Which procedure requires the addition of TISAB to a fluoride sample?

The addition of TISAB, which stands for Total Ionic Strength Adjuster Buffer, is crucial when performing ion-selective electrode measurements for fluoride. TISAB helps to stabilize the ionic strength of the sample, ensuring accurate and reliable readings from the fluoride ion-selective electrode. By doing so, it minimizes interferences from other ions present in the sample and allows for proper calibration and response of the electrode specifically to fluoride ions. This step is essential because the performance of ion-selective electrodes can be affected by various factors such as competing ions or variations in sample composition.

In contrast, other procedures listed do not necessitate the use of TISAB for fluoride analysis. Standard colorimetric analysis typically involves different reagents and methods for detecting fluoride that do not require such a buffer adjustment. Conductivity testing focuses on measuring the overall conductivity of a solution rather than specific ion concentrations. Lastly, a chlorine residual test is concerned with measuring chlorine levels and doesn't involve fluoride measurement at all. Therefore, TISAB is specifically relevant to the ion-selective electrode method due to its role in enhancing measurement accuracy for fluoride ion detection.