The ionic identification of surfactants only introduces some test methods.
Methylene blue-chloroform test.
1) Methylene blue reagent. Weigh 0.03g of methylene blue, dissolve it with water first, add 12g of concentrated H2SO4 and 450g of anhydrous Na2SO, and dilute it with distilled water to 1L.
2) Chloroform (chemically pure).
3) 0.05% anionic surfactant solution. Prepare 0.05% octyl sulfosuccinate sodium salt (penetrating agent OT).
4) Test methods.
Add 8 mL of methylene blue solution and 5 mL of chloroform into a 25 mL test tube with a stopper, and add 0.05% anionic surfactant drop by drop. After each drop is added, cover the stopper and shake vigorously to separate the layers;
Continue adding drops until the upper and lower layers have the same depth of color (usually 10 to 12 drops are needed). Then, add 2 mL of 0.1% sample solution, shake again, let it stand to separate the layers, and observe the color depth of the upper and lower layers.
5) Result judgment.
The color of the chloroform layer becomes darker, while the water layer is almost colorless, indicating that the sample is an anionic surfactant.
If the color of the water layer becomes darker, the sample is a cationic surfactant. If the color of the two layers is roughly the same and the water layer is emulsion-like, it indicates the presence of nonionic surfactants in the sample.
6) A few notes.
Inorganic substances (including silicates and phosphates) have no interference with this test. If there is any doubt (that is, the color is not easy to decompose), 2 mL of distilled water can be used instead of the sample for a control test.
If there is no penetrating agent OT, other anionic surfactants can be used instead. Since the reagent is acidic, pure carboxylate anionic surfactants are difficult to identify. The dosage and concentration of the sample solution should not be too large, otherwise it will easily affect the identification of nonionic surfactants.
Mixed indicator color reaction test.
1) Preparation of mixed indicator solution.
A: 0.5g Dimidil bromide is dissolved in 30mL 10% hot ethanol solution.
B: 0.25g blue disulfide VN is dissolved in 30mL 10% hot ethanol solution.
When the two solutions A and B are quantitatively transferred to a 250mL volumetric flask, add 10% ethanol solution to dilute to the mark.
Take 20 mL of the above solution, place it in a 500 mL volumetric flask, add 200 mL of water and 20 mL of 2.5 mol/L sulfuric acid, and dilute to the mark with water.
2) Test method.
Dissolve a small amount of surfactant sample in a few milliliters of water (or active substances extracted with ethanol) and place it in a test tube with a stopper. Add 5 mL of mixed indicator and 5 mL of chloroform, shake it thoroughly, and then let it stand still to allow stratification.
If the chloroform layer appears pink, it indicates the presence of anionic surfactant;
If the chloroform layer appears blue, it indicates the presence of cationic surfactant;
If the two phases are difficult to separate and an emulsion forms, nonionic surfactants may be present.
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