Epton titration

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The 2-phase titration according to Epton (also briefly Epton titration ) is a commonly used in the chemical analysis method.

Historical

The method was introduced by Epton in 1947 using methylene blue as an indicator. An improved method from Reid uses benzethonium chloride as the cationic titration solution and dimidium bromide / disulfine blue VN 150 as the indicator mixture .

definition

An anionic surfactant that is soluble in water is determined by titration with methylene blue as an indicator against cationic surfactant . Anion and cation must form a 1: 1 salt that is soluble in dichloromethane . The end point is determined by the change in the indicator or indicator mixture. Similarly, a cationic surfactant can be titrated with an anionic surfactant.

principle

The 2-phase titration is based on the fact that a water-soluble anion (or cation) surfactant forms a 1: 1 salt soluble in dichloromethane with a cation (or anion) surfactant titrant.

The end point is indicated in the organic phase by a color change of a mixed indicator, which consists of a cationic and an anionic dye (dimidium bromide / disulfine blue).

When determining anionic surfactants , the dichloromethane phase is colored pink due to the dimidium anionic surfactant salt. This color disappears towards the end of the titration from the dichloromethane phase through the formation of the dye anion-cation surfactant salt. An excess of cationic surfactant titrant in dichloromethane results in a gray-blue tint, produced by the formation of the cationic surfactant-dye anion salt, which marks the end point.

Procedure

The titration is carried out in a glass vessel with a strong mechanical stirrer. The vessel should be equipped with a drain cock.

The sample solution, the mixed indicator solution and the dichloromethane are presented. The titrant is then added with a burette while stirring vigorously. To assess the color of the dichloromethane phase, switch off the stirrer and wait until the aqueous and organic phases have separated. This process must be repeated several times towards the end of the titration.

Titrant

(also standard solution ) To determine anionic surfactants :
0.004 N Hyamine ® 1622 solution, titer adjustment with pure sodium lauryl sulfate

For the determination of cationic surfactants :
0.004 N sodium lauryl sulfate solution, titer adjustment with adjusted 0.004 N hyamine solution

scope of application

The method can be used for alkylbenzenesulfonates with a chain length of six carbon atoms or more and sulfated ethoxylates of alkylphenols and fatty alcohols with eight or more carbon atoms. In addition, alkanesulfonates , alkyl sulfates , fatty acid aminoethanesulfonates and quaternary ammonium compounds can be determined.

Disruptions

Free chlorine must be destroyed with hydrogen peroxide before the determination.

Alternatives

Because of the volatile and ecologically questionable dichloromethane, methods have been developed in which other solvents can be used. These recognize the end point from a turbidity maximum. But there are also special electrodes for the determination of ionic surfactants.

literature