Surfactants methylene blue test

Many anionic surfactants can react with a cationic dye such as methylene blue to form strong ion pairs that can be extracted by a suitable organic solvent and can be determined using colorimetric techniques. The anionic surfactants that respond to the methylene blue test are primarily the sulfonate (RS03 Na+) and the sulfate ester (R0S03 Na+) type substances. On the other hand, soaps and the alkali salts of fatty acids (C-10 to C-20) used in certain detergents do not respond to the above test. The various anionic surfactants and their characteristic structural features are presented in Figure 2.32.1. 

The test is conducted as in Section 4.3.1, acidic methylene blue test. If the aqueous layer is emulsified to a milk-like state, or both layers have the same colour, the existence of non-ionic surfactants can be confirmed. 

Amphoteric surfactants contain both anions and cations. They should show positive results when tested using either the basic methylene blue test for anionic surfactants or the alternative bromophenol blue test for cationic surfactants. 

This work intends to show the complexity of the dynamic adsorption process and to evaluate capacity of some granular carbons of various firms to remove pollutants from water. Adsorbents have been tested by various methods, and static and dynamic adsorption have been compared. Characteristics of carbons has been evaluated by the determination of porous structure, specific surface, content of ashes (mineral substances) and crushing strength and abrasion resistance. Adsorption capacity of activated carbon has been determined by means of phenol, iodide, methylene blue, sodium lauryl sulphate and molasses indicators for static conditions, and surfactant has been used for dynamic conditions. Analysis of some factors influencing adsorption has been accomplished and directions of further studies have been shown. 

Add -5 ml of 1 % sample surfactant solution into a mixture of 10 ml methylene blue solution and. 5 ml chloroform in a test tube shake vigorously then allow it to stand until two layers are formed. If the chloroform layer (bottom layer) shows blue, add another 2-3 ml of the surfactant solution. Shake well and leave for layers to form. The chloroform shows as dark blue and the water layer is almost colourless. This is a positive result of the existence of anionic surfactant in the sample solution. This test is suitable for alkylsulphate and alkylbenzolsulphonate surfactants. Soap cannot be tested because it would precipitate in the strong acidic medium. 

Add 1 drop of 5% sample solution to a mixture of 5 ml 0.1% methylene blue solution, 1 ml 1 N NaOH solution and 5 ml chloroform. Shake weU and observe the colour of the chloroform layer. If a blue-purple colour is shown, there is an anonic surfactant in the sample. This test is suitable for any type of anionic surfactant. 

Cationic surfactants can also be tested using methylene blue solution. First add 2 drops of a known anionic surfactant solution to a mixture of 5 ml methylene blue solution and 5 ml chloroform, shake well and leave to stand until the chloroform layer shows as blue. Then add a few drops of the sample solution, shake well and leave for layers to form. If the blue colour in the chloroform layer becomes lighter or colourless, the existence of cationic surfactants in the sample solution can be confirmed. 

The blue complex of the anionic detergent (surfactant) with methylene blue is in the chloroform solution. Measure the extinction of the chloroform phase in a photometer at 650 nm in cuvettes with 1, 2 or 5 cm path length against chloroform. Treat and measure a blank test with 100 ml of distilled water in exactly the same way (with a 1-cm cuvette the measured value should be below 0.02). 

In general, any of the visible spectrophotometric methods described in Chapter 12 can be converted to a qualitative test by replacing the instrumental measurement of color intensity with a visual inspection. A typical procedure is use of a mixture of methylene blue (a blue, cationic dye) and pyrocatechol violet (a yellow, anionic dye) to provide a general test for surfactants (29). The aqneons sample (pH 5-6), mixed indicator, and petroleum ether are mixed. Test results are as follows ... 

---