For Calibration of pH Combination Electrodes (standardized to 20 °C)
The electronics of the pH meter must be adapted to the special characteristics of the pH meter used by calibration. Not only different meters vary from each other (zero point, slope), but the same meter also changes due to ageing processes.
Precise and reproducible PH measurements are therefore only possible when calibration is carried out regularly and carefully.
The frequency of this procedure naturally depends on the place of use and the purpose. If measurement is carried out under greatly changing conditions, calibration is recommended before every measurement.
In the case of constant measurement in the aquarium, it is sufficient to calibrate every 2 - 4 weeks if nothing else has changed. A new meter or one at the end of its service life will change to a greater degree and therefore calibration will be necessary more often than in the middle of the expected service life.
Selection of the buffers
For two-point calibration, the two buffer solutions are selected in such a way that the pH to be measured is between the values of the two buffers where possible.
The ability to calibrate pH 4 and pH 9 to cover the whole pH range is only seemingly advantageous. Due to non-linearities of the meter and of the measuring amplifier, however, it leads to significant errors.
Temperature-dependence of the pH at various temperatures
As can be seen, it is very important to observe the correct temperature during calibration.
|Temperature||Buffer pH 4||Buffer pH 7||Buffer pH 9|
Handling buffer solutions
To ensure exact calibration, buffer solutions must be handled carefully. All dirt is to be avoided.
Therefore, never pour buffer solutions back into the storage bottle or calibrate directly in the storage bottle. Used buffer solutions are disposed of and not used a second time.
In the case of alkaline buffer solutions (pH > 7), the CO2 in the air causes a gradual neutralisation of the buffer. The storage bottles should therefore be resealed as soon as possible after use and contact with air reduced to a minimum.
Composition of the buffer solutions offered
pH 4 (20 °C) d204 = 1.007 (~ 0.056 m citric acid solution / ~ 0.068 m NaOH solution / ~ 0.044 m NaCl solution)
pH 7 (20 °C) d204 = 1.006 (~ 0.029 m NaOH solution / ~ 0.050 m KH2PO4 solution)
pH 9 (20 °C) d20 4 = 1.001 (~ 0.013 m Na2B4O7 solution / ~ 0.0046 m HCl solution)
(NaOH - sodium hydroxide, NaCl - sodium chloride, KH2PO4 - potassium dihydrogen phosphate, Na2B4O7 - sodium borate or borax, HCl - hydrochloric acid)
The solutions and chemicals used are non-toxic (at least in small quantities). Nevertheless, the electrodes should be thoroughly rinsed with demineralised water before being re-used in the aquarium. The buffers and rinsing solution can be poured directly down the drain without reservations.