Determination of surfactant type

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Determination of surfactant type I. Anionic surfactants A. Methylene blue-chloroform method 1. Reagents i. Methylene blue solution: Dilute 0.03 grams of methylene blue, 12 grams of concentrated sulfuric acid, and 50 grams of anhydrous sodium sulfate in water to a volume of 1 liter. ii. 0.05% anionic surfactant solution. iii. Chloroform. 2. Procedure: Add 3 ml of the methylene blue solution and 5 ml of chloroform to a test tube with a stopper. Add one drop of the 0.05% anionic surfactant solution, seal the tube, shake it thoroughly until layers form. Add drops of the anionic surfactant solution until both layers reflect light of the same color; generally, 10–20 drops are required. Then add 2 ml of the 10.1% sample solution, shake again to allow layers to form, and observe the colors of the two layers after letting them settle. 3. Results and interpretation: If the chloroform layer is darker in color while the water layer is almost colorless, it indicates the presence of anionic surfactants. Since the reagents are acidic, if soap is present, its fatty acids have already been decomposed, so soap cannot be detected. If the color of the water layer is darker, it indicates the presence of cationic surfactants; since the reagent is acidic, amphoteric surfactants generally give (weakly) positive results. If the two layers still have more or less the same color, or if the lower layer turns milky white or a very light color, it indicates the presence of non-ionic surfactants. If in doubt, a control test can be conducted using 2% blood solution instead of the sample. Inorganic salts such as silicates and phosphates do not cause interference. Improved method: Add 10 ml of methylene blue solution and 5 ml of chloroform to 5 ml of a 1% sample solution. Shake the mixture for 2–3 minutes to allow it to separate into layers, then observe the colors of the two layers. If the chloroform layer turns blue, it indicates the presence of anionic surfactants. Adding more of the sample solution will make the blue color of the chloroform layer darker.
Thymol blue test: 1. Reagents: Add 3 drops of thymol blue solution to 0.005N hydrochloric acid solution. 2. Add 5 ml of this reagent to 5 ml of a neutral 0.01–0.1% sample solution; a purplish-red color indicates the presence of anionic surfactants.
Cationic surfactants – Bromophenol blue method: 1. Reagents: Mix 75 ml of 0.2N sodium acetate solution, 925 ml of 0.2N acetic acid, and 20 ml of 0.1% bromophenol blue ethanol solution. The pH of this solution must be adjusted to 3.6–3.9. 2 Operating steps: Adjust the pH of the 1% sample solution to 7. Add 2–5 drops of the sample solution to 10 milliliters of reagent; if a dark blue color appears, it indicates the presence of cationic surfactants. All cationic surfactants yield such a result. Long-chain amino acids and betaines produce a slight blue or purple fluorescence. The presence of non-ionic surfactants does not interfere when they are present together with cationic surfactants.

**Non-ionic surfactants – Method A: Ammonium cobaltocyanate method**
This test is suitable for qualitatively detecting polyoxyethylene non-ionic surfactants in mixed samples.
1. **Reagent**: Ammonium cobaltocyanate reagent – Dissolve 174 grams of ammonium cyanate and 28 grams of cobalt nitrate in 1 liter of water.
2. **Procedure**: Add 5 milliliters of the ammonium cobaltocyanate reagent to 5 milliliters of a 1% sample solution. Wait for 2 hours and then observe the color of the solution. If a blue color appears, it indicates the presence of non-ionic surfactants. A red-purple to purple color indicates a negative result. If a blue precipitate forms along with a red-purple solution, it indicates the presence of cationic surfactants.

**Method B: Oxalic acid test**
1. **Reagents**:
– Hydroxylamine hydrochloride reagent: Dissolve 7 grams of hydroxylamine hydrochloride in 15 milliliters of water, then add 78 grams of 2-methylpentanediol-2,4.
– Hydrochloric acid solution: Mix 44 milliliters of 2-methylpentanediol-2,4 solution with 4 milliliters of 12N hydrochloric acid. Alcoholic potassium hydroxide solution: Dissolve 3.3 grams of 85% potassium hydroxide in 2 milliliters of water, then add 45 grams of 2-methylpentanediol-2,4 solution and a 10% ferrous chloride solution. Procedure: Add 1 milliliter of hydroxylamine hydrochloride reagent to 0.1 gram of anhydrous sample; heat to dissolve or disperse it. After cooling, add the alcoholic KOH solution or alcoholic hydrochloric acid solution until the Congo red test strip turns acidic. Boil gently for 3 minutes, then cool the mixture. Add 2 drops of ferrous chloride solution; a purple or dark color indicates the presence of fatty acid alkylamide non-ionic surfactants. Note: Fatty acid alkylamide sulfates exhibit the same reaction