Fluoride is widely added to drinking water systems to help prevent tooth decay. It is normally added in the form of liquid hydrofluorosalicic acid, which can be easily handled and applied using standard metering pumps. While the fluoridation process is often controlled by simple flow proportional feed, monitoring of final fluoride concentrations is useful in providing an alarm in the case of overfeed problems. Optimum fluoride levels of around 1 PPM are safe, but control system problems resulting in concentrations above 2 PPM are considered excessive and need to be detected as early as possible. Loss of chemical feed can also be detected quickly and reliably with on-line fluoride monitoring.
ATI’s Model A15/82 Fluoride Monitor provides continuous measurement of free fluoride concentration in potable water. The system employs a fluoride sensitive ion selective electrode (ISE), which provides reliable measurements down to 0.1 PPM and as high as 1000 PPM. A chemistry module provides sample conditioning for the sensor, and the measured fluoride concentration is displayed on a separate electronics module that also provides alarm and analog output functions.
Fluoride ISE sensors measure F-ion in solution the same way that a pH sensor measures hydrogen ions. A lanthanum fluoride crystal on the tip of the sensor develops a voltage that is proportional to fluoride ion activity. An integral reference electrode is used as a comparator to measure the developed voltage, with the measurement made at a differential input amplifier. Since the activity of fluoride ions in solution is a function of pH and ionic strength, a small amount of buffer solution is added to the measured sample. This creates a stable condition in which the concentration of fluoride ion and the activity of fluoride ion are directly proportional.
The A15/82 Fluoride Monitor requires very little maintenance. One gallon of buffer will operate the system for approximately 45 days. The system also performs an automatic calibration routine to correct for electrode “zero” drift. The fluoride standard is automatically pumped into the chemistry module and the analyzer zero is automatically adjusted. The frequency of this calibration is user-selectable from every 1 to 999 hours. Calibration standards are easily mixed from a stock solution provided with the unit, and the buffer is prepared using common vinegar.