POTENTIOMETRIC DETERMINATION OF FLUORIDE

2.1 Total solubilized fluoride is determined potentiometrically using a fluoride ion-selective electrode (ISE) in conjunction with a standard single-junction reference electrode, or a fluoride combination ISE, and a pH meter with an expanded millivolt scale or an ISE meter capable of being calibrated directly in terms of fluoride concentration. 2.2 Standards and samples are mixed 1:1 with a total ionic strength adjustment buffer (TISAB). TISAB adjusts ionic strength, buffers pH to 5-5.5, and contains a chelating agent to break up metal-fluoride complexes. Calibration is performed by analyzing a series of standards and plotting mV vs. fluoride concentration on semilog paper or by calibrating the ion meter directly in terms of fluoride concentration.