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

Instruction Manual Potassium Electrode
9
TABLE 2: Stepwise Calibration for Low Level Potassium Measurements
Added Concentration
Step Pipet Volume (ml) M ppm
1 A 0.1 1.0X10-6 0.1
2 A 0.1 2.0X10-6 0.2
3 A 0.2 4.0X10-6 0.4
4 A 0.2 6.0X10-6 0.6
5 A 0.4 9.9X10-6 1.0
6 B 2 2.9X10-5 2.9
7 B 2 4.8X10-5 4.8
Pipet A = 1 ml graduated pipet
Pipet B = 2 ml pipet
Solutions: additions of 1.0x10
-3
M or 100 ppm standard to 100 ml of distilled water
7.
On a semi-logarithmic graph paper, plot the mV reading (linear axis) against the
concentration (log axis) as in Figure 1.
8.
Rinse the electrode(s) in distilled water and blot dry.
9.
Measure out 100 ml of the sample into a 150 ml beaker, add 1 ml of low level ISA, and
place the beaker on the magnetic stirrer. Begin stirring. Lower the electrode tip(s) into the
solution.
10. 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 1-2
hours by repeating Steps 3-7 above.
Low Level Potassium Determination (using an ion meter)
Follow the procedure given for normal potassium determinations using an ion meter and the blank
correction procedure.
ELECTRODE CHARACTERISTICS
Reproducibility
Direct 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.
Interferences
Table 3 lists some common cations that, if present in high enough levels, will cause electrode
interferences and measurement errors or electrode drift when using the potassium ion electrodes.