BrewWater
What is carbonate hardness?
- carbonate hardness
- Carbonate hardness is the fraction of total hardness associated with carbonate and bicarbonate ions, which precipitates when water is heated. It is also called temporary hardness.
Short answer
Carbonate hardness is the portion of hardness bound to carbonate and bicarbonate, which precipitates out when water is heated. It is also called temporary hardness because boiling removes it, leaving permanent non-carbonate hardness behind. This is the fraction that becomes limescale inside a boiler.
| Measure | Value | Unit | Source |
|---|---|---|---|
| Upper bound of soft water; above this, water is at least moderately hard | 60 | mg/L as CaCO3 | World Health Organization · verified |
| Target total alkalinity for brewing water | 40 | mg/L as CaCO3 | Specialty Coffee Association of America · verified |
Why is it called temporary hardness?
Because heating removes it. Hardness is caused by cations, but it is conventionally discussed as carbonate and non-carbonate fractions. The carbonate fraction is soluble when cold and stops being soluble when hot, so boiling drives it out of solution. What remains afterwards is the permanent, non-carbonate portion.
Why does this matter for an espresso machine?
Because the carbonate fraction is precisely the part that becomes scale. In heated water applications, insoluble metal carbonates form, coat surfaces and reduce the efficiency of heat exchangers. In a machine, the hottest surface collects the deposit first, which means the element and the boiler wall rather than the group head.
Is carbonate hardness the same as alkalinity?
They overlap heavily but are measured from different sides. Alkalinity measures acid-neutralising capacity, most of which comes from bicarbonate. Carbonate hardness measures the cations paired with those carbonate species. In most drinking water the two track each other closely, which is why alkalinity is a useful practical proxy for scale risk.