Effect of humic substances aggregation on the determination of fluoride in water using an ion selective electrode

• Humic substances (HS) tend to aggregate at low pH and high ionic strength.
• HS aggregates are shown to interfere in fluoride measurements.
• An “open cage” model explains the interaction between fluoride and HS.

The control of drinking water quality is critical in preventing fluorosis. In this study humic substances (HS) are considered as representative of natural organic matter (NOM) in water. We show that when HS aggregate the response of fluoride ion selective electrodes (ISE) may be perturbed. Dynamic light scattering (DLS) results of both synthetic solutions and natural water sample suggest that low pH and high ionic strength induce HS aggregation. In the presence of HS aggregates, fluoride concentration measured by ISE has a reduction up to 19%. A new “open cage” concept has been developed to explain this reversible phenomenon. The interference of HS aggregation on fluoride measurement can be effectively removed by centrifugation pretreatment.

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