A.O.C.S Official Method Cd 1-25
Revised 1988
Reapproved 1989

    Iodine Value of Fats and Oils

    Definition:
    The iodine value is a measure of the unsaturation of fats and oils and is expressed in terms of the number of centrigrams of iodine absorbed per gram of sample (% iodine absorbed).

    Scope:
    Applicable to all normal fats and oils which do not contain conjugated double bonds (see Notes, 1).

    Apparatus:
  1. Glass stoppered iodine flasks, 500 mL.
  2. Glass stoppered volumetric flask, conforming to National Bureau of Standards (NBS) tolerances and accurately calibrated to contain 1,000 mL.
  3. Pipet, 20 mL.
  4. Two pipets, 25 mL.
    Note - One 25 mL pipet is reserved for use with the standard potassium dichromate solution This pipet must conform to NBS tolerances anci be accurately calibrated to deliver 25 mL.
  5. Bottles, borosilicate, actinic glass, with glass stoppers, 1,000 mL.
  6. Filter paper, Whatman No. 41H, or equivalent.
    7.
    Reagents:
  1. Glacial acetic, ACS grade. The permanganate test should be applied to be sure that this specification in met.
    (a) Permanganate test - Dilute 2 mL of the acid with 10 mL of distilled water and add 0.1 mL of 0.1 .N KMnO4. The pink color must not be entirely discharged within 2 hours.
  2. Potassium iodide (KI), reagent grade.
  3. Chlorine, 99.8% (see Notes, Caution). Satisfac- tory commercial grades are available in cylinders, but the gas must be dried by passing through sulfuric acid (sp. gr. 1.84) before introducing into the iodine solution. Note - Prepared Wijs (see Notes, Caution and 2) solution is available commercially, and may be used in place of the laboratory prepared solution.
  4. Soluble starch, tested for sensitivity. Make a paste with 1 g of starch and a small amount of cold distilled water. Add, while stirring, 200 mL of boiling water. Place 5 mL of this solution in 100 mL of water and add 0.05 mL of 0.1 N iodine solution. The deep blue color produced must be discharged by 0.05 mL of 0.1 N sodium thiosulfate.
  5. Potassium dichromate, reagent grade. The potassium dichromate is finely ground and dried to constant weight at about 110 C before using.
    Note - A standard sample of potassium dichromate with a certificate of analysis may be obtamed from the National Bureau of Standards in Washington, D.C. This sample, or equivalent, is strongly recommended for the primary standard for this method. Treat as directed in directed in the certificate of analysis accompanying the sample.
  6. Sodium thiosulfate (Na2S2O3.5H20), reagent grade.
  7. Iodine, reagent grade.
  8. Carbon tetrachloride, reagent grade (see Notes, Caution and Recommendations).
  9. Hydrochloric acid (HCl), reagent grade, concentrated, sp. gr. 1.19 (see Notes, Caution).
  10. Sulfuric acid (H25O4), reagent grade, concentrated, sp. gr. 1.84 (see Notes, Caution).l
  11. Iodine monochloride, reagent grade.
    12.
    Solutions:
  1. Potassium iodide (KI) solution, prepared by dis- solving 150 g KI in distilled water and making up to 1 liter.
  2. Starch indicator solution, prepared and tested as noted in Reagents, 4. Salicylic acid (1.25 g per liter) may be added to preserve the indicator. If long storage is required, the solution must be kept in a refrigerator at 4 to 10 C (40 to 50 F). Fresh indicator must be prepared when the endpoint of the titration from blue to colorless fails to be sharp. If stored under refrigeration, the starch solution should be stable for about two to three weeks.
  3. Sodium thiosul fate (Na2S2O3.5H20) solution, 0.1 N, accurately standardized vs. potassium dichromate primary standard as follows -
    (a) Sodium thiosulfate solution 0.1 N, prepared by dissolving 24.9 g of sodium thiosulfate in distilled water and diluting to 1 liter.
    (b) The potassium dichromate primary standard should be finely ground, dried at 105 C for 2 hours and cooled in a desiccator. Accurately weigh 0.16 to 0.22 g +- 0.0001 g of potassium dichromate into a 500 mL flask or bottle by difference from a weighing bottle. Dissolve in 25 mL of watei; add 5 mL of concentrated hydro- chloric acid, 20 mL of potassium iodine solution and rotate to mix. Allow to stand for 5 minutes and then add 100 mL of distilled water. Titrate with sodium thiosulfate solution, shaking con- tinuously until yellow color has almost disap- peared. Add 1 to 2 mL of starch indicator and continue the titration, adding the thiosulfate solution slowly until the blue color just disappears. The strength of the sodium thiosulfate solution is expressed in terms of its normality.

