tags: [] - coffee/brewing - coffee/brewing/water aliases: - Carbonate hardness - KH water - Temporary hardness

KH (Carbonate Hardness)¶

Tags: #coffee/brewing #coffee/brewing/water Aliases: Carbonate hardness, KH water, Temporary hardness Related: Water in Coffee MOC | Alkalinity | Hardness | Bicarbonate in Coffee Water | Scale Formation Status: ✅ Complete

Overview¶

KH (Karbonathärte, the German term for carbonate hardness) is a water quality parameter that measures the concentration of bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions in water — the fraction of total hardness associated with these anions. KH is numerically equivalent to total alkalinity in most natural water where bicarbonate dominates. In coffee water management, KH is the most critical single parameter for flavour: it quantifies the water's acid-buffering capacity, which directly determines how much of coffee's natural organic acid character is neutralised during brewing. High KH produces flat, dull, bitter coffee; low-to-moderate KH allows the cup's acidity and brightness to express fully.

KH and Alkalinity¶

In water chemistry, KH and total alkalinity are numerically equivalent when bicarbonate is the dominant buffering anion — which is the case in virtually all natural and municipal water used for coffee brewing. The two terms are used interchangeably in coffee water management:

KH = carbonate hardness = the bicarbonate-associated hardness component
Alkalinity = the water's total acid-neutralising capacity, dominated by bicarbonate in most coffee-relevant water

Both are expressed in the same units and measure the same thing in coffee contexts. Some sources express KH as German degrees of carbonate hardness (°dKH), where 1°dKH = 17.85 mg/L as CaCO₃.

Effect on Coffee¶

KH directly determines how much coffee acid is neutralised during brewing:

KH (mg/L as CaCO₃)	Effect on coffee
< 20	Very low buffer; may produce sharp or sour cups; very low scale risk
20–50	Ideal range for specialty coffee; slight buffer without significant acid suppression
50–100	Moderate buffering; light roast acidity begins to be suppressed
100–150	High buffering; noticeable flatness; dark roasts relatively more acceptable
> 150	Very high; significant acid neutralisation; cups taste bitter and one-dimensional

Temporary Hardness¶

KH is also called "temporary hardness" because it can be partially removed by boiling:

Ca²⁺ + 2HCO₃⁻ → CaCO₃↓ + H₂O + CO₂↑

Boiling drives off CO₂, shifts the carbonate equilibrium, and causes calcium carbonate to precipitate as scale. The resulting water has lower KH (and lower calcium) after boiling. This is the mechanism behind limescale formation in boilers and kettles — it is the KH component of hardness that forms scale on heating. See Scale Formation.

Measuring KH¶

KH is measured by acid titration: - Titration kits: Commercial aquarium test kits (designed for reef aquarium water management) work equally well for coffee water — drop reagent into water sample until colour changes from blue to yellow; each drop corresponds to a defined amount of carbonate hardness - Laboratory analysis: Ion chromatography for precise bicarbonate concentration

Key Facts¶

KH (carbonate hardness) = bicarbonate concentration in water; numerically equivalent to total alkalinity in most coffee-relevant water
The most flavour-critical water parameter for coffee: KH neutralises coffee's extracted organic acids during brewing, suppressing acidity and brightness
SCA target alkalinity (= KH): 40 mg/L as CaCO₃ (approximately 2.2°dKH)
KH is "temporary hardness" — it precipitates as calcium carbonate scale when water is heated; is the scale-forming component of total hardness
Measured by acid titration; commercial aquarium test kits are practical and inexpensive for café water management

References¶

Changelog¶

Date	Change
2026-04-28	Note created

This article is part of All-About-Coffee.com - The comprehensive coffee knowledgebase.