of dry
& filtered oil/fat sample mentioned in the table above, in a clean dry
500-ml iodine value flask, to which add 25-ml chloroform or carbon
tetrachloride.
2. Mix the content well. The weight of the
sample shall be such that there is an excess of 50 to 60 percent of Wij’s
solution over that actually needed.
3. Pipette 25 ml of Wij’s solution
immediately by bulb pipette.
4. Stopper the flask using the quick fix
stopper moistened with potassium Iodide solution.
5. Swirl for proper mixing and keep the
flasks in dark for half an hour for non-drying and semi-drying oils and one
hour for drying oils.
6. Carry out a blank simultaneously.
7. At the end of the period remove the
flask, add 15 ml of 10% Potassium Iodide solution and 100 ml of recently boiled
and cooled water, washing down the stopper also.
8. Immediately titrate the liberated
iodine with 0.1 N Sodium Thiosulphate solution using starch solution as
indicator towards the end until the blue colour formed disappears after through
shaking with the stopper on.
9. Vigorous shaking is necessary during
and towards the end of the titration.
10. Carry out a blank determination in the
same manner as test sample excluding the sample.
Note:
Slight variations in temperature appreciably affect titre of I 2 solution as
chloroform has a high coefficient of expansion. It is thus necessary that
blanks and sample determinations are made at the same time.
CALCULATION:
Calculate
Iodine Value or Percentage Iodine absorption of the sample using the relation.
1
ml of 0.1 N Sodium Thiosulphate = 0.01269 g of Iodine
Iodine value = 12.69 (B – S) N
W
B
= volume in ml of standard sodium thiosulphate solution required for the blank.
S
= volume in ml of standard sodium thiosulphate solution required for the
sample.
N
= normality of the standard sodium thiosulphate solution.
W
= weight in g of the sample.
Caution:
• Carbon tetrachloride is a known
carcinogen. It is toxic by ingestion, inhalation and skin absorption. It is a
narcotic. It should not be used to extinguish fires. It decomposes to phosgene
gas at high temperature. The TLV is 5 ppm in air. A fume hood should be used at
all times when using carbon tetrachloride.
• Wijs solution causes severe burns and
vapour can cause lung & eye damage. Use of fume hood is recommended.
• Sulphuric acid & Hydrochloric
acid is a strong acid & will cause severe burns. Protective clothing should
be worn when working with acid.
