150
mg/L as CaCO3
50
mg/L as CaCO3
8.4
dKH
75
%
150
mg/L as CaCO3
50
mg/L as CaCO3
8.4
dKH
75
%
The Carbonate Hardness Calculator separates total water hardness into its two components: carbonate hardness (temporary hardness, KH) and non-carbonate hardness (permanent hardness). Carbonate hardness represents the portion of calcium and magnesium hardness that is associated with bicarbonate and carbonate anions, while non-carbonate hardness is associated with sulfate, chloride, and nitrate anions. This distinction is critically important in water softening — carbonate hardness can be removed by boiling (hence "temporary"), while non-carbonate hardness requires chemical treatment (hence "permanent"). Understanding this breakdown is essential for designing lime-soda softening processes, managing aquarium water chemistry, and assessing scaling potential in boilers and cooling systems. The calculator also provides the result in German degrees of carbonate hardness (°dKH), widely used in aquatics.
Carbonate hardness is determined by comparing total hardness with total alkalinity:
$$KH = \min(\text{Total Hardness}, \text{Total Alkalinity})$$
$$\text{Non-Carbonate Hardness} = \begin{cases} TH - Alk & \text{if } TH > Alk \\ 0 & \text{if } TH \leq Alk \end{cases}$$
Where all values are expressed as mg/L CaCO₃. The logic is:
German degrees of carbonate hardness: $$°dKH = \frac{KH \,(mg/L)}{17.848}$$. One degree dKH equals 17.848 mg/L CaCO₃, which corresponds to 10 mg/L CaO. This unit is commonly used in the aquarium hobby and European water chemistry.
Carbonate hardness is the hardness that can be removed by boiling, which decomposes bicarbonates: Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂. In lime softening, it is removed by adding Ca(OH)₂. Non-carbonate hardness requires soda ash (Na₂CO₃) for removal. When alkalinity exceeds hardness, the difference represents sodium/potassium alkalinity — these do not contribute to hardness. In aquariums, KH of 3-8 °dKH is recommended for freshwater, while reef aquariums require 7-12 °dKH. Low KH indicates poor pH buffering, while excessive KH may keep pH undesirably high.
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With hardness exceeding alkalinity, 150 mg/L is carbonate (temporary) hardness and 50 mg/L is non-carbonate (permanent). KH of 8.4°dKH provides good buffering.
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All hardness (80 mg/L) is carbonate type. The excess 120 mg/L alkalinity comes from sodium bicarbonate, which doesn't contribute to hardness.
Carbonate hardness is called temporary because it can be removed by boiling. When heated, calcium bicarbonate decomposes: Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂↑. The calcium carbonate precipitates as white scale (limescale in kettles), effectively removing hardness. This is why boiled water is softer than raw water.
Non-carbonate hardness (associated with sulfate, chloride, and nitrate ions) cannot be removed by boiling because calcium sulfate and calcium chloride do not decompose at boiling temperatures. Chemical treatment (adding soda ash, Na₂CO₃) or ion exchange is required to remove permanent hardness.
When alkalinity exceeds hardness, all hardness ions are associated with carbonate/bicarbonate, and the excess alkalinity comes from alkali metal (Na⁺, K⁺) bicarbonates. There is no non-carbonate hardness in this case. This excess is sometimes called "non-hardness alkalinity" and doesn't cause scaling issues.
KH (carbonate hardness) is the primary pH buffer in aquariums. Low KH (<2°dKH) can cause dangerous pH crashes — especially overnight when CO₂ from fish respiration accumulates. Maintaining KH above 3-4°dKH provides a stable buffer. Reef aquariums need 7-12°dKH to support coral growth and stable alkalinity.
In lime-soda softening, lime (Ca(OH)₂) removes carbonate hardness by converting bicarbonate to insoluble carbonate precipitate. Soda ash (Na₂CO₃) removes non-carbonate hardness by providing carbonate ions to precipitate calcium and magnesium. The amounts required are calculated directly from the carbonate and non-carbonate hardness values.
Carbonate hardness is the primary cause of scale in hot water systems — heating drives off CO₂ and precipitates CaCO₃. Non-carbonate hardness also contributes to scale but through different mechanisms (CaSO₄ precipitation at higher temperatures). Boiler water treatment specifically targets carbonate hardness reduction.
Common conversions: 1°dKH = 17.848 mg/L CaCO₃; 1°fH (French) = 10 mg/L CaCO₃; 1°eH (English/Clark) = 14.3 mg/L CaCO₃; 1 mmol/L = 100 mg/L CaCO₃; 1 gpg = 17.12 mg/L CaCO₃. The German degree (°dKH) is most common in the aquarium industry.
Yes, in waters with zero alkalinity (pH below about 4.5), there are no bicarbonate or carbonate ions, so carbonate hardness is zero. In this case, all hardness is non-carbonate (permanent). Such waters are rare in nature but can occur in acid mine drainage or industrial wastewater.
Add sodium bicarbonate (baking soda) at a rate of about 1 teaspoon per 20 gallons to raise KH by approximately 1°dKH. Commercial KH buffers are also available. Add slowly and test frequently. Crushed coral or aragonite substrate provides a slower, more natural KH increase through dissolution.
GH (general hardness) measures total hardness from Ca²⁺ and Mg²⁺, while KH (carbonate hardness) measures the bicarbonate/carbonate buffering. GH affects organism osmoregulation, while KH affects pH stability. They are measured independently — you can have high GH with low KH or vice versa, depending on the dissolved mineral composition.
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