Dicyclohexylamine (DCHA) and methyldicyclohexylamine (MDCHA) in cooling lubricant applying Inline Dialysis

Analytical Determination of Dicyclohexylamine (DCHA) and Methyldicyclohexylamine (MDCHA) in Cooling Lubricant Emulsions by Inline Dialysis Ion Chromatography

Importance of the Topic

The use of cooling lubricants in abrasive metalworking processes is critical for temperature control, lubrication, and corrosion protection. Amines such as DCHA and MDCHA are commonly added to maintain an alkaline pH, but accurate quantification is challenging due to oil and surfactant interference. Inline dialysis coupled to ion chromatography (IC) addresses this by removing hydrophobic matrix components, ensuring reliable analysis of amines directly in emulsions.

Objectives and Overview

This study aims to develop and validate a robust IC method with inline dialysis for the selective determination of DCHA and MDCHA in cooling lubricant emulsions. Two real samples were analyzed to demonstrate sensitivity, accuracy, and suitability for routine quality control in industrial laboratories.

Methodology and Instrumentation

Sample Preparation:

• Weigh 1 g of emulsion into 10 mL of diluent.
• Further dilute 1:2 with ultrapure water.
• Introduce samples through Metrohm Inline Dialysis to eliminate oil contaminants.

Chromatographic Conditions:

• Column: Metrosep C 4-100/4.0 with Metrosep C 4 Guard/4.0.
• Eluent: 1.7 mmol/L nitric acid, 0.7 mmol/L dipicolinic acid, 15 % acetone.
• Diluent: 4 mmol/L nitric acid, 10 % acetone.
• Acceptor (Inline Dialysis): 2 mmol/L nitric acid, 10 % acetone.
• Flow rate: 0.9 mL/min; Injection volume: 20 µL; Column temperature: 30 °C.
• Dialysis duration: 10 min; System pressure up to 25 MPa; Total run time: 24 min.

Used Instrumentation

• 930 Compact IC Flex (2.930.2360) with Oven/ChS/PP/Deg unit
• IC Conductivity Detector (2.850.0010)
• 858 Professional Sample Processor (2.858.0020)
• Inline Dialysis module (6.5330.100) with polyamide membrane (6.2714.030)

Main Results and Discussion

Analysis of two cooling lubricant samples yielded:

• Sample 1: DCHA at 1900 mg/kg (101 % recovery); MDCHA below detection limit.
• Sample 2: MDCHA at 1730 mg/kg (100 % recovery); DCHA below detection limit.

Inline dialysis effectively prevented oil-related fouling, ensuring stable baselines and reproducible quantification. The method provided clear separation of DCHA and MDCHA with direct conductivity detection.

Benefits and Practical Applications

• Rapid, direct analysis of cooling lubricant emulsions without extensive cleanup.
• High accuracy and recovery near 100 % for target amines.
• Minimal maintenance due to inline removal of hydrophobic matrix components.
• Suitable for routine quality control in manufacturing and R&D laboratories.

Future Trends and Applications

Potential developments include:

• Extension to other non-volatile amines and cationic additives in complex matrices.
• Automation of sample dilution and dialysis steps for higher throughput.
• Integration with mass spectrometric detection for structural confirmation.
• Application to environmental monitoring of amine-based corrosion inhibitors.

Conclusion

The inline dialysis-IC method offers a reliable, accurate, and efficient approach for quantifying DCHA and MDCHA in cooling lubricant emulsions. It minimizes sample preparation and instrument downtime while delivering robust performance for industrial quality control.