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

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Abstract

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 Ca2+ and Mg2+ 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 CaCO3) and in Vila Praia Âncora (15 ± 1 mg L−1 CaCO3), slightly hard in Leiria (59 ± 1 mg L−1 CaCO3), moderately hard in Aveiro (74 ± 1 mg L−1 CaCO3) and in Esposende (104 ± 1 mg L−1 CaCO3) and very hard in Tomar (225 ± 1 mg L−1 CaCO3). 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 Ca2+ and Mg2+, this new methodology also has the advantage of being less reagent, sample and time consuming.

Introduction

Calcium and magnesium dissolved in water are the two most common minerals that make water “hard”. The degree of hardness becomes higher as the calcium and magnesium content increases, and is related to the concentration of multivalent cations dissolved in the water. Hardness is most commonly expressed as milligrams of calcium carbonate equivalent per litre. Water containing less than 17.1 mg of calcium carbonate per litre is generally being considered as soft, slightly hard with less than 60 mg L−1, moderately hard from 60 to 120 mg L−1, hard from 120 to 180 mg L−1 and very hard with 180 or more mg L−1 of calcium carbonate.

Hard water does not present a health risk, as long as the minerals are not heavy metal salts [1], [2]. Therefore, why is hardness such a matter of concern? The answer is that extremely hard water may shorten the life of plumbing and less the effectiveness of certain cleaning agents. Calcium reacts with bicarbonate ions to form insoluble calcium carbonate (scale).Ca2+ (aq) + 2HCO3 (aq)  CaCO3 (s) + H2O + CO2Therefore, depending on pH, hard water may cause scale deposition in the distribution system, heat exchange equipment and boilers. Besides, soap is less effective in hard water because it reacts to form the calcium or magnesium salt of the organic acid of the soap. These salts are insoluble and form grayish soap scum, but no cleansing lather.

Hard water can also leave an unpleasant film on hair, fabrics, and glassware.

In contrast, soft or moderate hard water, with hardness less than about 100 mg L−1, has a greater tendency to cause corrosion of pipes, which may result in the presence of certain heavy metals, such as cadmium, copper, lead and zinc, in drinking-water. The degree to which this corrosion and solubility of metals occurs also depends on the pH, and dissolved oxygen concentration. Some hardness is therefore needed in plumbing systems to prevent corrosion of pipes.

Hardness of water has traditionally been determined by a complexometric titration with ethylene diaminetetraacetic acid (EDTA). Using the indicator Eriochrome Black T, magnesium and calcium will be determined together. Indicator change colour slowly at the end point and thus titration should be performed slowly and carefully.

As the complexometric titration is a laborious and time consuming methodology, there has been attempts to employ a semi-automatic end point detection [3] as well as to develop a flow-batch photometric system which classifies water in the four hardness categories, by monitoring the absorbance at a fix wavelength after the addition of fixed amounts of titrant [4].

Traces of iron and copper block the indicator, and masking agents may be necessary. Fritz et al. [5] recommended a change of indicator.

Ion selective electrodes seam to be an interesting alternative to titration, and Numata et al. [6] used an electrode sensitive both to Ca2+ and Mg2+. In 2002, Saurina et al. [7] have published a potentiometric sensor array for total hardness determination in natural waters. Ion selective electrodes respond to the logarithm of activity rather than concentration and activity coefficients depend on ionic strength. In order to work with concentration, ionic strength must be kept constant, which is usually achieved adding an electrolyte, both to standards and samples. As support electrolyte adds, typically K+, Li+ or amine compounds, Saurina et al. [7] did avoid it.

As acoustic wave sensors respond to mass and ionic strength maintenance is not an issue, they can be an excellent alternative to ion selective electrodes. Besides, acoustic wave sensors can be used without a reference, while ion selective electrodes cannot. Frequency changes are proportional to metal sample concentrations and do not exhibit a logarithm dependence as potentiometric sensors, which is clearly another advantage.

An array of acoustic wave sensors to measure water hardness will be presented here, and results will be compared with those obtained, for the same water samples, by complexometric titrations.

Section snippets

Reagents

Tetrahydrofuran (THF, Merck 8114), magnesium ionophore 1,3,5-tris [10(1-adamantyl)-7,9-dioxo-6,10-diazaundecyl] benzene (Fluka 63112), dioctyl sebacate (Fluka 84818), calcium ionophore 10,19-bis[bis(octadecylcarbamoyl) methoxyacetyl]-1,4,7,13,16-pentaoxa-10,19-diaza cycloheneicosane (Fluka 21203), poly vinyl chloride (PVC, Fluka 81388) and o-nitrophenyloctyl ether (Fluka 73732) were used. Potassium tetrakis (p-chlorophenyl) borate (KTpClPB, Fluka 60591) was added to the membrane composition as

Results and discussion

The sensor for calcium was prepared with one of the membrane compositions previously reported [9]: PVC (34.5%, w/w), DOS (62.1%, w/w), calcium ionophore (3.4%, w/w) and KTpClPB in a relative molar proportion of salt/ionophore of 60%. Fig. 2 shows the selectivity coefficients for the calcium sensor over some other metals, calculated by the fixed interference method.

A new magnesium acoustic wave sensor is here reported for the first time. The sensor for magnesium was coated with a THF solution of

Acknowledgments

This project was financed by the Portuguese Foundation for Science and Technology (FCT), POCTI and FEDER.

Marta I.S. Veríssimo has a post-doc position at the University of Aveiro. She received a BS degree in chemistry in 1998 from the University of Aveiro and a PhD in analytical chemistry in 2003 from the same university. Her current research interests are in chemical sensors and analytical chemistry.

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Marta I.S. Veríssimo has a post-doc position at the University of Aveiro. She received a BS degree in chemistry in 1998 from the University of Aveiro and a PhD in analytical chemistry in 2003 from the same university. Her current research interests are in chemical sensors and analytical chemistry.

João A.B.P. Oliveira is an associate professor at the University of Aveiro. He received his BS degree in chemical engineering from the Technical University of Lisbon in 1976 and a PhD in analytical chemistry from the University of Virginia in 1985. His current research interests are chemical sensors, chemometrics, and laboratory automation.

M. Teresa S.R. Gomes is an associate professor at the University of Aveiro. She received her BS degree in chemical engineering from the University of Coimbra in 1983 and a PhD in analytical chemistry from the University of Aveiro in 1997. Her current interests are chemical sensors and analytical chemistry.

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