Water analysis

Importance of the Topic

Accurate analysis of drinking water is vital for human health and environmental monitoring. Key parameters such as pH, alkalinity and total hardness indicate water stability, corrosion potential and scale formation risk. Reliable measurement of these parameters in low ionic strength and weakly buffered waters presents analytical challenges that must be addressed to ensure consistent water quality control.

Study Objectives and Overview

This study compares conventional glass pH electrodes with a specialized Metrosensor Aquatrode Plus membrane for potentiometric determination of pH, carbonate alkalinity and total hardness in tap water. It aims to demonstrate improved response time, reduced stirring dependency and enhanced precision for drinking water samples using the new electrode designs and ion‐selective techniques.

Methodology and Instrumentation

The work follows EN ISO 9963-2 for carbonate alkalinity titration with 0.05 M HCl to pH 5.4, and potentiometric hardness titration using a calcium ion‐selective electrode (Ca ISE). Instrumentation included:

• Aquatrode Plus combined pH/glass electrode with fixed ground‐joint diaphragm, maintenance‐free gel inner electrolyte and rapid‐response glass membrane.
• Standard pH electrode with ceramic pin diaphragm for comparison.
• Polymer‐membrane Ca ISE for total hardness determination over 5×10⁻⁷ to 1 M Ca²⁺, with high Mg²⁺ selectivity (log K_ij = –6.8).
• Thermostatted titration vessel under argon and Merck buffer solutions at pH 4.005, 6.863 and 9.184 for electrode calibration.

Main Results and Discussion

The Aquatrode Plus electrode consistently measured near the theoretical pH 9.98 and showed negligible variance with stirring rates, while the standard electrode undervalued pH at higher stirring speeds by up to 0.2 units. In alkalinity titrations, rapid membrane response was critical for accurate endpoint detection in low ionic strength matrices. Total hardness titrations exhibited two distinct potential jumps: the first corresponding to Ca²⁺ and the second to combined Ca²⁺ and Mg²⁺. Detection limits for both ions were around 0.01 mmol/L, and reliable quantification was achieved across calcium/magnesium ratios up to 10:1. Precision for Mg²⁺ improved when samples were spiked with a known standard.

Benefits and Practical Applications

The enhanced electrode performance enables faster, more precise pH and alkalinity analyses in weakly buffered water, reducing measurement uncertainty caused by stirring and slow membrane response. The Ca ISE method simplifies total hardness determination, allowing simultaneous quantification of calcium and magnesium with low detection limits, which is valuable in quality assurance, regulatory compliance and instrumented process control.

Future Trends and Applications

Future developments may focus on integrating fast‐response membranes into multi‐parameter probes for online and inline water monitoring systems. Advances in ion‐selective membrane chemistry could extend detection to trace contaminants and optimize selectivity. Coupling these electrodes with automated titrators and digital data platforms will further streamline water quality management in industrial, environmental and municipal contexts.

Conclusion

The Aquatrode Plus pH electrode and polymer‐membrane Ca ISE offer significant improvements over conventional sensors for comprehensive water analysis. Their rapid response, low maintenance and high selectivity support accurate determination of pH, alkalinity and hardness in low ionic strength samples, enhancing routine monitoring and research applications.

Reference

• EN ISO 9963-2:1996-02, Water quality – Determination of alkalinity.
• Metrohm AG poster 8.000.6021EN, Water analysis with Aquatrode Plus and Ca ISE.