Fully automatic determination of the total content of Ba, Ca, Mg, Pb, and Zn in unused lubricating oils

Importance of the Topic

Complex metal analysis in lubricating oils is essential for quality control, additive verification and wear monitoring in machinery. Techniques that enable rapid, accurate quantification of alkaline earth and heavy metals help manufacturers ensure product performance, comply with specifications and detect potential contaminants.

Objectives and Study Overview

This study aims to develop a fully automatic titrimetric method for determining the total content of barium, calcium, magnesium, lead and zinc in unused lubricating oils. The approach uses EDTA complexation and back-titration of excess chelator with a magnesium chloride solution, detecting the endpoint via an Optrode at 610 nm and Eriochrome Black T indicator.

Methodology and Instrumentation

The analytical workflow comprises two main steps:

• Sample preparation: 10 mL of lubricating oil is dissolved in a mixture of toluene and isopropyl alcohol, buffered to pH 10 and reacted with an excess of Na2EDTA.
• Back-titration: Unreacted EDTA is titrated with 0.025 mol/L MgCl2 using an automated dosing system until the Eriochrome Black T endpoint is reached, monitored by a 610 nm optical sensor.

Used Instrumentation:

• 907 Titrando with MET U titration mode
• Multiple Dosino dosing units (2 mL, 5 mL, 10 mL, 50 mL)
• 802 Rod Stirrer and sample rack (250 mL beakers)
• Optrode sensor at 610 nm for endpoint detection
• Robotic USB sample processor

Key titrant and reagent preparations:

• 0.025 mol/L MgCl2 titrant prepared from MgCl2·6H2O
• 0.025 mol/L Na2EDTA auxiliary solution
• Eriochrome Black T indicator with ascorbic acid
• NH4Cl/NH3 buffer at pH 10

Main Results and Discussion

Replicate analyses (n = 6) yielded a mean metal content of 1.101 ± 0.014 mmol/kg. The automated titration demonstrated high precision, stable endpoint detection and minimal operator intervention. Dissolution in mixed solvent prevented emulsions, and the pH control ensured complete metal chelation.

Benefits and Practical Applications

• Fully automatic workflow reduces manual handling and variability.
• Simultaneous determination of multiple metals accelerates sample throughput.
• High precision supports compliance testing and additive verification.
• Sensitivity to trace impurities aids quality assurance in oil production.

Future Trends and Potential Applications

Advancements may include integration with in-line sampling for real-time monitoring in production plants. Expansion to used oil analysis could support wear particle investigation. Coupling with chemometric data processing may further enhance multielement deconvolution and routine diagnostics.

Conclusion

The described EDTA complexometric back-titration method provides a robust, accurate and fully automated solution for total Ba, Ca, Mg, Pb and Zn determination in unused lubricating oils. Its precision and throughput make it well suited for industrial quality control and research laboratories.

Reference

Application Note T–91, Metrohm Ltd.