Determination of the total hardness in tap water using acoustic wave sensors

The determination of hardness in water is a useful parameter to control the quality of water for households and industrial uses. The present work proposes a new methodology as an alternative to the conventional EDTA titration. The determination of Ca 2+ and Mg 2+ is performed using two different coated piezoelectric quartz crystals. Poly(vinyl chloride) membranes, incorporating a plasticizer and a Mg or Ca ionophore were used to coat the piezoelectric quartz crystals. Results obtained by this new methodology shows that the tap water found in Portugal can be considered soft in Gouveia (9.4 ± 0.8 mg L −1 CaCO 3) and in Vila Praia Âncora (15 ± 1 mg L −1 CaCO 3), slightly hard in Leiria (59 ± 1 mg L −1 CaCO 3), moderately hard in Aveiro (74 ± 1 mg L −1 CaCO 3) and in Esposende (104 ± 1 mg L −1 CaCO 3) and very hard in Tomar (225 ± 1 mg L −1 CaCO 3). The results obtained by the proposed method are not significantly different ( α = 0.05), both in terms of accuracy and precision, from the ones obtained by the conventional EDTA titration method. Besides providing individual concentration of Ca 2+ and Mg 2+, this new methodology also has the advantage of being less reagent, sample and time consuming.