SOP: Saponification Value of Oils and Fats

GIFSQ — Global Institute for Food Safety & Quality

Principle

When a fat is boiled with an excess of alcoholic KOH, the triglycerides hydrolyse and glycerol and soap are formed. The alkali consumed in this hydrolysis, expressed as the number of milligrams of KOH required to saponify completely one gram of the oil or fat, is the saponification value (S.V.). It is also a measure of the mean molecular weight of the fatty acids originally bound as triglycerides. The saponification equivalent of the fat is 56,000/S.V., which is one-third of the mean molecular weight of the fat.

Apparatus required

• 250 ml conical flask with a ground glass joint
• Glass beads or pieces of pumice
• Air condenser at least 65 cm long (preferably 202 cm)

Reagents

a. 0.5 N alcoholic KOH: dissolve 30 g of KOH in 20 ml of distilled water and add sufficient aldehyde-free alcohol to make to 1000 ml. Allow to stand overnight, decant the clear liquid and store in a bottle with a rubber stopper.

b. 0.5 N HCl (standard).

c. Phenolphthalein indicator. For samples giving a dark coloured soap solution, use thymolphthalein or alkali blue 6B in place of phenolphthalein, or an indicator containing 1 ml of a 0.1% (w/v) solution of methylene blue in water made to 100 ml with phenolphthalein (mixed before titration).

d. Hot ethyl alcohol, neutral to phenolphthalein.

Procedure

Melt the sample, if it is not already a liquid, and filter; ensure that the sample is free of impurities and moisture

↓

Weigh 1 to 2 g of the sample into a 250 ml conical flask having a ground glass joint; pipette 25 ml of alcoholic KOH solution and add a few glass beads or pieces of pumice

↓

Connect the flask to an air condenser at least 65 cm (or preferably 202 cm) long; reflux the sample on a water bath or a hot plate for not more than 1 hr, boiling gently but steadily until the sample is completely saponified, as shown by the absence of any oily matter and the appearance of a clear solution

↓

Allow the flask and condenser to cool; wash the inside of the condenser with about 10 ml of hot ethyl alcohol neutral to phenolphthalein

↓

Add 1 ml of phenolphthalein indicator and titrate with standard HCl; conduct a blank determination alongside

Calculation

                 (Blank titre − Sample titre) × N of KOH × 56.01
S.V. = ─────────────────────────────────────────────────────────────
                               Wt of sample (g)

Note: the saponification equivalent of the fat is 56,000/S.V. (one-third of the mean molecular weight of the fat). The source’s printed expression for the mean molecular weight (M) of the fatty acids is garbled in the available text and is not reproduced here.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.