Several methods are used to measure pH to prevent it dropping to critical values. A simple but inaccurate way is to observe the colour change of pH paper, also known as litmus paper, when it is dipped into water. This paper contains a chemical, the colour of which is pH dependent. Similarly, a small water sample can be taken, a specially prepared liquid added to it and the colour change observed.
The pH can also be measured by a standard chemical analysis with titration and observing the change in colour, which is pH dependent. This system may be implemented for daily measure-ments on a fish farm and has the advantages of requiring low maintenance and being reliable. The disadvantages are, however, that this takes some time and experience of working with chemical equipment, and measurements in a laboratory are necessary.
A pH meter is constructed on the same principle as the oxygen meter. Here the probe is constructed with two electrodes, one for measuring the concentration of H+ and the other as a reference electrode. In this case the pH electrode consists of a membrane permeable to hydrogen ions. Here also the membrane creates a cell that encloses the electrolyte. Between the electrodes a current will pass that depends on the concentration of H+. The voltage between the pH electrode and the reference electrode is then measured, and the pH calculated.
Instruments for measuring pH must be calibrated before use. This is performed by placing the probe in different solutions of known pH and then adjusting the instrument. The pH probe is compact, in contrast to the O2 probe; therefore it must be completely replaced if it ceases to function. A disadvantage of the pH probe is its limited duration, normally from 3 months up to 1 year in special cases. Practical experience with the use of pH meters in fish farming shows that maintenance is of extreme importance. Both calibration and changing of electrodes must be done regularly to achieve reli-able measurements.
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