Thermo Fisher Scientific Ion Selective Electrodes Fluoride - Manual (Page 8)

Instruction Manual Fluoride Electrode
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3.
Place the mid-range solution on the magnetic stirrer and begin stirring. After rinsing the
electrodes with distilled water, blot dry, and immerse the electrodes in the solution. When
the reading has stabilized, record the mV value.
4.
Place the most concentrated solution on the magnetic stirrer and begin stirring. After
rinsing the electrodes with distilled water, blot dry, and immerse the electrodes in the
solution. When the mV reading has stabilized, record the mV value.
5.
Using the semi-logarithmic graph paper, plot the mV reading (linear axis) against the
concentration (log axis). Extrapolate the curve down to about 1.0X10
-5
M. A typical
calibration curve can be found in Figure 1.
A calibration curve is constructed on semi-logarithmic paper when using the pH/mV
meter in the millivolt mode. The measured electrode potential in mV (linear axis) is
plotted against the standard concentration (log axis). In the linear region of the
curve, only three standards are necessary to determine a calibration curve. In the
non-linear region, additional points must be measured. The direct measurement
procedures given are for the linear portion of the curve. The non-linear portion of the
curve requires the use of low level procedures.
6.
To a clean, dry, 150 ml plastic beaker, add 50 ml of sample and 50 ml of TISAB 1 or
TISAB 2. Place the beaker on the magnetic stirrer and begin stirring. Rinse the electrodes
with distilled water, blot dry, and lower the electrode tips into the solution. When the
reading has stabilized, record the mV reading. Using the calibration curve, determine the
sample concentration.
7.
The calibration should be checked every 1-2 hours. Assuming no change in ambient
temperature, place the electrode tips in the mid-range standard. After the reading has
stabilized, compare it to the original reading recorded in Step 3 above. A reading differing
by more than 0.5 mV or a change in the ambient temperature will necessitate the repetition
of Steps 2-5 above. A new calibration curve should be prepared daily.