pH value
The pH value describes the acid-base balance of the water and directly controls the HOCl share (active chlorine) of the free chlorine. A working range of about 7.0–7.4 combines a still high disinfecting power with acceptable compatibility for materials and skin. Above pH 7.5, the share of active chlorine (and thus of the actual disinfectant in the water) drops significantly.
Carbonate hardness / alkalinity / TA
Carbonate hardness / alkalinity / TA (in °kH or ppm) is the buffer capacity against pH changes. It determines how much acid or base is needed to shift the pH. Manufacturer information on pH adjusters typically refers to about 15 °kH (approx. 270 ppm); the actual dosage therefore differs greatly depending on the real carbonate hardness (alkalinity).
Carbonate hardness that is too high makes pH correction consume a lot of product. Hardness that is too low – below about 2 °kH (approx. 36 ppm) – leads to an unstable pH. Even small amounts of pH minus, rainfall or the use of the pool itself can then cause large pH changes or trigger a pH crash. In addition, photometer measurements become unreliable: phenol red photometer readings and electrode readings can differ by 0.2–0.5 pH units. In this case, a calibrated pH electrode is always preferable, as its measurement does not depend on the carbonate hardness of the water.
How carbonate hardness drops and how to raise it
Every dose of pH minus reduces carbonate hardness. Without regular replacement with fresh water (backwashing and topping up) or deliberate raising, alkalinity drops towards zero within a few weeks. Sodium bicarbonate is suitable for raising it: about 30 g/m³ raises carbonate hardness by 1 °kH (approx. 17.8 ppm).
Caution with softened fill water
Caution is required with fill water from water softeners (ion exchangers): photometric alkalinity measurements (ALKA-M indicator) also detect sodium ions, which do not contribute to the buffer capacity – the result will then suggest a seemingly sufficient carbonate hardness. Here, determination by titration (drop test for carbonate hardness, not alkalinity) is the preferable method.
Conclusion
pH value and carbonate hardness are therefore not secondary parameters, but the prerequisite for a sufficient share of active chlorine to form from the measured free chlorine at all and to be kept stable.