Chemistry parameter
Alkalinity
The capacity of water to neutralise acid without its pH changing much. Alkalinity and pH are routinely confused: pH is the current acidity, alkalinity is the resistance to change.
Key facts
- Unit
- mg/l as CaCO₃
- Metric
- Alkalinity
What it measures
The sum of bases available to accept hydrogen ions, in most natural waters dominated by bicarbonate with smaller contributions from carbonate and hydroxide.
How it is measured
By titration with standard acid to defined endpoints, reported as calcium carbonate equivalent.
How to read a value
| Value (mg/l as CaCO₃) | What it indicates |
|---|---|
| Below 50 | Poorly buffered. pH swings readily; typical of upland soft water, and associated with acidification sensitivity. |
| 50 – 150 | Moderately buffered. |
| Above 150 | Well buffered. Typical of chalk and limestone groundwater; stable pH. |
Bands are conventional interpretation aids, not regulatory limits. Where a legal standard exists it is stated on the relevant standards page.
What causes it
- Dissolution of carbonate minerals
- Soil carbon dioxide driving carbonate weathering
- Deliberate dosing during treatment to stabilise water chemistry
Why it matters
- Low alkalinity water is more corrosive and is stabilised during treatment before distribution
- Alkalinity determines how much acid a lake or river can absorb before its pH falls, which is why acid rain damaged granite catchments and left limestone ones largely unaffected
- Coagulation consumes alkalinity, so low-alkalinity waters need supplementation during treatment
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