Official Method Cd 3b-76
The saponification value is the amount of alkali necessary to saponify
a definite quantity of the sample. It is expressed as the number of
milligrams of potassium hydroxide (KOH) required to saponify 1 gram
of the sample.
to vegetable oil deodorizer distillates and sludges.
- Erlenmeyer flasks,
alkali resistent, 250 or 300 mL, with T 24/40 ground glass joint.
- Air condensers,
minimum 65 cm long, with T 24/40 ground glass joint to fit Erlenmeyer
- Water bath, or
a hot plate with variable heat control.
- Distillation flask,
2 liter, with T ground glass joint, fitted with water cooled condenser,
for refluxing and distilling 95% ethyl alcohol as noted in Reagents,
- Hydrochloric acid, 0.5 N, accurately standardized. See AOCS Specification
- Alcoholic potassium
hydroxide (KOH), prepared by placing 5 to 10 g of reagent grade KOH
(see Notes, Caution) pellets in a 2 liter flask and adding 1 liter to
1.5 liters of 95% ethyl alcohol (see Notes, Caution) and boil on a water
bath under reflux for 30 to 60 minutes. Distill and collect the alcohol
in a clean, glass stoppered receiver. Dissolve 40 g of potassium hydroxide,
low in carbonate, in 1 liter of the distilled alcohol (see Notes, Caution)
with cooling to keep the temperature at or below 15 C. This solution
should remain clear.
indicator solution, 1.0% in 95% ethyl alcohol.
- Toluene, reagent
grade (see Notes, Caution).
- Heat the sarmple
(see Notes, 1) at 100 C until liquid and then mix thoroughly.
- Accurate weigh
about 2g +-
0.1 mg of the melted sample into the Erlenmeyer flask.
- Add 25 mL of toluene
and heat gently to dissolve the sample.
- Add 50 mL of the
alcoholic KOH with a pipet and allow the pipet to drain for a definite
period of time (see Notes, 2).
- Prepare and conduct
blank determinations simultaneously with the sample and similar in all
respects, except omitting the fat or oil.
- Add several boiling
chips to the flask and connect the air condenser. Boil gently, but steadily,
until the sample is completely saponified. This usually requires about
1 hour for normal samples. Make certain that the vapor ring in the condenser
does not rise to the top of the condenser, or loss may occur.
- After the flask
and condenser have cooled somewhat, but not sufficiently to form a jell,
wash down the inside of the condenser with a small quantity of distilled
water. Disconnect the condenser, add about 1 mL of phenolphthalein indicator
and titrate with 0.5 N HCl until the pink color just disappears. Record
the volume of 0.5 N HCl required for the titration (see Notes, 3).
Saponification value = [(B - S) x (N)]/ W x
B = mL 0.5 N HCl required to titrate blank
S = mL 0.5 N HCl required to titrate sample
N = normality of HCl solution
= weight of sample in grams
- Two single determinations
performed in one laboratroy should not differ by more than 3.2 units.
- Two single determinations
performed in different laboratories should not differ by more than 6.4
Potassium hydroxide, like all alkalies, can burn skin, eyes, respiratory
tract severely. Wear heavy rubber gloves and face shield to protect against
concentrated alkali liquids. Use effective fume removal device or gas
mask to protect respiratory tract against alkali dusts or vapors. When
working with extremely caustic materials like potassium hydroxide, always
add pellets to water and not vice versa. Alkalies are extremely exothermic
when mixed with water. Take precautions to contain the caustic solution
in the event the mixing container breaks from the extreme heat generated.
Ethyl alcohol (ethanol) is flammable. Use a fume hood when heating or
evaporating this solvent. 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.
- When the saponification
value is required on the fatty acids, the preparation and separation
are performed as directed in AOCS Official Method Cd 6-38.
- For consistent
results, a 50 mL class A volumetric pipet, calibrated "To Deliver" should
be used. The pipet should be allowed to drain for the time specified.
- Some samples may
yield a, solution too dark in color to permit satisfactory endpoint
detection. To overcome this problem, use a smaller sample size, about