It has been explained before that no pH electrode assembly responds ideally to the Nernst potential. The reason lies in the behavior of the various potential sources of the electrode assembly as well as their response to temperature change and the changes of these parameters with time.
In order to use a single electrode assembly (separate measuring electrode and reference electrode) or a combination pH sensor effectively and to perform an accurate and repeatable pH measurement, the pH meter/transmitter has to be adapted to the characteristics of the
applied electrode assembly.
Consider that it is not actually the pH sensor that is calibrated, it is the mV measurement instrument which is adjusted in order to compensate for the imperfection of any utilize sensor. The performance properties of any portion of electrode assembly cannot be changed at will.
The adaptation of the pH measuring instrument to the performance characteristics of the electrode assembly is done by calibrating the pH measuring system (pH sensor plus instrument) against liquid buffer solutions having definite pH values.
A brief description of the calibration procedure for modern microprocessor-based pH meters follows below.
Please note: For the purposes of this article the pH electronics and the sensor assembly will be treated separately. Hamilton has gone one step further and incorporated the microprocessor directly into the sensor head of our Arc Intelligent pH sensors. Arc pH sensors have no separate pH meter but instead rely upon ArcAir Software for all configuration and calibration procedures for the sensor. Consult your local Hamilton contact to learn more.
Calibration of Microprocessor-based pH Meters
Microprocessor-based pH meters have largely replaced traditional analog pH meters in laboratories and as on-line instruments in industrial plants. Even portable pH meters are nowadays microprocessor controlled. Their handling is relatively simple as there are no potentiometers to adjust. Normally the operator is menu-guided through the procedure of his task. These instruments give less reasons for wrong handling and therefore less measuring errors.
The calibration procedure uses two buffer solutions that should have a difference of at least 2 pH units or greater. It is not necessary to calibrate the zero point with buffer 7. The two buffer solutions required should approximate the start and the end of the selected measuring span for the process. The zero point, the slope and even the isopotential point are all calculated by the microprocessor during the different stages of calibration.
In general the two point pH calibration is performed as follows:
- Two buffer solutions of different pH values are selected. The pH value of one buffer solution should lie somewhere in the region of the beginning of the desired measurement span and the pH value of the second buffer solution should lie somewhere near the end of the measurement span. Hamilton Buffer Solutions can be found here.
- Rinse the pH sensor in deionized water prior to immersing in the first buffer solution. This removes any residual liquids that could influence the calibration accuracy. After the calibration mode of the pH meter is activated, the calibration menu guides the operator through the calibration procedure. It is typical that the sensor has to stay in the buffer solution for a short time (normally one to three minutes) until the meter can settle on the actual pH value. Sometimes the buffer values still have to be entered manually, however very often the microprocessor identifies the buffer values automatically from a preset table stored in the device memory.
- After the first buffer calibration, the sensor should be rinsed again with deionized water and dried with tissue paper. When drying the sensor, care must be taken not to rub the membrane, i.e. only dab the electrode with tissue paper. Under no circumstances must an electrode be rubbed. This could introduce static electricity into the glass shaft of the sensor which could upset the accuracy of the pH calibration for hours.
Additional notes on this procedure:
The temperature of the buffer solutions must be measured; either with the integral temperature element within the pH sensor or separately with a dedicated RTD. The temperature values are either entered manually into the meter or, detected automatically with the incorporated temperature element.
In order to keep the temperature error as low as possible, especially errors due to the diffusion potential and the isopotential point, it is recommended that the calibration be executed at the temperature at which the actual pH measurement will be performed.
After the successful calibration the used buffer solution should be discarded. Never reuse a buffer solution and never return it to its original storage container.