The number of milligrams of potassium hydroxide required to saponify completely one gram of oil or fat. When fat is saponified by refluxing with a known excess of alcoholic potassium hydroxide solution, the triglycerides hydrolyze, glycerol and soap are formed.
The alkali consumed for this hydrolysis is a measure of the saponification value, which is determined by titrating the excess alkali with standard hydrochloric acid.
- Conical Flasks -250 to 300 ml capacity.
- Reflux Condenser-at least 65 cm long.
- Water-Bath or Electric Hot-Plate with Rheostat Control
- Alcoholic Potassium Hydroxide Solution – Dissolve 35 to 40 g of KOH in 20 ml of distilled water, and add sufficient aldehyde-free rectified spirit to make up to 1000 ml.
- Allow to stand overnight, decant the clear liquid and keep in a bottle closed tight with a cork or rubber stopper.
- Aldehyde-Free Rectified Spirit -Reflux 1.2 litres of rectified spirit for 30 min. in a round-bottom flask with 10 g of KOH and 6 g of granulated aluminium (or aluminium foil).
- Distil and collect one litre after discarding the first 50 ml.
- Phenolphthalein Indicator Solution – Dissolve 1.0 g of phenolphthalein in 100 ml of rectified spirit.
- Note – When testing oils or fats which give dark-coloured soap solutions, the observation of the end point of the titration may be facilitated either (a) by using thymolphthalein, or alkali blue 6B in place of phenolphthalein or (b) by adding 1 ml of a 0.1 % (w/v) solution of methylene blue in water to each 100 ml of phenolphthalein indicator solution before the titration.
- Standard Hydrochloric Acid (0.5 N).
- Melt the sample, if it is not already liquid, and filter through a filter paper to remove any impurities and the last traces of moisture.
- Make sure that the sample is completely dry.
- Mix the sample thoroughly, and weigh accurately by difference about 1.5 to 2.0 g of the sample in a conical flask.
- Add 25 ml of the alcoholic KOH solution and connect the reflux air condenser to the flask.
- Heat the flask on a water-bath or an electric hot plate for not more than 1 hr.
- Boil gently but steadily until the sample is completely saponified as indicated by absence of any oily matter and appearance of clear solution.
- After the flask and condenser have cooled somewhat, wash down the inside of the condenser with about 10 ml of hot ethyl alcohol neutral to phenolphthalein.
- Add about 1 ml of phenolphthalein indicator solution, and titrate with standard hydrochloric acid.
- Prepare and conduct a blank determination at the same time.
B = volume, in ml, of HCl required for the blank,
S = volume, in ml, of HCl required for the sample,
N = normality of HCl, and
W = weight, in g, of the material taken for the test.
RESULTS AND INFERENCE
The mean of the results of two determinations should be reported.
The difference between the results of two determinations carried out simultaneously or in rapid succession by the same analyst (repeatability) shall not exceed 0.5. The saponification value of commonly used edible oils is given as below.
• Alcoholic KOH should be prepared overnight.
• Refluxing should not be done for more than one hour.
• Condenser should be carefully washed with alcohol.
• Blank determination should be carried out simultaneously along with the sample.