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

Instruction Manual Chloride Electrode
3. Place the mid-range solution (1.0x10
-3
M or 100 ppm) on the magnetic stirrer and begin
stirring. After rinsing the electrodes with distilled water, blot dry and immerse the
electrode tips in the solution. When the reading has stabilized, record the mV reading.
4.
Place the most concentrated solution (1.0x10
-2
M or 1,000 ppm) on the magnetic stirrer and
begin stirring. After rinsing the electrodes with distilled water, blot dry and immerse the
electrode tips in the solution. When the reading has stabilized, record the mV reading.
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 or 1 ppm. 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 100 ml of sample and 2 ml of ISA. Place the beaker on
the magnetic stirrer and begin stirring. Place the electrode tips in the solution. When the
reading has stabilized, record the mV reading. Determine the concentration directly from
the calibration curve.
7.
The calibration should be checked every two 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 Step 2-5 above. A new calibration curve should be prepared daily.
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