Foxcroft FX-300-F, Инструкция по эксплуатации

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2
2.0 Fluoride Ion Selective Electrodes
The combination fluoride sensor includes both the measuring and reference electrodes with
electrolyte, electronics and a high-impedance PVC organic membrane system in a sealed plastic
body. As such there are no replaceable or serviceable items in the sensor. The only service required
is occasional cleaning and calibration.
The recommended pH range of the general purpose fluoride sensors is 5.5 to 9.5pH on a continuous
basis for optimum sensor lifetime. We recommend a pH less than 9.5 to minimize aging on the
crystal. Sensors will start to have some loss of linearity at pH 10.
Solutions at pH 11 will destroy
the fluoride crystal.
It is not recommended to operate the general purpose drinking water sensors below 5.5 pH. If
the fluoride sensor must operate below 5.5 pH you must contact the factory for assistance in
selecting a different sensor or sample point for your application.
At pH levels below 5.5, the fluoride sensor will not detect the total fluoride content, as some of the
fluoride ion will be converted into the form of dissolved HF gas. To convert the measured fluoride ion
activity (free fluoride) into a total fluoride reading the free fluoride measurement must be
compensated for the effect of pH using the FX-300-TOT module and a pH sensor with FX-300 pH
measurement module. See the pH dependent extent of ionization curve for HF dissolved gas and
fluoride ions (F) for a graphical representation of this phenomenon.
If you have purchased the FX-300-TOT module, you will be able to compensate for the pH induced
effects on the extent of ionization to find the “total unbound free fluoride” as defined by the sum of the
free ionized fluoride ion species together with the protonated HF bound form.
Please also note that these pH effects are a temperature dependent phenomenon. The provided
extent of ionization curve is only completely valid for pure two component systems with deionized (DI)
water. Real world water solutions of a much more complex makeup may vary somewhat from these
idealized curves, although the deviation is not expected to be vast for most typical systems.
2 Introduction