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Determination of unsaponifiable matter in oils and fats

INTRODUCTION

  • The unsaponifiable matter is that fraction of oil and fat, which is not saponified with caustic alkali but is soluble in non-polar solvents.
  • The unsaponifiable matter in oil or fat consists of hydrocarbons, higher alcohols, oil-soluble vitamins, and sterols, which are not soluble in water after esterification.
  • Most oils and fats of normal purity contain less than 2% unsaponifiable matter.
  • A higher value indicates the possibility of adulteration with mineral oil.
  • Adulteration of oils and fats with paraffin hydrocarbons will appear in the unsaponifiable matter

PRINCIPLE

  • The material is completely saponified with alcoholic potassium hydroxide solution and extracted with petroleum ether.
  • The petroleum ether extract is washed with aqueous alcohol and then again with water.
  • The washed ether extract is evaporated and the residue is weighed.
  • The unsaponifiable matter is this residue minus the fatty acid present in it, which is determined by titration with sodium hydroxide solution in the alcoholic medium.

REQUIREMENTS

Apparatus
Flat-Bottomed or Conical Flask – 250 to 300 ml capacity. An ordinary round, flat-bottomed flask, fitted with a long glass tube that acts as a condenser, may also be used.
Separating Funnels -500 ml capacity.

Reagents

  • Alcoholic Potassium Hydroxide Solution – Dissolve 70 to 80 g of potassium hydroxide in an equal quantity of distilled water, and add sufficient aldehyde-free ethyl alcohol (95 %v/v by volume), to make up to 1 liter.
  • Allow to stand overnight, decant the clear liquid and keep in a bottle closed tightly with a cork or rubber stopper.
  • Ethyl Alcohol – 95% v/v
  • Phenolphthalein Indicator Solution – Dissolve 1 g of phenolphthalein in 100 ml of ethyl alcohol.
  • Petroleum Ether – 60-80°C.
  • Aqueous Alcohol – containing 10 % v/v of ethyl alcohol.
  • Standard Sodium Hydroxide Solution – approximately 0.02 N.
  • Acetone – free from evaporation residue.

PROCEDURE

  • Weigh accurately about 5 g of the well-mixed sample into the flask.
  • Add 50 ml of alcoholic potassium hydroxide solution.
  • Boil gently but steadily under a reflux condenser for one hour or until the saponification is complete.
  • Wash the condenser with about 10 ml of ethyl alcohol.
  • Cool the mixture and transfer it to a separating funnel.
  • Complete the transfer by washing the flask first with some ethyl alcohol and then with cold water.
  • Altogether, add 50 ml of water to the separating funnel followed by the addition of 50 ml of petroleum ether.
  • Insert the stopper and shake vigorously for at least one minute and allow to settle until both the layers are clear.
  • Transfer the lower layer containing the soap solution to another separating funnel, and repeat the ether extraction at least six times more using 50 ml of petroleum ether for each extraction.
  • If any emulsion is formed, add a small quantity of ethyl alcohol or alcoholic potassium hydroxide solution.
  • Collect all the ether extracts in a separating funnel.
  • Wash the combined extracts in the funnel three times with 25 ml portions of aqueous alcohol shaking vigorously and drawing off the alcohol-water layer after each washing.
  • Again wash the ether layer successively with 20 ml portions of water until the wash water no longer turns pink on the addition of a few drops of phenolphthalein indicator solution.
  • Do not remove any of the ether layers.
  • Transfer the ether layer to a tared flask containing a few pieces of pumice stone, and evaporate to dryness in a water bath under a gentle stream of clean dry air.
  • To remove the last traces of ether, place the flask in an air oven at 80 to 90°C for about 1 hr.
  • To remove the last traces of moisture, add a few ml of acetone and pass a gentle stream of clean dry air over the surface of the material or evacuate using a water vacuum pump at about 50°C for about 15 min.
  • Cool in a desiccator and weigh. Repeat the evacuating, cooling, and weighing until a constant weight is obtained.
  • After weighing, take up the residue in 50 ml of warm neutral ethyl alcohol, containing a few drops of phenolphthalein indicator solution, and titrate with standard sodium hydroxide solution.

CALCULATION

Where,
A = weight, in g, of the residue,
B = weight, in g, of the fatty acids in the extract (B = 0.282×V×N),
V = volume, in ml, of NaOH solution,
N = normality of NaOH solution, and
W = weight, in g, of the material taken for the test.

RESULTS AND INFERENCE

  • The difference between the results of two determinations carried out simultaneously or in rapid succession by the same analyst (repeatability) shall not exceed 0.1% by mass.
  • Higher content of unsaponifiable matter may indicate the presence of mineral oils.
  • The unsaponifiable matter content in commonly used edible oils is given below.

PRECAUTIONS

  • The unsaponified portion must be washed free of alkali and this should be ascertained with phenolphthalein.
  •  As a check residue is dissolved in 10 ml of accurately neutralized alcohol and titrated with 0.1N NaOH solution using phenolphthalein as an indicator. If more than 0.1 ml of alkali is consumed it demands repetition of the experiment.
  •  Sufficient time should be given for the complete separation of ether and aqueous phases.
  • Formation of the emulsion should be prevented by adding small quantities of alcohol or conc. KOH and NaCl solution.
  • The formation of emulsions that are difficult to break leads to a loss of quantitative accuracy.

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