THE MEASUREMENT OF FLUORIDE IN WATER

A rapid and simple method for the measurement of fluoride concentration in water in the range of 0.1 - 100ppm.

Equipment Required

1.   Model DR359TX Ion Meter

2.   3221 Fluoride Combination ISE

3.   21333 Fluoride 1000ppm Standard Solution

4.   30333 Total Ionic Strength Adjustment Buffer (TISAB)

5.   Glassware. Beakers 250ml, Volumetric Flask 100ml. 10ml pipette

6.   Deionised water.

Standard Preparation

Prepare standards of 100, 10, 1 and 0.1ppm  Fluoride  by serial dilution of the 21333 Fluoride 1000 ppm standard. This is achieved by pipetting 10ml of the  top standard into a 100ml volumetric flask and diluting to the mark with deionized water. This is now a 100ppm Standard solution. Repeat in series to prepare the required standards.

Method

To 50ml of each of the prepared standards and samples add 10ml of 30333 TISAB and mix the contents. Be sure the beakers are clean and make sure not to touch the inside of the beakers with bare hands as contamination from sweat etc. is common. Beakers that have been washed with softened water or tap water will be contaminated. In these cases rinse the beakers with deionised water.

Immerse the 3221 Fluoride Combination electrode in each of the standards in increasing concentration steps in the calibration mode of the Ion Meter, rinsing the electrodes with distilled water and dabbing off the excess water between standards. 

Using the DR359TX Ion meter will allow the result to be read directly on the display. It also allows for up to a 5 point calibration if required.

If you do not have an ION meter you can read the mV values using a pH/mV meter such as an EDT model FE257. Record the mV response and plot a graph of mV vs log of Concentration.

Calculations

As both standards and samples have been diluted by the same amount the result obtained from the meter is the concentration of the original sample.