Methylene blue method

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The methylene blue method is used to determine the cation exchange capacity (KAK value) of clay minerals . The clay minerals used on a large scale in industry consist largely of aluminum smectites , as well as admixtures of quartz , sheet silicates, iron oxohydroxides, carbonates, sulfates and pyrite . Because of these "impurities", the clay minerals are both negatively charged and capable of exchanging cations.

Measurement

The following information comes from a recommendation by the Association of German Foundry Experts (VDG):

To carry out the measurement, 500 milligrams of the sample are suspended in 50 ml of distilled water and 5 ml of a saturated sodium diphosphate solution are added as a deflocculant. After boiling and stirring for ten minutes, 10 ml of 0.5 molar sulfuric acid are added after cooling . The sulfuric acid is supposed to neutralize any negative charges along the crystal surface.

After addition of a 0.02 molar solution of methylene blue which occurs adsorption of the positively charged methylene blue cations on the surface of the Tonkristalle by cation exchange. Care must be taken that the amount of dye added does not exceed the adsorption capacity of the sample, because then smaller amounts of methylene blue are added until the "free" greenish dye indicates that the titration limit has been reached. The amount of methylene blue determined in this way is compared with reference values ​​so that conclusions can be drawn about the nature of the clay minerals.

Despite its inaccuracy, the method is one of the most important procedures for quality control of clay minerals. The simultaneous use of photometry provides better comparable and more precise results .

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