Official Method Cd 3a-63
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
Applicable to crude and refined animal, vegetable, and marine fats and
oils, and various products derived from them.
- Erlenmeyer flasks,
250 or 300 mL.
- 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,
- 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.
indicator solution, 1.0% in isopropyl alcohol.
- 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.
- Determine the
sample size from Table 1 -
(+- 10%), grams
|0 to 1
|1 to 4
|4 to 15
|15 to 75
|75 and over
Weigh the specified amount of well mixed liquid sample into an Erlenmeyer
125 mL of the neutralized solvent mixture. Be sure that the sample
is completely dissolved before titrating. Warming may be necessary
in some cases.
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.
The acid value, mg kOHig of sample = [(A -
B) x N x 56.1]/W
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
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.
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.
procedure for highly colored samples:
- Glass electrodecalomel electrode pH meter for electrometric titration.
A sleeve type calomel electrode should be used (see Notes, 2).
speed mechanical stirrer with glass stirring paddle.
- 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.
- Beakers, 250 mL.
- Stand and mountings
for electrodes1 stirrer, and buret.
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.
- Determine the sample
size from Table 1 and weigh the sample into a 250 mL beaker.
- Add 125 mL of solvent
- 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.
- 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.
- Remove titrated
solution, rinse electrodes with isopropyl alcohol, and immerse in distilled
water. 6. Perform a blank titration, using 125 mL of solvent mixture.
The acid value, mg KOHIg of sample = [(A - B)
x N x 56.1]/W
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.
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
- 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.
- 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.