    4. Normality of Na2S2O3 solution = [ 20.394 x Wt. of K2Cr2O7 ] / [mL of sodium thiosulfate]

  4. Wijs solution (see Notes, Caution and 2), labora- tory preparation -
    Note - Prepared Wijs solution is available commercially, and may be used in place of the laboratory prepared solution. Dissolve 13.0 g of iodine in 1 liter of glacial acetic acid. Gently heat may be necessary to promote solution. Cool remove a small quantity (100-200 mL) and set aside in a cool place for future use. Pass dry chlorine gas into the iodine solution until the original titration is not quite double. A characteristic color change takes place in the Wijs solution when the desired amount of chlorine has been added. This may be used to assist in judging the endpoint. A convenient procedure is to add a small excess of chlorine and immediately bring back to the desired titration by addition of some of the original iodine solution which was taken out at the beginning. The orig- inal solutioh and finished Wijs solution are both titrated with Na2S2O3 solution as directed in Procedure, 6 and
  5. The Wijs solution may be prepared from commercial iodine monochloride as follows -
    (a) Stock Solution - Add 317 +- 0.1 g of iodine monochloride to 1 liter of glacial acetic acid and filter through Whatman No. 41H filter paper or equivalent into a clean and dry actinic glass bottle. Filter rapidly to prevent contamination with moisture and store in a cool place. Discard the solution ifa precipitate forms on standing.
    (b) Wijs Solution - Using a.graduate pour 117.0 +- 0.1 mL of the stock solution into a standard 5 pound bottle of glacial acetic acid, and mix well by shaking.
  6. The I/Cl ratio of the Wijs solution should be within the limits of 1.10 +- 0.1. The procedure for determining the ratio is as follows - Iodine Content -
    (a) Pour 150 mL of saturated chlorine water into a 500 mL Erlenmeyer flask and add a few glass beads.
    (b) Pipette 5 mL of the Wijs solution into the flask containing the saturated chlorine water. Shake, and heat to boiling.
    (c) Boil briskly for 10 minutes, cool, and add 30 mL of 2% sulfuric acid and 15 mL of 15% potassium iodide solution.
    (d) Mix well, and titrate immediately with 0.1 N sodium thiosulfate solution to a starch endpoint.
    7.

    Total Halogen Content -
    (a) Pour 150 mL of recently boiled distilled water into a clean, dry 500 mL Erlenmeyer flask.
    (b) Add 15 mL of 15% potassium iodine solution.
    (c) Pipette 20 mL of Wijs solution into the flask, and mix well.
    (d) Titrate with 0.1 N sodium thiosulfate solution to a starch endpoint. Calculation of Halogen

    Ratio -
    Halogen Retio R = [2A] / [3B - 2A]

    A = Titration of iodine content as mL sodium thiosulfate.
    B = Titration of total halogen content as mL sodium thiosulfate.

    Procedure:
  1. Melt the sample, if it is not already liquid (the temperature during melting and filtering should not exceed the melting point of the sample by more than 10 C), and filter through filter paper to remove any solid impurities and the last traces of moisture. The sample must be absolutely dry.
    Note - All glassware must be absolutely clean and completely dry.
  2. Weigh the sample (see Notes, 3) accurately into a 500 mL flask or bottle to which has been added 20 mL of carbon tetrachloride (see Notes, Caution) or other solvent (see Notes, Recommendations). The weight of sample must be such that there will be an excess of Wijs solution of 100 to 150% over the amount absorbed. Table 1 is a guide to the size of sample to weight.
  3. Pipette the 25 mL of Wijs solution into flask containing the sample, stopper the flask, and swirl to insure an intimate mixture.
  4. Prepare and conduct at least two blank determinations with each group of samples simultaneously and similar in all respects to the samples.
  5. Store the flasks in the dark for 30 minutes at a temperature of 25 +- 5 C (see Notes, 4)
  6. Remove the flasks from storage and add 20 mL of Kl solution, followed by 100 mL of distilled water.

