Determination of Calcium and Magnesium in Brine

Significance of the Topic

Membrane based chlor-alkali processes demand brine with extremely low levels of calcium and magnesium to prevent membrane fouling and maintain efficient current flow. Achieving hardness below 50 µg/L extends membrane life and reduces operational costs. Conventional off line sample preparation is time consuming and prone to contamination. The presented on line ion chromatography method enables rapid, sensitive and reproducible analysis of trace alkaline earth metals in high salt matrices.

Aims and Study Overview

This study introduces an ion chromatography approach to quantify calcium and magnesium at low µg/L concentrations in a 30 percent sodium chloride brine. Key objectives include selective concentration of analytes, effective removal of the sodium matrix and robust separation with suppressed conductivity detection. The workflow integrates on line matrix elimination and high salt sample handling to deliver accurate results within a 35 minute analysis cycle.

Methodology

The method employs a four step sequence:

1. Fill a 100 µL sample loop with brine adjusted to pH 11.5 using 10 mM sodium hydroxide
2. Load the loop contents onto a MetPac CC-1 concentrator column via a 1 mM HCl rinse supplied by a dedicated pump
3. Remove the sodium matrix by washing the concentrator with 1 mM HCl for 20 minutes at 2 mL/min
4. Elute retained calcium and magnesium in reverse flow with a 20 mM methanesulfonic acid eluent at 1 mL/min and separate them on IonPac CG12A guard and CS12A analytical columns

Instrumentation Used

• Dionex DX-500 system including GP40 gradient pump, CD20 suppressed conductivity detector and LC20 enclosure with Rheodyne valves
• DQP rinsing pump delivering 1 mM HCl
• TMC-1 trap column for rinse solution purification
• MetPac CC-1 concentrator column
• IonPac CG12A guard column (4×50 mm) and CS12A analytical column (4×250 mm)

Main Results and Discussion

System background conductivity stabilized at 0.3–3 µS with peak to peak noise around 10 nS. Calibration for both calcium and magnesium was linear from 0 to 200 µg/L (R2 = 0.9999). A brine blank showed only lithium, sodium and potassium, with no interference in quantification. Replicate injections of 5 µg/L standards yielded relative standard deviations below 2.5 percent.

Benefits and Practical Applications

• Eliminates off line sample preparation and minimizes contamination risk
• High sensitivity and selectivity for divalent cations in high salt matrices
• Enables monitoring of trace hardness in real time to protect membranes
• Suitable for routine quality control in chlor-alkali production facilities

Future Trends and Potential Applications

Emerging trends include coupling microbore concentrators with faster column chemistries to reduce analysis time, integration with mass spectrometry for multi element profiling and development of portable IC systems for on site brine monitoring. Adapting the approach to other high salt industrial streams and extending to additional trace metals could further enhance process control across diverse sectors.

Conclusion

The on line ion chromatography method delivers reliable quantification of calcium and magnesium at sub ppb levels in 30 percent sodium chloride brine. Its combination of matrix elimination, precise separation and sensitive detection offers a robust solution for maintaining membrane integrity and ensuring operational efficiency in chlor-alkali plants.

References

• David Hildebrand, personal communication, Vulcan Chemical, Wichita, Kansas, USA
• Dionex Corporation. Determination of Trace Anions in Isopropyl Alcohol, Application Note 85
• Kaiser E, Wojtusik MJ. J Chromatogr A. 1994;671:253–258