Colorimetric water hardness sensor optode containing neutral ionophore and chromoionophore

Hardness (total calcium and magnesium ion concentration) is one of the most important indicators of water quality. In this study, we report a simple hardness sensor membrane that enables quantitative determination of water hardness through color changes. The sensor membrane contains a high-selectivity neutral ion carrier and a chromoionophore that we designed, which has weaker basicity (pKa=10.8) compared to commonly used Nile blue derivatives. By leveraging the ion-selective optode principle, this membrane responds to calcium and magnesium ions. The key advantages of this sensor include its high selectivity for these ions while maintaining similar responses for both calcium and magnesium, as well as the relatively low pKa of our chromoionophore. These features allow for effective sensing across a wide pH range, covering the typical environmental range from submillimolar to tens of millimolar, with detection limit of 0.11 μM at pH 7.0. Testing with real tap water revealed that the quantitative results obtained using our sensor closely match those from traditional complexometric titration. Therefore, we believe that this sensor membrane has the potential to outperform conventional methods in terms of time efficiency and portability for hardness measurements.