pH: the position
pH measures the concentration of hydrogen ions on a logarithmic scale from 0 to 14. Neutral is 7; each whole unit is a tenfold change, so pH 5 is ten times more acidic than pH 6. SANS 241 wants drinking water between 5 and 9.7; most treatment processes are happiest between 6.5 and 8.5.
Low pH water is aggressive: it dissolves copper and cement, pits geysers, and leaches metals from fittings. High pH water tastes flat, scales more readily, and sharply reduces the effectiveness of chlorine disinfection (at pH 8, chlorine needs roughly twice the contact time it needs at pH 7).
Alkalinity: the resistance
Alkalinity is the water's capacity to neutralise acid — its buffering. It is mostly bicarbonate (HCO₃⁻), reported in mg/L as CaCO₃ like hardness. Two waters can sit at the same pH with wildly different alkalinity, and they behave completely differently the moment you dose anything:
- High alkalinity + acid dosing: the pH barely moves, then you keep dosing, then it suddenly crashes once the buffer is consumed. Budget chemical accordingly.
- Low alkalinity (soft, acidic waters): the pH swings wildly at the smallest dose. This is the classic Western Cape surface water problem — pH correction on unbuffered water needs careful control, not bigger pumps.
Why the pair matters in practice
- Scale prediction. The Langelier index needs pH and alkalinity and hardness — pH alone predicts nothing. High-alkalinity water scales at a pH where low-alkalinity water would still be corrosive.
- pH correction media. Calcite (calcium carbonate) media beds raise pH by dissolving into acidic water — self-dosing and self-limiting, but they add hardness and alkalinity as they work, and the bed is consumed and must be topped up.
- RO permeate. RO strips alkalinity, so permeate is unbuffered and slightly acidic — aggressive to copper and mild steel. Downstream pH correction or blending is normal practice, not a system fault.
- Coagulation and membranes. Coagulants consume alkalinity; membrane antiscalant selection reads alkalinity directly.
Reading the analysis together
| Situation | pH | Alkalinity | What it means |
|---|---|---|---|
| Hard borehole | 7.5 – 8.2 | High | Scaling risk; check LSI before heating or RO |
| Cape surface water | 5.5 – 7 | Very low | Corrosive, unstable pH; correction needs care |
| RO permeate | 5.5 – 6.5 | ~0 | Aggressive; correct before copper pipework |
| Treated municipal | 7 – 9 | Moderate | Usually stable; verify chlorine, not pH |
Rule of thumb: never plan pH correction — chemical or media bed — from the pH value alone. Alkalinity tells you how much work the correction has to do, and it is the difference between a 2 L/h and a 20 L/h dosing pump on the same pH reading.