Standardization of cationic surfactants by argentometric titration

AN-T-246

2026-01

Accurate assay determination of TEGO®trant with silver nitrate including near-infrared (NIR) quantification model

The standardization of the cationic surfactant TEGOtrant is performed using potentiometric titration as well as near-infrared spectroscopy (NIRS) in this application.

A well-known frequently used cationic surfactant (TEGOtrant, also known as 1,3-didecyl-2-methylimidazolium chloride) is standardized in this Application Note. The stoichiometric chloride is titrated argentometrically with standardized silver nitrate, enabling an extremely precise determination of the assay. Therefore, the result determined is equivalent to the total surfactant content, allowing the accurate calculation of the cationic surfactant concentration.

Furthermore, a quantification model was developed that uses NIR measurements in addition to titration. This enables users to quickly determine cationic surfactant content using only NIR spectroscopy.

There are no primary or secondary standard methods for titrating or determining the active substance content of anionic and cationic surfactants. A cationic titrant is standardized using an anionic titrant, and vice versa. In short, the result of this titration is a sum parameter usually specified as «total surfactant content». This value is then used to determine the content in the actual sample.

The major weakness of this type of standardization is the significant margin of error one must accept. Because the exact concentration of the titrant is unknown, conclusions about the analyzed sample can only be made with limited accuracy.

This application is demonstrated on 25 different batches of TEGOtrant A100 (1,3-didecyl-2-methylimidazolium chloride). The sample is ground into a fine powder before analysis.

Figure 1. OMNIS Sample Robot S – WSM with OMNIS Professional Titrator and OMNIS Dosing Modules equipped with a dAg Titrode as well as an OMNIS NIR Analyzer Solid.

The determination is carried out using an OMNIS Sample Robot S – WSM, an OMNIS Professional Titrator equipped with OMNIS Dosing Modules, as well as a dAg Titrode along with an OMNIS NIR Analyzer Solid (Figure 1).

Titration

An appropriate amount of sample is weighed into the titration beaker, to which deionized water and a nitric acid solution are added. A titration is performed until after the first equivalence point with standardized silver nitrate.

NIR spectroscopy

A sufficient amount of sample is added to the small cup NIR accessory. This is placed in the corresponding holder which is already mounted on the OMNIS NIR Analyzer Solid.

Five determinations were carried out automatically in triplicate for each near-infrared measurement.

This method offers very accurate results, as displayed in Table 1. The comparative measurement of the control sample, summarized in Table 2, shows that the values from the titration and the NIR measurement differ by only 0.5%. Exemplary titration and NIR measurements of TEGOtrant are given in Figure 2 and Figure 3, respectively.

**Table 1.** Selected results of the potentiometric determination of TEGOtrant.
Sample (n = 3), batch number	Assay mean value in %	SD(rel) in %
20780065	96.16	0.3
11020053	94.11	0.1
11070002	95.65	0.3
00470397	95.19	0.1
00530513	93.71	0.3

**Table 2**. Summary of results for assay determination of the TEGOtrant «control sample» by titration and NIR.
Control sample, batch number	Assay mean value in %	SD(rel) in %
20650022 by titration (n = 6)	97.61	0.2
20650022 by NIR (n = 15)	98.13	0.3
Difference	0.52%	–

Figure 2. Titration curve of TEGOtrant with silver nitrate and the dAg Titrode.

Figure 3. NIR spectrum of TEGOtrant between the wavelengths of 1000 nm and 2250 nm.

Potentiometric titration is an accurate and precise method that can be used to standardize the cationic surfactant TEGOtrant.

The OMNIS system used in this study is fully automated and enables fast and reliable cationic surfactant titration standardization. The argentometric determination with the digital Ag Titrode is highly accurate. Furthermore, the Ag₂S-coated silver ring increases sensitivity, delivering even better results.

When used with the corresponding OMNIS NIR Analyzer Solid, OMNIS Software can easily create a quantification model on a single platform, offering users real added value for the TEGOtrant standardization.

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