Analysis of anionic surfactants
A
Metrohm UK product story
Edited by the Laboratorytalk editorial team
Feb 7, 2007
The determination of the content of surface-active agents (surfactants) plays an important role in many sectors - from wastewater analysis up to quality control in production processes
No matter how much fun we have squishing our toes in the mud, we love to be clean.
We want our clothes to be fresh, our dishes to be spotless, and our cars to be shiny.
We continue to invent new ways to make things clean, and soap was probably discovered only shortly after cooking, as the fats from the food hit the ashes from the fire.
Detergents have molecules with one side that prefers water (hydrophilic), and another side that prefers oils and fats (hydrophobic).
The hydrophilic side attaches to water molecules, and the hydrophobic side attaches to oil molecules.
This action allows the oil droplets to break up into smaller droplets, surrounded by water.
These smaller droplets are no longer stuck to the material to be cleaned, and are washed away.
Surfactants have a hydrophilic side of the molecule attaches to water, and a hydrophobic side of the molecule that avoids water.
In the absence of oils, the hydrophobic side sticks out of the surface of the water drop.
There is no longer any water at the surface to form a strong surface tension, so the water no longer beads up, but spreads.
The hydrophobic end of the molecule is also free to attach to grease, fat, or oil on the surface, aiding in the spreading.
Some detergents and surfactants are used as emulsifying agents.
An emulsifier keeps oil droplets and water droplets from joining together, so a thick mixture of oil and water will not separate.
Examples of emulsions are mayonnaise, butter, cream, homogenised milk, and salad dressings.
The determination of the content of surface-active agents (surfactants) plays an important role in many sectors - from wastewater analysis up to quality control in production processes.
A method that is still in widespread use is the so-called Epton titration according to ISO 2271, a complicated and time-consuming manual titration.
The new Din EN 14480 standard now describes an alternative determination method that is much simpler and faster.
Potentiometric two-phase titration according to DIN EN 14480: more precise, faster and much healthier.
The solution of the anionic surfactant is pipetted into a titration vessel, to which a two-phase mixture of water and methyl isobutyl ketone/ethanol 1:1 and an emulsifier are added.
Under vigorous stirring the emulsion formed is titrated with a cationic surfactant, in this case benzethonium chloride (Hyamine 1622).
The ion pair formed, which is insoluble in water, is extracted immediately into the organic phase.
The determination of the equivalence point does not take place visually, but potentiometrically using surfactant-sensitive electrodes.
The Surfactrodes from Metrohm are used for this purpose; they have a high affinity for surface-active agents.
In comparison with the Epton titration you profit from the following advantages:.
Use of unproblematic solvents instead of chloroform.
Short determination times: only a few minutes per titration.
Objective, computer-supported determination of the equivalence point and therefore improved precision.
Can easily be automated.
In an extensive Europe-wide interlaboratory test the potentiometric two-phase titration was compared with the Epton method and was found to yield the same analytical results with a better repeatability.
A detailed evaluation of the interlaboratory test can be found in the appendix of Din EN 14480.phase titration.
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