Instruction Manual
Cadmium Electrode
7
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 the sample and 2 ml of ISA. Place the beaker
on the magnetic stirrer and begin stirring at a constant rate. Rinse the electrode tips with
distilled water, blot dry and lower the electrode tips into the solution. After 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.
Direct Measurement of Cadmium (using an ion meter)
1.
By serial dilution of the 0.1M or 1,000 ppm cadmium standard, prepare two cadmium
standards whose concentration is near the expected sample concentration. Measure out 100
ml of each standard into individual 150 ml beakers and add 2 ml of ISA to each.
2.
Place the more dilute solution on the magnetic stirrer and begin stirring at a constant rate.
Assure that the meter is in the concentration mode. Lower the electrode tips into the
solution.
