Determination of complexing agents in detergents

Importance of the Topic

Complexing agents such as EDTA, NTA and citrate have become essential in detergent formulations as environmentally friendly builders, replacing traditional phosphates regulated by legislation. Accurate quantification of these chelators ensures compliance with legal limits and supports optimal cleaning performance.

Objectives and Overview

This bulletin aims to present potentiometric titration methods for the determination of EDTA, NTA and citrate in detergent matrices at concentrations above 100 mg·L−1. The focus lies on replacing photometric endpoints with an ion-selective copper electrode (Cu-ISE), streamlining workflow and improving robustness against sample turbidity.

Methodology and Instrumentation

Direct potentiometric titration with CuSO4 is applied for EDTA and citrate, while NTA is quantified via back-titration against an excess of cupric nitrate. All measurements employ a metrological setup controlling pH and potential endpoints under defined stirring rates, buffer systems and pH setpoints. Calculations derive titers from standard solutions and sample consumption using established formulas.

Used Instrumentation

• Titrator with SET (pH-stat) and MET (direct titration) modes
• Ion-selective copper electrode (Cu-ISE, 6.0502.140) with integrated Pt1000 reference (Unitrode, 6.0258.600)
• Multiple burets (10 mL and 20–50 mL) for titrants and standards
• Magnetic stirrer and beakers for sample handling

Key Results and Discussion

Potentiometric titration curves exhibit sharp inflection points unaffected by sample turbidity or surfactant background. The Cu-ISE reliably detects equivalence points for EDTA and citrate at pH 5.0 and 8.5, respectively, and NTA via back-titration at pH 4.65. Interferences from other copper-binding species (e.g. phosphate, citrate in EDTA analysis) are noted, highlighting the need for separate assays when multiple chelators coexist.

Benefits and Practical Applications

• Eliminates the need for photometric indicators, reducing waste and simplifying sample handling
• Unambiguous endpoint detection regardless of sample clarity
• Compliant with ASTM D1767, ISO 4325, ASTM D4954 and ASTM D4608 standards for detergent analysis
• Applicable to routine QA/QC workflows in industrial and research laboratories

Future Trends and Potential Applications

Advances may include automation of titration protocols within integrated platforms, miniaturized flow-through ISE sensors for on-line monitoring, and development of multi-ion arrays for simultaneous detection of several chelators. Green chemistry initiatives may drive the adoption of biodegradable or lower-toxicity titrants and buffers.

Conclusion

Potentiometric copper-complex titration provides a robust, accurate and standardized approach for quantifying EDTA, NTA and citrate in detergent formulations. The methodology supports regulatory compliance, enhances laboratory efficiency and offers clear endpoints independent of sample turbidity.

References

• ASTM D1767-89(2009) – EDTA in Soaps and Synthetic Detergents
• ISO 4325:1990 – Determination of Chelating Agent Content in Detergents
• ASTM D4954-89(2009) – Nitrilotriacetates in Detergents
• ASTM D4608-89(2009) – Citrate in Detergents
• Metrohm Application Bulletin AB-206 – Titer Determination in Potentiometry
• Metrohm Application Bulletin AB-076 – Polarographic Determination of NTA and EDTA