Green Soap

This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition

Issued and maintained by the United States Pharmacopeial Convention (USP)

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1 DEFINITION

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.

Prepare Green Soap as follows.

Mix the Vegetable 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 hot to the hot oil mixture. Stir the mixture vigorously until emulsified, then heat with continuous 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 bring the preparation to nal weight. Continue stirring until the Soap is homogeneous.

2 SPECIFIC TESTS

2.1 Water Determination, Method II〈921〉

Sample: 5 g, quickly weighed, in the ask of the toluene moisture apparatus

Analysis: Place 250 mL of toluene and 10 g of anhydrous barium chloride in the ask. Proceed as directed in the chapter. Acceptance criteria: The volume of water found corresponds to NMT 52.0% by weight of the Soap taken.

2.2 Alcohol-Insoluble Substances

Sample: 5 g, rapidly and accurately weighed

Analysis: Dissolve the Sample 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 h. Retain the solution for the test for Free Alkali Hydroxides, and retain the residue for the test for Alkali Carbonates.

Acceptance criteria: The weight of the residue so obtained is NMT 3.0% of the weight of the Soap taken.

2.3 Free Alkali Hydroxides

Sample: Combined filtrate and washings obtained in the test for Alcohol-Insoluble Substances

Analysis: To the Sample 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 potassium hydroxide.

Acceptance criteria: The volume of 0.1 N sulfuric acid VS consumed corresponds to NMT 0.25% of potassium hydroxide. • Alkali Carbonates

Sample: Residue obtained in the test for Alcohol-Insoluble Substances

Analysis: Wash the filter containing the Sample with 50 mL of boiling water, cool, add methyl orange TS, and titrate the ltrate with 0.1 N sulfuric acid VS.

Acceptance criteria: NMT 0.5 mL of 0.10 N sulfuric acid per g of Soap taken is required (0.35% as potassium carbonate). • Unsaponified Matter

Sample: A solution of Soap in hot water (1 in 20)

Acceptance criteria: Solution is nearly clear.

2.4 Fats and Fixed Oils, Acid Value〈401〉

Sample: 30 g

Analysis: Dissolve the Sample 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 pass the acids through a dry filter in a warm oven. Determine the acid value of about 1 g, accurately weighed, of the fatty acids.

Acceptance criteria: NMT 205

2.5 Fats and Fixed Oils, Iodine Value〈401〉

Sample: 30 g

Analysis: Dissolve the Sample 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 pass the acids through a dry filter in a warm oven. Determine the iodine value of 150–200 mg, accurately weighed, of the fatty acids.

Acceptance criteria: NLT 85

3 ADDITIONAL REQUIREMENTS

Packaging and Storage: Package in well-closed containers.