BS 6068-2.41:1993

1.1 Field of application

This part of ISO 10359 specifies a method for the determination of dissolved fluoride in fresh, potable and low contaminated water, and some surface waters, using an electrochemical technique.

The method is directly suitable for measuring fluoride concentrations from 0,2 mg/l to 2,0 g/l. After the addition of a known amount of fluoride, concentrations as low as 0,02 mg/l can be detected (see 7.3).

The method is not suitable for waste waters and industrial effluents; this determination will be the subject of ISO 10359-2.

1.2 Interferences

The electrode will respond directly to hydroxide ions. The formation of HF under acidic conditions will reduce the measured fluoride concentration. Therefore, buffer all test aliquots to a pH between 5 and 7 to prevent such interference. Cations such as calcium, magnesium, iron and aluminium form complexes with fluoride or precipitates to which the electrode does not respond. Therefore the buffer solution also contains trans-1,2-diaminocyclohexane-N,N,Nʹ,Nʹ-tetraacetic acid (CDTA) as a decomplexing agent to free bound fluoride. The boron tetrafluoride anion, BF⁻₄, is not decomplexed by the addition of buffer.