ISEmax CAS40D
Calibration
Hauser
13
5
Calibration
When determining ion concentrations potentiometrically, the voltage supplied by the
electrochemical measuring cell (consisting of the ion-selective electrode and a reference electrode)
within the "linear" or preferably "NERNST“ range is proportional to the logarithm of the
concentration (or activity) of the ions to be determined.
5.1
Calibration modes
The following types of calibration are possible:
• pH electrode:
– Two-point calibration
– Sample calibration
• Ion-selective electrodes:
– Single-point calibration
– Two-point calibration
– Standard addition
– Sample calibration
– Data entry
• ORP sensor:
– Single-point calibration
• Temperature adjustment by entering a reference value
5.2
Calibration and adjustment of ion-selective electrodes
5.2.1
Sequence for calibration/adjustment of measuring point
Some measured values from other electrodes or sensors are used for measured value compensation
of ion-selective electrodes:
• Measured value of temperature sensor for temperature compensation
• pH measured value for pH compensation of ammonium (optional)
• Potassium or chloride measured value for compensation of interference ions in the case of
ammonium or nitrate (optional)
For this reason, there is a sequence for calibration and adjustment that must be followed in order to
achieve a reliable measurement:
1.
Temperature adjustment
2.
Calibration and adjustment of pH electrode
3.
Calibration and adjustment of ion-selective compensation electrodes (potassium, chloride)
4.
Calibration and adjustment of ion-selective measuring electrodes (ammonium, nitrate)
5.2.2
Specific information
In general
For sample calibration, single- and two-point calibration, the following minimum concentrations
apply:
• 1 mg/l ammonium or ammonium nitrogen
• 1 mg/l nitrate nitrogen or 4 mg/l nitrate
• 5 mg/l potassium
• 10 mg/l chloride
The values are benchmarks which may change over time due to the influence of interference ions
or the aging of ion-selective electrodes.
