Photometric determination of ionic surfactants by two-phase titration
AN-T-245
2026-01
de
Accurate and reliable results with the Optrode according to the classical Epton titration
Accurate, reliable determination of ionic surfactants with the Epton two-phase titration method can be achieved using an OMNIS system as shown in this study.
The two-phase Epton titration method is the standard approach for titrating ionic surfactants. It is used whenever the delivery conditions specify this method or when potentiometric surfactant titration cannot be carried out, such as with powdered washing agents.
This Application Note presents the two-phase titration of ionic surfactants according to Epton. The titration is optimized for the OMNIS Software. Specifically, the concentration of sodium dodecyl sulfate (SDS) in shower gel is determined by titration with hyamine using an Optrode M2.
Two-phase titrations were first developed in the early 1930s and still continue to be improved upon. In 1947, chemist S. R. Epton developed the Epton titration method. In this first iteration, an anionic sample is mixed with methylene blue in diluted sulfuric acid (as an indicator), and chloroform. Vigorous shaking forms an ionic salt that is extracted into the chloroform, turning the phase a blue color. Titrating with the classical cationic surfactant cetylpyridine bromide causes the color to slowly migrate into the upper aqueous phase. The endpoint is reached when both phases turn blue.
In the 1960s, Holness and Stone developed the method into its current form. They replaced methylene blue with a mixed indicator, dimidium bromide-disulfine blue, and used Hyamine® 1622 as cationic titrant. This new indicator clearly improved endpoint recognition because the color change is only observed in the chloroform phase. The chloroform phase is red in the presence of excess anionic surfactant ions and blue in the presence of excess cationic surfactant ions.
Depending on the titrant (e.g., hyamine or SDS), either anionic or cationic surfactants can be titrated. Despite this, the term «Epton titration» has stuck and is still used today.
This application is demonstrated on shower gel. Sample preparation is not required.
?qlt=85&ts=1765198913913&dpr=off&bfc=on "The OMNIS Professional Titrator with OMNIS Dosing Modules (shown here) are ideal for two-phase titrations of ionic surfactants when equipped with the Optrode M2.")
The determination is carried out on an OMNIS Professional Titrator equipped with OMNIS Dosing Modules (Figure 1) as well as an Optrode M2.
An appropriate amount of sample is weighed into the titration beaker, then deionized water, mixed indicator (dimidium bromide-disulfine blue), and chloroform are added. The solution is titrated with standardized hyamine until after the first equivalence point is achieved using a wavelength of 520 nm.
The Epton titration method offers very accurate results, as displayed in Table 1. An exemplary titration curve of SDS in shower gel with hyamine is given in Figure 2, showing the sharp jump in voltage at the equivalence point.
| Sample (n = 6) | SDS (%) | SDS (mmol/100g) |
|---|---|---|
| Mean value | 5.09 | 17.65 |
| SD(abs) | 0.05 | 0.19 |
| SD(rel) in % | 1.1 | 1.1 |
Precise and reliable determination of ionic surfactants according to the Epton two-phase titration can be achieved using an OMNIS system. The Optrode M2, with its glass shaft, is fully resistant against solvents and is therefore ideal for surfactant titrations with chloroform.
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