Calcium, magnesium, and total hardness in water

Significance of the topic

Water hardness, defined by calcium and magnesium concentrations, is a key quality parameter in drinking water, industrial processes and environmental monitoring. Accurate hardness determination is essential for boiler protection, scaling prevention and compliance with regulatory standards.

Objectives and study overview

This application note demonstrates an automated, potentiometric approach for separate quantification of total, calcium and magnesium hardness in tap water. The study replaces conventional photometric, subjective methods with a robust titration protocol using a copper ion-selective electrode (Cu-ISE) and two complexing agents.

Methodology and instrumentation

Samples of unfiltered tap water from Herisau, Switzerland, were analyzed without pretreatment. Two titrations were performed per sample:

• Total hardness by titration with EDTA and Cu-ISE detection
• Calcium hardness by titration with EGTA and Cu-ISE detection

Magnesium hardness was calculated as the difference between total and calcium values. Equivalence points were identified by monitoring the potential change of the Cu-ISE vs. a long-life reference electrode.

Used instrumentation

• 907 Titrando: high-end titrator with ion-selective electrode measurement capability
• 815 Robotic USB Sample Processor XL: automated sample handling and dosing
• Copper ion-selective electrode (Cu-ISE) with crystal membrane
• Long-life silver/silver-chloride reference electrode (LL ISE)

Main results and discussion

Both titrations produced sharp, well-defined potential jumps at the endpoints.
Reproducibility (n=5) for tap water samples:

• Total hardness: 3.517 mmol/L ± 0.020 mmol/L (0.57% RSD)
• Calcium hardness: 2.547 mmol/L ± 0.012 mmol/L (0.47% RSD)
• Magnesium hardness: 0.971 mmol/L ± 0.009 mmol/L (0.94% RSD)

The low relative standard deviations demonstrate high precision and reliability compared with manual colorimetric methods.

Benefits and practical applications

• Objective endpoint detection avoids analyst bias inherent in color change observations.
• Automated sample preparation and titration streamline workflow and reduce hands-on time.
• Easy electrode maintenance through surface polishing extends sensor life and performance.
• High precision supports quality control in water treatment, beverage production and laboratory settings.

Future trends and opportunities

Integration of the potentiometric protocol with multi-channel robots can enable high-throughput screening of water quality. Adaptation for on-line monitoring in industrial loops or coupling with flow-injection systems could further reduce analysis time. Extension of the electrode approach to other metal ions and scaling to field-deployable devices represents additional development paths.

Conclusion

The automated Cu-ISE titration method offers a fast, reproducible and user-independent solution for determining total, calcium and magnesium hardness in water. Its simplicity, precision and compatibility with robotic systems make it an attractive alternative to traditional photometric assays.

References

• Metrohm AG. Application Note AN-T-131: Calcium, magnesium, and total hardness in water. Automated determination using the Cu-ISE and two different titrants.