Thermo Fisher Scientific Ion Selective Electrodes Sodium - Manual (Page 7)

Instruction Manual Sodium Electrode
5
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 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 a 150 ml beaker, add 100 ml of sample and 2 ml of ISA. Place the beaker
on the magnetic stirrer and begin stirring. After rinsing the electrode tips with electrode
rinse solution, blot dry and place the electrode tips in 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, 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
difference 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 Sodium (using an ion meter)
1. By serial dilution of the 0.1M or 1,000 ppm sodium standard, prepare two sodium
standards whose concentration is near the expected sample concentration. Add 2 ml of ISA
to each 100 ml of standard.
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.
3.
Lower the electrode tips into the solution. Adjust the meter to the concentration of the
sodium 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 electrode rinse solution and blot dry. Place the more concentrated
solution on the magnetic stirrer and begin stirring at a constant rate.
5.
Lower the electrode tips into the solution.
6.
Adjust the meter to the concentration of the sodium standard and fix the value in the
memory according to the meter manufacturer's instructions after stabilization of the
reading.
7.
For low level measurements, place the rinsed, dried electrodes into a solution containing
100 ml of distilled water and 2 ml of ISA. After stabilization, fix the blank value in the
meter according to the meter manufacturer's instructions.
8.
Place 100 ml of the sample and 2 ml of ISA in a 150 ml beaker. Place the beaker on the
magnetic stirrer, and begin stirring.