A.O.C.S Official Method Cd 3a-63
Revised 1987
Reapproved 1989

    Acid Value

    Definition:
    The acid value is the number of milligrams of potassium hydroxide necessary to neutralize the free acids in 1 gram of sample. With samples that contain virtually no free acids other than fatty acids, the acid value may be directly converted by means of a suitable factor to percent free fatty acids.

    Scope:
    Applicable to crude and refined animal, vegetable, and marine fats and oils, and various products derived from them.

    Apparatus:
  1. Erlenmeyer flasks, 250 or 300 mL.
    2.
    Reagents:
  1. Potassium hydroxide (KOH), 0.1 N, accurately standardized and carbonate free. See AOCS Specification H 15-52 for guidance. Add 6 g reagent grade KOH to 1 liter of water in a 2 liter Erlenmeyer flask, boil 10 minutes with stirring, add 2 grams reagent grade barium hydroxide (Ba(OH)2), boil an additional 5 to 10 minutes, cool, stopper flask and let stand several hours. Pilter through sintered glass funnel and store in an alkali- resistant bottle protected from C02. Standardize by titration with primary standard grade potassium acid phthalate, using phenolphthalein indicator (see Notes, 1).
  2. Solvent mixture consisting of equal parts by volume of isopropyl alcohol (AOCS Specification H18-58) and toluene (AOCS Specification H 19-58). See Notes, Caution. The mixture must give a distinct and sharp end point with phenolphthalein in the titration as noted in Procedure, paragraph 5.
  3. Phenolphthalein indicator solution, 1.0% in isopropyl alcohol.
    Procedure:
  1. Add indicator solution to the required amount of solvent in ratio of 2 mL to 125 mL and neutralize with alkali to a faint but permanent pink color.
  2. Determine the sample size from Table 1 -
    3. Acid Value

    Wt. ofsample
    (+- 10%), grams

    		 Weighing accuracy,
    +- grams
    0 to 1 20 0.05
    1 to 4 10 0.02
    4 to 15 2.5 0.01
    15 to 75 0.5 0.001
    75 and over 0.1 0.0002

  3. Weigh the specified amount of well mixed liquid sample into an Erlenmeyer flask.
  4. Add 125 mL of the neutralized solvent mixture. Be sure that the sample is completely dissolved before titrating. Warming may be necessary in some cases.
  5. Shake the sample vigorously while titrating with standard alkali to the first permanent pink color of the same intensity as that of the neutralized solvent before the latter was added to the sample. The color must persist for 30 seconds.

    Calculations:
    The acid value, mg kOHig of sample = [(A - B) x N x 56.1]/W

    Where -
    A = mL of standard alkali used in the titration
    B = mL of standard alkali used in titrating the blank
    N = normality of standard alkali
    W = grams of sample

    To express in terms of free fatty acids as percent oleic, lauric, or palmitic, divide the acid value by 1.99, 2.81, or 2.19, respectively.

    Precision:
    Single determinations performed in two different laboratories should not differ by more than 0.22 for values less than 4 nor by more than 0.36 for values in the range 4 to 20.

    Alternate procedure for highly colored samples:

    Apparatus:

  1. Glass electrodecalomel electrode pH meter for electrometric titration. A sleeve type calomel electrode should be used (see Notes, 2).
  2. Variable speed mechanical stirrer with glass stirring paddle.
  3. Buret, 10 mL, graduated in 0.05 mL divisions with a tip drawn to a fine opening and extending at least 10 cm below the stopcock.
  4. Beakers, 250 mL.
  5. Stand and mountings for electrodes1 stirrer, and buret.
    6.
    Reagents:
    Same as for the phenolphthalein titrimetric procedure except that the standard alkali should be standardized by electrometric titration of pure potassium acid phthalate, and no indicator solution is necessary.
    Procedure:
  1. Determine the sample size from Table 1 and weigh the sample into a 250 mL beaker.
  2. Add 125 mL of solvent mixture.
  3. Mount the beaker in the titration assembly so that the electrodes are half immersed. Start the stirrer and operate at speeds that will give vigorous agitation without spattering. Immerse the tip of the buret to 1 cm below the surface of the sample.
  4. Titrate with suitable increments of alkali. After each addition of alkali, wait until the meter reading is essentially constant (usually within 2 minutes), then record buret and meter readings graphically. Limit increments of alkali so that changes in meter readings are 0.5 pH units (0.03 volts) or less; when inflections in titration curve occur, add alkali in 0.05 mL portions.
  5. Remove titrated solution, rinse electrodes with isopropyl alcohol, and immerse in distilled water. 6. Perform a blank titration, using 125 mL of solvent mixture.
    Calculation:
    The acid value, mg KOHIg of sample = [(A - B) x N x 56.1]/W

    Where -
    A = mL of standard alkali used in titrating to middle of inflection in titration curve of sample.
    B = mL of standard alkali used in titrating to same pH meter reading for the blank.
    N = normality of standard alkali.
    W = grams of sainple.

    To express in terms of free fatty acids as percent oleic, lauric, or palmitic, divide the acid value by 1.99, 2.81 or 2.19, respectively.

    Notes:
    Caution
    Isopropyl alcohol is flammable and a dangerouls fire risk. The explosive limits in air are 2% to 12%. It is toxic by ingestion and inhalation. The TLV in air is 400 ppm.
    Toluene is flammable and a dangerous fire risk. Explosive limits in air are 1.27 to 7%. It is toxic by ingestion, inhalation and skin absorption. The TLV is 100 ppm in air. A fume hood should be used at all times when using toluene.

    Numbered Notes
  1. A standard methanolic potassium hydroxide (0.1N) solution (see AOCS Specification H 15-52) may be used as an alternate titrant in place of the standard aqueous solution. The methanolic potassium hydroxide is reported to provide a complete solvent system, having a distinct, clear endpoint. See JAOAC 59: 658 (1976) regarding the ruggedness of the acid value method.
  2. The pH meter should be standardized to pH 4.0 with standard buffer solution. Immediately before using, wipe the electrodes thoroughly with clean cloth or tissue and soak for several minutes in distilled water. At weekly intervals, or more often if necessary, clean the glass electrode in a suitable cleaning solution. Also, drain calomel electrode and refill with fresh potassium chloride (KCl) electrolyte at weekly intervals. Both electrodes should be stored in distilled water when not in use.
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