What must be added to samples prior to using an ion selective electrode to measure fluoride?

Using an ion selective electrode (ISE) to measure fluoride requires specific sample preparation to ensure accurate readings. The Total Ionic Strength Adjustment Buffer (TISAB) is crucial in this process. TISAB is added to samples to help maintain a constant ionic strength and to release fluoride ions from any complexes or precipitates they may be forming with other substances in the sample.

By including TISAB, the electrode can more accurately respond to the fluoride ions because it creates a consistent environment that accounts for the varying concentrations of other ions present in the sample. This is vital for reliable measurements, as fluctuations in ionic strength can lead to erroneous readings. TISAB typically contains buffers to adjust the pH and other components to ensure that the fluoride is in a form that the ion-selective electrode can accurately detect.

Other options involve various scenarios with different ions or chemical adjustments, but they do not provide the same level of control over ionic strength or fluoride release as TISAB does in the context of fluoride measurement.