Determination of Calcium and Magnesium in Marine Brines

Significance of the Topic

The precise quantification of calcium and magnesium in marine brines is essential for environmental monitoring, desalination processes, and industrial water treatment. Thermometric titration offers rapid, accurate endpoint detection without the need for colorimetric indicators, making it particularly suitable for high-salinity samples.

Objectives and Study Overview

This application note outlines a direct thermometric titration method using 1 mol/L Na4EDTA to determine Ca2+ and Mg2+ concentrations in seawater, and evaluates the influence of caustic soda and spent aluminate solutions on ionic levels. The addition of acetylacetone improves endpoint sharpness by adjusting metal-EDTA stability constants.

Methodology and Instrumentation

The procedure involves pipetting 50 mL of marine brine into a titration vessel, adding NH3/NH4Cl buffer and acetylacetone, then titrating with standardized Na4EDTA while monitoring temperature changes.

• Titrant: 1 mol/L Na4EDTA, standardized against 0.2 mol/L Mg2+.
• Buffer: NH3/NH4Cl solution for pH control.
• Complexing agent: Acetylacetone (2,4-pentanedione) to enhance endpoint definition.
• Titration parameters:
  
  • Delivery rate: 4 mL/min
  • Stirring speed: setting 15 on 802 stirrer
  • Data smoothing factor: 80
  • Start delay: 10 s
  • Endpoints: one exothermic (Ca) and one endothermic (Mg)

Used Instrumentation

• Thermometric titrator equipped with temperature sensor and automated second-derivative endpoint detection.

Main Results and Discussion

Uncontaminated seawater yielded average concentrations of Ca = 0.517 ± 0.008 g/L and Mg = 1.237 ± 0.007 g/L (n = 11). Samples spiked with 2–10 dm3 spent aluminate liquor per m3 showed proportional decreases in Ca and Mg levels. Blank determinations, obtained by regressing endpoint volume against sample volume (20–50 mL), produced intercepts of +0.1115 mL for Ca titrations and –0.1167 mL for Mg, correcting for mutual chelation interference.

Benefits and Practical Applications of the Method

• High throughput: rapid titrations without indicator addition.
• Robustness: unaffected by sample turbidity or color.
• Versatility: applicable to industrial process control and environmental quality assurance.

Future Trends and Potential Applications

• Automated, continuous monitoring in desalination and chemical processing plants.
• Extension to other metal ion determinations using tailored complexing agents.
• Development of portable thermometric titration systems for field analysis.

Conclusion

Thermometric titration with Na4EDTA and acetylacetone provides a fast, reliable approach for calcium and magnesium analysis in marine brines. Its automation potential and independence from optical indicators make it well suited for modern analytical laboratories.

Reference

• Thermo Scientific. Application Note No. H-074: Determination of Calcium and Magnesium in Marine Brines.