pH
A measure of how acidic or alkaline water is, on a logarithmic scale where 7 is neutral. pH governs almost every other chemical behaviour in water — which metals dissolve, how effective disinfection is, and whether pipes corrode or scale.
Key facts
- Unit
- pH units
- Metric
- pH
What it measures
The activity of hydrogen ions. Because the scale is logarithmic, each whole unit represents a tenfold change: pH 5 water has a hundred times the hydrogen ion activity of pH 7 water. This is why averaging pH values arithmetically is strictly incorrect.
How it is measured
By glass electrode, calibrated against buffer solutions. pH changes as water warms and as dissolved carbon dioxide escapes, so a sample measured hours after collection may read meaningfully differently from the source.
How to read a value
| Value (pH units) | What it indicates |
|---|---|
| Below 6.5 | Acidic. Aggressive toward copper and lead plumbing, and a common cause of elevated metals at the tap in soft-water areas. |
| 6.5 – 7.5 | Slightly acidic to neutral. Typical of soft upland surface water. |
| 7.5 – 8.5 | Slightly alkaline. Typical of groundwater from chalk and limestone, and the usual range for treated public supply. |
| Above 8.5 | Strongly alkaline. Can reduce chlorine disinfection efficiency and impart a soapy taste. |
Bands are conventional interpretation aids, not regulatory limits. Where a legal standard exists it is stated on the relevant standards page.
What causes it
- Carbon dioxide dissolving from the air and from soil respiration, forming carbonic acid
- Dissolution of carbonate rock, which raises pH and buffers it
- Organic acids from peat and vegetation, which lower it
- Deliberate adjustment during treatment, usually upward to control corrosion
Why it matters
- Low pH increases dissolution of lead and copper from plumbing
- Chlorine disinfection is markedly more effective below pH 7.5, because the active hypochlorous acid form dominates
- High pH combined with high hardness and alkalinity promotes limescale
- pH is an indicator parameter in most regulations, set for acceptability and corrosion control rather than direct health effect
Related intelligence
Sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- National Institute of Standards and Technology — NIST Chemistry WebBook — thermophysical properties of water. Open · US Government work — public domain