Potentiometric determination of ionic surfactants by two-phase titration using Surfactrodes

Significance of the Topic

The accurate determination of ionic surfactants is critical for quality control in sectors such as detergents, cosmetics, pharmaceuticals, metal plating and industrial analytics. Traditional methods often rely on toxic solvents and manual extraction, leading to safety and reproducibility issues.

Objectives and Overview

This study presents a fully automated two-phase potentiometric titration approach using Metrohm Surfactrode electrodes, designed to measure anionic and cationic surfactants directly in raw materials and complex formulations. The method aims to replace chloroform with safer organic solvents and to align results with classical Epton titration.

Methodology and Instrumentation

The procedure employs:

• Metrohm titrator in differential potentiometric (DET) mode with 20 mL buret and propeller rod stirrer
• Surfactrode Resistant and Surfactrode Refill electrodes with Ag/AgCl reference
• Titrants: TEGO trant A100 (cationic) and sodium dodecyl sulfate (anionic)
• Organic solvents: MIBK, cyclohexane, n-hexane and small fractions of ethanol or methanol
• Additive TEGO add to stabilize emulsions and maintain electrode performance

Sample pH is adjusted between 2 and 10 based on surfactant class, and titrant strengths of 0.005–0.02 mol/L are used.

Main Results and Discussion

Titration curves demonstrate clear potential jumps and yield results that correlate closely with classical methods. The approach handles a variety of matrices, including shampoo concentrates, cooling lubricants, washing powders (with soluble and insoluble builders), scouring dispersions and household cleaners, achieving relative standard deviations around 1% and high reproducibility over many runs.

Benefits and Practical Applications

The two-phase potentiometric titration offers:

• Complete automation and reduced manual handling
• Elimination of pre-extraction steps and toxic solvents like chloroform
• Fast turnaround (typically 5–10 minutes per analysis)
• Minimal sample preparation and adaptability to complex formulations
• Consistent performance over many titrations due to TEGO add

Future Trends and Potential Applications

Advancements may include integration with inline process monitoring, miniaturized sensor platforms for field use, expanded detection of amphoteric surfactants and coupling with spectroscopic methods for enhanced selectivity. Environmental monitoring and real-time quality control represent promising directions.

Conclusion

The two-phase potentiometric titration method with Surfactrode electrodes provides a robust, versatile and safer alternative for ionic surfactant analysis. It delivers accurate, reproducible results across diverse sample types while streamlining laboratory workflows.

Instrumentation Used

• Metrohm DET titrator with 20 mL buret
• Propeller rod stirrer
• Surfactrode Resistant (6.0507.130) and Surfactrode Refill (6.0507.140)
• Ag/AgCl reference electrode (6.0726.107)

References

• R. Schulz, “Titration of Surfactants and Pharmaceuticals,” VCH, 1996.
• R. Schulz & R. Gerhards, “New Specialized Surfactant Electrodes,” LaborPraxis, 1996.
• R. Schulz & J. Thiede, “Titration of Ionic Surfactants in Cooling Lubricants,” LaborPraxis, 1997.
• Metrohm Application Bulletins No. 233, No. 268 and No. 275.