Soft test for hard water

A comparison of definitive atomic absorption spectroscopic and complexiometric titration results with a new approach to determining the hardness of water reveals the new technique to be simpler and less costly to carry out but just as accurate, according to a study by researchers in Spain. The research could ultimately lead to an inexpensive and portable hard water sensor.

Teresa Mairal Lerga and Ciara K. O'Sullivan of the Nanobiotechnology & Bioanalysis Group, of the Chemical Engineering Department, at the University of Rovira I Virgili, and ICREA, Spain, hoped to find an alternative to AAS that could be used in quality control for bottled waters. They turned to molecular biology as one possible solution to the issue of hard water determination.

Hard and soft water differ in their mineral content. Hard water usually has a relatively high concentration of calcium and magnesium ions as well as counterions, usually in the form of bicarbonates and sulfates. Claims as to the benefits of hard water on health as well as other factors have led to a growing market for bottled water of specific mineral content.

Moreover, in industry water hardness can reduce profits as the formation of limescale deposits in boilers, cooling towers and other industrial equipment reduces efficiency and leads to premature breakdowns. As such, the hardness of an industrial water supply has to be monitored constantly to help mitigate such effects. As such, there is a QC/QA context for a simple analytical method for determining overall hardness.

"The standard method for the determination of water hardness is complexiometric titration," say the researchers, "but this is affected by other metal ions, is time-consuming, and laborious." Likewise, "AAS is also used for the determination of hardness in water but is lengthy and necessarily laboratory based technique."

Of course, anyone who has gone from a hard water area to a soft water area of vice versa will be well aware of the most straightforward test of water's hardness while bathing - soap will form lather very readily in soft water but not in hard water.

Mairal Lerga and O'Sullivan have used a doubly labelled synthetic oligonucleotide as a fluorescent beacon for measuring total hardness of tap and bottled waters. Artificial oligonucleotides have been used for many years in therapeutics, diagnostics and bioanalytical applications and are easy to handle. Additionally, these molecules are temperature resistant, do not degrade with pH or in the presence of standard reagents. The molecule in question is a modified thrombin binding aptamer (5'-NH-C3-GGTTGGTGTGGTTGG-C3-SH-3') bearing a fluorescent 6-carboxyfluorescein (FAM) and 7-amino-4-methyl-coumarin labels at the 5' end and the 3', respectively.

In the presence of the main hard water ions, calcium and magnesium, this molecule forms a complex with those ions, which quenches its fluorescence. A change that is easily measured and thence correlated with the total concentration of those ions.

Their approach precludes interference from sodium, potassium and other "non-hardening" ions in water, something that earlier attempts at hardness detectors have not been able to do wholly satisfactorily. "The detection limit of the aptamer beacon is 0.04 mmol per litre," the researchers say, which compares well with EDTA titration and AAS results. "The molecular beacon provides a reagent less, reproducible, sensitive, easy to use assay, with the entire assay being completed within 30 seconds" the researchers add. The cost of the test is around 0.05 euros per assay.

Critically, the assay is highly reproducible and works equally well with real tap and bottled water samples and the correlation with AAS and complexiometric titration is "excellent", say the researchers. They add that their molecular beacon can be easily formatted for use in situ. They explain that they are now focusing on finding a method of immobilizing the aptamer so that their test could be developed into a simple re-usable sensor for hard water determination.