Potentiometric two-phase titration of anionic surfactants in washing powders and liquid washing agents

Importance of Topic

Accurate quantification of anionic surfactants in washing powders and liquid detergents is crucial for quality assurance, regulatory compliance and environmental monitoring. The potentiometric two-phase titration offers a rapid, automatable alternative to classical visual indicator methods, enhancing precision and reducing operator bias.

Objectives and Study Overview

This bulletin presents the development, validation and application of a potentiometric two-phase titration method for anionic surfactants. Key aims include:

• Establishing optimal pH conditions and electrode configurations
• Comparing potentiometric results with the classical Epton titration (DGF H-III 10)
• Testing method robustness in diverse detergent matrices

Methodology and Instrumentation

Sample Preparation

• Powders: Homogenize by division or ultracentrifugal milling, dissolve ~200 mg in water, adjust pH, add modifier and solvent mixture.
• Liquids: Direct titration without interference from non-ionic surfactants or betains.

Reagents and Solvent System

• Hyamine® 1622 titrant (approx. 0.005 mol/L)
• Sodium dodecyl sulfate standard (0.005 mol/L)
• Buffer adjustments with HCl or NaOH (0.5 mol/L)
• Solvent: methyl isobutyl ketone / ethanol (1 : 1, v/v)
• TEGO add for improved phase formation

Instrumental Setup

• Automated titrator in differential electrode titration (DET) mode
• 20 mL buret, rod stirrer
• Electrodes: Surfactrode Refill for pH 2.0, Surfactrode Resistant for pH 3.0, Ag/AgCl reference

Main Results and Discussion

The potentiometric titration yielded recoveries between 98 and 102 % relative to the DGF Epton method, with standard deviations of 0.2–0.8 mmol/100 g. Key findings include:

• Nonionic surfactants show no interference, whereas insoluble builders (zeolites, phosphates) reduce potential jumps and flatten titration curves.
• Optimal pH values of 2.0–3.0 ensure clear endpoints without precipitation.
• Dual titration at pH 2.0 and pH 11.5 enables separate quantification of soaps and anionic surfactants.

Benefits and Practical Applications

This potentiometric protocol provides:

• Automated, reproducible results with minimal sample handling
• Rapid analysis suitable for routine QA/QC in detergent manufacturing
• Applicability across diverse product types: universal, colored-fabric, delicate-fabric detergents and liquid shampoos

Future Trends and Potential Applications

Emerging directions include:

• Integration with flow-injection systems for high-throughput screening
• Development of novel titrants (e.g., TEGO®trant A100) to extend analysis to more hydrophilic surfactants
• Miniaturized electrodes and green solvent alternatives for reduced environmental impact

Conclusion

The potentiometric two-phase titration method delivers reliable, accurate and high-throughput determination of anionic surfactants in complex detergent matrices. It streamlines quality control workflows and offers a robust alternative to manual indicator titrations.

References

• Deutsche Gesellschaft für Fettwissenschaften (DGF), Manual for Two-Phase Titration According to Epton, DGF H-III 10, ISBN 978-3-8047-3024-3