    7. Table 1. Sample weights.

    Sample Weight
    Weighing accuracy,
    Iodine Value
    150% excess
    100% excess
    g
    g
    g, +-
    Lessthan3
    10
    10
    0.001
    3
    8.4613
    10.5760
    0.005
    5
    5.0770
    6.3460
    0.0005
    10
    2.5384
    3.1730
    0.0002
    20
    0.8461
    1.5865
    0.0002
    40
    0.6346
    0.7935
    0.0002
    60
    0.4321
    0.5288
    0.0002
    80
    0.3966
    0.3173
    0.0001
    100
    0.3173
    0.2538
    0.0001
    120
    0.2644
    0.2115
    0.0001
    140
    0.2266
    0.1813
    0.0001
    160
    0.1983
    0.1587
    0.0001
    180
    0.1762
    0.1410
    0.0001
    200
    0.1586
    0.1269
    0.0001

  7. Titrate with 0.1 N Na2S2O3 solution, adding it gradually and with constant and vigorous shaking (see Notes, 5). Continue the titration until the yellow color has almost disappeared. Add 1 to 2 mL of starch indicator solution and continue the titration until the blue color has just disappeared.

    Calculations:
    The iodine value = [(B - S) )x N x 12.69] / [Weight of sample]

    Where -
    B = Titration of blank
    S = Titration of sample
    N = Normality of Na2S2O3 solution

    Notes:

    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 athigh temperature. The TLV is 5 ppm in air. A fume hood should be used at all times when using carbon tetrachloride.
    Chlorine a poisonous gas. The TLV in is 1 ppm in air. It is a strong oxidizing agent and should not be allowed to come in contact with organic materials, hydrogen, powered metals and reducing agents. A fume hood should be used at all times when using chlorine.
    Wijs solution causes severe burns and vapors can cause lung and eye damage. Use of a fume hood is recommended. Wijs solution without carbon tetrachioride is available commercially.
    Sulfuric acid is a strong acid and will cause severe burns. Protective clothing should be worn when working with this acid. It is an oxidizing agent and should not be stored in the vicinity of organic mat- erials. Use great caution in mixing with water due to heat evolution that can cause explosive spattering. Always add the acid to water, never the reverse. Hydrochloric acid is a strong acid and will cause severe burns. Protective clothing should be worn when working with this acid. It is toxic by ingestion and inhalation and a strong irritant to eyes and skin. The use of a properly operating fume hood is recommended. When diluting the acid, always add the acid to the water, never the reverse.

    Recommendations
    At the time of the revision of this method in 1989, studies were under way to find a replacement for carbon tetrachloride in this method. The most satis- factory replacement found to date has been cyclohexane (see AOCS Recommended Practice Cd lb- 87 and JAQCS 65:745 (1988)), although erratic results may be obtained for oils with iodine values 100 - 120, and especially marine oils. Because of environmental concerns, 1,1,2-trichloro-1,2,2- trifluoroethane (Freon 113), is not recommended. Acetic acid alone and a mixture of acetic + cyclohexane, 1:4, respectively, have been shown to be unsatisfactory.

    Numbered Notes
  1. When the iodine value is determined on materials having conjugated Systems, the result is not a measure of total unsaturation, but rather is an empirical value indicative of the amount of unsaturation present. Reproducible results are obtained which afford a comparison of total unsaturation. When the iodine value is required on fatty acids, the preparation and separation are performed as directed in AOCS Official Method Cd 6-38.
  2. All WUs solutions are sensitive to temperature, moisture, and light. Store in a cool and dark place and never allow to come to a temperature above 25-30 C.
  3. In the case of tung oil, weigh 0.1 to 0.2 g of sample, using an excess of 55 +- 3% Wijs solution, and allow the absorption to proceed for 1 hour at 20 to 25 C. In the case of dehydrated castor oils and dehydrated castor oil fatty acids, weigh 0.11 to 0.13 g.
  4. Store samples with an iodine value greater than 150 for 1 hour in the dark at 25 C +- SC.
  5. Mechanical stirring is very satisfactory for agitating during the addition of thiosulfate.
    6.
© 1999 IMPERIAL INDUSTRIAL CHEMECALS (THAILAND) Co., Ltd. All rights reserved.
Disclaimer Inquiries or Comments, contact us