Green Soap
» Green Soap is a potassium soap made by the saponification of suitable vegetable oils, excluding coconut oil and palm kernel oil, without the removal of glycerin.
Green Soap may be prepared as follows:
The Vegetable Oil 380 g
Oleic Acid 20 g
Potassium Hydroxide (total alkali 85 percent) 91.7 g
Glycerin 50 mL
Purified Water, a sufficient quantity to make about 1000 g
Mix the oil and the Oleic Acid, and heat the mixture to about 80. Dissolve the Potassium Hydroxide in a mixture of the Glycerin and 100 mL of Purified Water, and add the solution, while it is still hot, to the hot oil. Stir the mixture vigorously until emulsified, then heat while continuing the stirring, until the mixture is homogeneous and a test portion will dissolve to give a clear solution in hot water. Add hot purified water to make the product weigh 1000 g, continuing the stirring until the Soap is homogeneous.
Packaging and storage— Preserve in well-closed containers.
Water— Place about 5 g of Soap, quickly weighed to the nearest centigram, in the distilling flask of the apparatus for Moisture Method by Toluene Distillation 921. (The Soap is most conveniently weighed in a boat of metal foil, of a size that will just pass through the neck of the flask.) Place 250 mL of toluene and 10 g of anhydrous barium chloride in the flask, connect the flask through a ground-glass joint to the distilling apparatus, fill the receiving tube with toluene, and determine the water as directed, beginning with “Heat the flask gently.” The volume of water found corresponds to not more than 52.0% by weight of the Soap taken.
Alcohol-insoluble substances— Dissolve about 5 g of Soap, rapidly and accurately weighed, in 100 mL of hot neutralized alcohol, collect the residue, if any, on a tared filter, thoroughly wash it with hot neutralized alcohol, and dry at 105 for 1 hour: the weight of the residue so obtained is not more than 3.0% of the weight of the Soap taken. Retain the solution for the test for Free alkali hydroxides, and retain the residue for the test for Alkali carbonates.
Free alkali hydroxides— To the combined filtrate and washings obtained in the test for Alcohol-insoluble substances add 0.5 mL of phenolphthalein TS. If a pink color is produced, titrate the solution with 0.1 N sulfuric acid VS until the pink color is just discharged. Each mL of 0.1 N sulfuric acid is equivalent to 5.611 mg of KOH. The volume of 0.1 N sulfuric acid VS consumed corresponds to not more than 0.25% of KOH.
Alkali carbonates— Wash the filter containing the Alcohol-insoluble substances with 50 mL of boiling water, cool, add methyl orange TS, and titrate the filtrate with 0.1 N sulfuric acid VS. Not more than 0.5 mL of 0.10 N sulfuric acid per g of Soap originally taken is required (0.35% as K2CO3).
Unsaponified matter— A solution of Soap in hot water (1 in 20) is nearly clear.
Characteristics of the liberated fatty acids— Dissolve about 30 g of Soap in 300 mL of hot water in a beaker, add gradually 60 mL of 2 N sulfuric acid, and heat on a steam bath until the liberated acids form a transparent layer. Decant the fatty acids into a separator, and wash them with 50-mL portions of hot water until the last washing, when cool, is neutral to methyl orange TS. Transfer the fatty acids to a dry beaker, and allow them to stand in a warm oven until any water that may be present has separated. Then filter the acids through a dry filter in the warm oven. Determine the Acid Value (see Fats and Fixed Oils 401) of about 1 g, accurately weighed, of the fatty acids: it is not more than 205. Determine the Iodine Value (see Fats and Fixed oils 401) of 150 to 200 mg, accurately weighed, of the fatty acids: it is not less than 85.
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Topic/Question Contact Expert Committee
Monograph Jeanne H. Sun
Assistant Scientific Liaison
(SM12010) Monographs - Small Molecules 1
USP35–NF30 Page 3377