Instruction Manual
Iodide 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 into the solution. When the reading has stabilized,
record the mV reading. Using the calibration curve, 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 Steps 2-5 above. A new calibration curve should be prepared daily.
Direct Measurement of iodide (using an ion meter)
1.
By serial dilution of the 0.1M or 1,000 ppm iodide standard, prepare two standards whose
concentration is near the expected sample concentration. Measure 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.
3.
Adjust the meter to the concentration of the standard and fix the value in the memory
according to the meter manufacturer's instructions after stabilization of the reading.
4.
Rinse the electrodes with distilled water and blot dry.