Thermo Fisher Scientific Ion Selective Electrodes Fluoride - Manual (Page 11)

Instruction Manual Fluoride Electrode
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Low Level Fluoride Measurements (using a pH/mV meter)
Use the following low level fluoride measurement procedure in the non-linear portion of the
calibration curve. (See Figure 1.) This procedure is used for fluoride samples containing less than
2X10
-5
M or 0.4 ppm fluoride and containing no fluoride complexing agents. Use low level TISAB
2 for both samples and standards. A longer response time should be expected for low level fluoride
measurements.
1. By serial dilution, prepare a 1.0X10
-3
M or 10 ppm fluoride standard by diluting the 0.1M,
1,000 ppm, or 100 ppm standard solution. Add 50 ml low level TISAB 2 to 50 ml of
standard solution.
2.
Using a 150 ml beaker, add 50 ml of distilled water and 50 ml of low level TISAB 2.
Place the beaker on the magnetic stirrer and begin stirring at a constant rate. Lower the
electrode tips into the solution. Assure that the meter is in the mV mode.
3.
Increments of the standard should be added to the beaker according to the steps outlined in
Table 2 below. After the reading stabilizes, record the mV reading for each addition.
TABLE 2: Low Level Measurement Calibration Curve
Added Concentration
Step Pipet Volume (ml) ppm M
1 A 0.1 0.01 1.0X10-6
2 A 0.1 0.02 2.0X10-6
3 A 0.2 0.04 4.0X10-6
4 A 0.2 0.06 6.0X10-6
5 A 0.4 0.10 1.0X10-5
6 B 2.0 0.29 2.9X10-5
7 B 2.0 0.48 4.8X10-5
Pipet A = 1 ml graduated pipet
Pipet B = 2 ml pipet
Solutions: additions of standard / TISAB 2 to 50 ml of distilled water
and 50 ml of low level TISAB 2.
4. On semi-logarithmic graph paper, plot the concentration (log axis) against the millivolt
reading (linear axis) as in Figure 1. Keep the final solution for checking the electrode
calibration.
5.
To a 150 ml plastic beaker, add 50 ml of sample and 50 ml of low level TISAB 2. Place the
beaker on a magnetic stirrer and begin stirring. After rinsing the electrodes and blotting
dry, place the electrode tips into the solution. After stabilization of the reading, read the
mV potential and determine the concentration from the calibration curve. A new low level
calibration curve should be prepared daily using fresh standards.
Low Level Fluoride Determination (using an ion meter)