Thermo Fisher Scientific Ion Selective Electrodes Lead - Manual (Page 6)

3. Adjust the meter to the concentration of the standard and fix the value in the memory
according to the meter manufacturer's instructions.
4.
Rinse the electrodes with distilled water and blot dry.
5.
To another 150 ml beaker, add 50 ml of methanol-formaldehyde solution and 50 ml of the
higher value standard. Add 2 ml of ISA. Place the beaker on a magnetic stirrer and begin
stirring at a constant rate. Lower the electrode tips into the solution.
6.
Adjust the meter to the concentration of the standard and fix the value in the memory.
7.
Read the electrode slope according to the meter manufacturer's instructions. Correct
electrode operation is indicated by a slope of 90-100%. See the
TROUBLESHOOTING
sections if the slope is not within this range.
MEASUREMENT
Measuring Hints
All samples and standards should be at the same temperature for precise measurement. A difference
of 1
o
C in temperature will result in a 4% measurement error.
Constant, but not violent stirring is necessary for accurate measurement. Magnetic stirrers can
generate sufficient heat to change the solution temperature. To counteract this effect, place a piece
of insulation material, such as styrofoam sheet, between the stirrer and beaker.
Use plastic lab-ware for all low level measurements in order to minimize absorption on container
walls. Always rinse the electrodes with distilled water and blot dry between measurements. Use a
clean, dry tissue to prevent cross-contamination.
To prevent oxidation of the membrane, always use methanol-formaldehyde solution to mix with all
standards and samples.
For samples with high ionic strength, prepare standards whose composition is similar to the sample.
Dilute concentrated samples (>0.1M) before measurement.
Use fresh standards for calibration. Use 2 ml of ISA for each 100 ml of sample or standard.
Always check to see that the membrane is free from air bubbles after immersion into the standard or
sample.
Sample Requirements
All samples must be aqueous and not contain organics which can dissolve the epoxy electrode body
and/or the cement bonding the sensing crystal to the electrode body. Infrequent measurements in
solutions containing methanol, benzene, or acetonitrile are permitted. Highly polar solvents slowly
attack the electrode. Please check with Eutech Instruments Pte Ltd. before using these electrodes in
other organic solvents.