Thermo Fisher Scientific Ion Selective Electrodes Sodium - Manual (Page 9)

Instruction Manual Sodium Electrode
7
8. On semi-logarithmic graph paper, plot the millivolt reading (linear axis) against the
concentration (log axis) as in Figure 1.
9.
Rinse the electrodes with electrode rinse solution and blot dry.
10. To a 150 ml plastic beaker add 100 ml of sample and 1 ml of low level ISA. Place the
beaker on the magnetic stirrer and begin stirring. Adjust the pH, if necessary, to above 9.
Lower the electrode tips into the solution. After the reading has stabilized, record the mV
reading and determine the concentration from the low level calibration curve.
11. Prepare a new low level calibration curve daily. Check the calibration curve every two
hours by repeating Steps 3-8.
Low Level Sodium Determination (using an ion meter)
Follow the procedure given for sodium measurements with an ion meter. Always use the blank
correction procedure. Use plastic lab-ware for low level sodium measurement.
ELECTRODE CHARACTERISTICS
Reproducibility
Electrode measurements reproducible to
±2% can be obtained if the electrode is calibrated every
hour. Factors such as temperature fluctuations, drift, and noise limit reproducibility. Reproducibility
is independent of concentration within the electrode's operating range.
Interference
Table 3 lists some common cations that, if present in high enough levels, will cause electrode
interference and measurement errors or electrode drift when using the sodium ion electrodes.
Most samples do not contain or contain very little of the cations shown in Table 3. The ammonium
ion (NH
4
+
), found in the recommended ISA, will not result in an error if all samples and standards
have the same level of ISA present.
Electrode drift and slow response could indicate the presence of high interference from the ions
listed. Soak the electrodes in electrode storage solution when this happens to restore proper
response. See
Measuring Hints
section.
TABLE 3: Levels of Interfering Ions Resulting in a 10% Error at Specified Levels of Sodium
Interference
1.0x10-
4
M 1.0x10-
3
M 1.0x10-
2
M
Li
+1
5X10-
4
M 5X10-
3
M 5X10-
2
M
K
+1
1X10-
2
M 1X10-
1
M 1M
Rb
+1
3X10-
1
M 3M --
NH
4
+1
3X10-
1
M 3M --
Ag
+1
3X10-
9
M 3X10-
8
M 3X10-
7
M
Tl
+1
5X10-
2
M 5X10-
1
M --
Interference 1 ppm 10 ppm 100 ppm
Li
+1
1.5 ppm 15 ppm 150 ppm
K
+1
17 ppm 170 ppm 1,700 ppm