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

5.
Using the semi-logarithmic graph paper, plot the mV reading (linear axis) against the
concentration (log axis). Extrapolate the calibration curve down to about 2.0X10
-6
M. A
typical calibration curve can be found in Figure 1.
A calibration curve is constructed on semi-logarithmic paper when using a
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 beaker, add 50 ml of the sample, 50 ml of methanol-formaldehyde
solution, and 2 ml of ISA. Place the beaker on the magnetic stirrer and begin stirring at a
constant rate. 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 two hours. Assuming no change in ambient
temperature, immerse 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.