Fluoride is considered as one of the essential microelements for humans to be healthy (Bhattacharya et al., 2016; Narsimha & Sudarshan, 2018). It presents in most, if not all body tissues, with the highest levels in bones, dentine and teeth. Smaller quantities in the order of 1.0 mg/L in ingested water are usually considered to have beneficial effects on the rate of avoidance of dental carries, particularly among children (Kanduti et al., 2016; Ullah et al., 2017;). However, excessive intake results in pathological changes to teeth and bones, such as mottling of teeth (dental fluorosis) followed by skeletal fluorosis (Hosokawa et al., 2016; Joseph & Johnson, 2016; Kurdi, 2016). Higher levels of fluoride lead to increases in the levels of dental mottling (de Oliveira et al., 2016; Sebastian et al., 2016; Akuno et al., 2019), and changes in bone structure, namely skeletal fluorosis (Oweis, 2018; Unde et al., 2018; Guth et al., 2020; Wang et al., 2020). Fluorosis is caused by intake of high fluoride predominantly through drinking water containing concentrations more than 1.0 mg/L (Maadid et al., 2017; Narsimha & Sudarshan, 2017; Demelash et al., 2019).
Neel et al. (2016) and Linhares et al. (2019) stated that fluoride accumulates in bones and teeth as fluorapatite and cause bones to become brittle. Other metabolic changes also have been reported in soft tissues such as thyroid, reproductive organs, brain, liver and kidneys (Chiniah, 2017; Luo et al., 2017; Shahab et al., 2017; Malin et al., 2019; Wei et al., 2019). Fluoride may induce periosteal reaction, hyperostosis, osteoporosis, osteosclerosis, osteophytosis or osteomalacia in various combinations (Fossey et al., 2016; Perumal. 2017).
The effect of fluoride is also observed on plants. Excessive accumulation of fluoride in leaves results in the appearance of necrosis at the tips and margins of leaves (Gheorghe & Ion, 2011; Rhimi et al., 2016). Fluoride may induce changes in metabolism, decreased growth and yield, leaf chlorosis and in extreme cases plant death (Choudhary et al., 2019).
Hence it is imperative to monitor the amount of fluoride in water bodies as well as the soil. The content of fluoride in samples can be determined by using several techniques, including potentiometry using a fluoride ion selective electrodes (ISE), ion chromatography (IC), inductively coupled plasma-mass spectrometry (ICP-MS), capillary electrophoresis, solvent-extraction coupled to fluorimetry, polarography and colourimetric techniques based on dyes. Methods based on flow injection analysis (FIA), using different detection methodologies have also been reported; each method with its own advantages and disadvantages (Yahyaviet al. 2016; Walia et al., 2017; Rocha et al., 2018). Potentiometry involves the usage of fluoride ion selective electrode (F−ISE), miniaturized analytical devices, which can deliver real-time and on-line information on the presence of fluoride ions in complex samples (Ameer et al., 2018; Cuartero et al., 2019).
In potentiometry TISABs are required to adjust the pH and ionic strength of the sample solution. In addition any polyvalent cations present in the solution that might interfere with the analysis, need to be removed by complexing them (Tokalioglu et al., 2004, ThermoFisher, 2016; Harhash et al., 2017; Mettler-Toledo, 2018). The theoretical slope of the graph obtained from the calibration standards is 59.2 mV at 25o C for monovalent anions (Dabrowskaet al., 2019; Fakih et al., 2020). The slope of the calibration graph is the mV response per decade of concentration change. Measured slope generally lie in the range 54 ± 5 mV/decade and will have a negative value for negative ions.