The carbonate hardness (KH) is part of the total hardness of the water. The total hardness is made up of the permanent hardness and the temporary hardness, the carbonate hardness described here. In German tap water, the carbonate hardness is usually around 60% of the total hardness.
The carbonate hardness is the content of alkaline earth ions, mainly magnesium (Mg) and calcium (Ca) with traces of barium and strontium, dissolved in the water together with carbonate ions (CO32-) and hydrogen carbonate ions (HCO3–). are. The carbonate hardness therefore only includes the alkaline earth ions, which are present as dissolved carbonates or hydrogen carbonates. Carbonates are chemical compounds containing carbon (C) and oxygen (O) and are salts of carbonic acid. In order to be considered a carbonate hardener, every carbonate and every hydrogen carbonate ion must be able to find a calcium or magnesium cation.
If e.g. B. Sodium hydrogen carbonate (NaHCO3) is added to water whose carbonate hardness is less than the total hardness, the carbonate hardness increases until it is equal to the total hardness. Because the added hydrogen carbonate ions cannot be assigned to calcium or magnesium ions, the carbonate hardness does not increase any further. The sodium ion added does not count for hardness because it is an alkali ion. The concentration of sodium bicarbonate can therefore be higher than the corresponding concentration of calcium and magnesium combined. But the carbonate hardness cannot increase further because there are no more alkaline earth ions.
Carbonate hardness is also called temporary hardness because it is the portion of the total hardness that is eliminated by boiling the water. During cooking, the hydrogen carbonates are converted into poorly soluble carbonates. Scale is deposited, a mixture of magnesium and calcium carbonate.
The optimum carbonate hardness
The value of carbonate hardness for aquaristics lies in the fact that it stabilizes the pH value. A pH value between 3 and 10° dKH is optimal for most aquariums. As a rule, a higher carbonate hardness results in a higher pH value (*other acids are conceivable*). If you want to keep fish that require a very low carbonate hardness, special measures must be taken to ensure that the pH value remains stable.
Measures to keep the pH stable at low carbonate hardness
- Filtering over peat
- Less fish stock because the metabolism of the fish reduces the pH value
- Few or no plants
- Low light level
Many of the acidic waters in South America also have very low fish density and almost no aquatic plants. Low fish density does not mean cm fish in a 2l tank volume, but really few fish!
Plants use carbon dioxide during photosynthesis. When the light intensity is high, photosynthesis is correspondingly strong. The pH fluctuates more during the day. It should not be forgotten that plants produce CO2 at night. This is less than what is consumed by photosynthesis during the day, but it is noticeable. This consumption during the day and the production at night lead to fluctuations if the pool does not have an appropriate buffer.
Some plants extract hydrogen carbonate ions from the water as a source of carbon dioxide (biogenic decalcification). Carbonate hardness can continue to drop until the pH is dangerously unstable. A loss of 1° kH per day is realistic. In order to consume nitrate or phosphate from the water, some houseplants – ivy or philodendron – can therefore also be used. Even with floating plants, e.g. B. mussel flowers, at least part of the CO2 from the air is used.
Biogenic decalcification
At pH values between 7 and 8, the calcium hydrogen carbonate breaks down according to the formula Ca(HCO3)2 <-> CaCO3 + H2CO3. If the pH is so high that both hydrogen carbonate ions and carbonate ions are present, lime (CaCO3) can precipitate. Because there must also be a lot of calcium (Ca) available, this can only happen at pH values above 8.3.
With strong assimilation of the plants, the lime can occur directly on the leaf surface. This effect is called biogenic decalcification. With biogenic decalcification, the carbonate hardness in the water drops while the pH values are high at the same time. Some plant species, e.g. B. waterweed, remove the required carbon dioxide (CO2) from the water. Biogenic decalcification is therefore an indication that an additional CO2 supply is necessary.
The biogenic decalcification can be recognized by the sandy-rough leaves. If the sheet, e.g. B. a Vallisnerie, pulled through the fingers, you can feel the many small calcite crystals like fine emery.
“Aquaristic” carbonate hardness = acid-binding capacity
In aquaristics, the term carbonate hardness is often misused.
Tests for carbonate hardness in aquaristics measure all carbonate and bicarbonate ions without regard to alkaline earth ions. The carbonate hardness defined above is not measured. The acid capacity is measured. Acid capacity is also known as acid-binding capacity (SBV).
The acid capacity measures the buffering capacity of water against acids. It determines how stable the pH value is. The acid capacity indicates how much acid is needed to reach a pH value of 4.3 with a certain amount of water. In practice, 0.1 mol/liter hydrochloric acid is usually used together with an indicator that changes color at pH 4.3.
In aquaristics, the carbonate hardness is therefore often referred to as the content of hydrogen carbonates, which can buffer the addition of acid. Because cations other than calcium and magnesium also bind hydrogen carbonates, e.g. B. sodium, the carbonate hardness is at least a part of the total hardness according to this definition. According to this definition, the carbonate hardness can also be greater than the total hardness. Actually, the acid-binding capacity is then greater than the total hardness.
In most natural waters, the carbonate hardness corresponds to the acid capacity up to pH 4.3. Exceptions are e.g. B. the soda waters in Central Africa.
Means for increasing the carbonate hardness, such as KH-Plus, therefore do not contain any calcium salt, which would increase the carbonate hardness. They contain sodium hydrogen carbonate and increase the acid-binding capacity. Since the total hardness is usually high enough, ie there are enough alkaline earth ions, the carbonate hardness also increases.
- The carbonate hardness is always less than or equal to the total hardness.
- The carbonate hardness is not always the same as the acid-binding capacity.
- The acid-binding capacity can be higher than the total hardness.
- In aquaristics, the acid-binding capacity is measured and referred to as carbonate hardness.
Because the acid-binding capacity or the stability of the pH value is actually the variable of interest for fish and plants in aquaristics, the incorrect designation as carbonate hardness causes confusion again and again, but usually does not lead to critical errors.
Calculate the carbonate hardness from the acid capacity
On request, the water suppliers will provide the water parameters of the drinking water free of charge. The carbonate hardness can be calculated from the acid capacity:
- Carbonate Hardness = Acid Capacity x 2.8
Increasing carbonate hardness
In some aquariums, the carbonate hardness is higher than the carbonate hardness in tap water. This is usually a normal occurrence and should not be a cause for alarm.
A biological equilibrium can only be achieved if only fully desalinated water is refilled in an aquarium in order to refill evaporated water. Fertilization, lighting and, above all, feeding constantly introduce new substances into the aquarium, which can have consequences for the water chemistry and change certain values.
Factors that increase carbonate hardness
- denitrification
- CO2 fertilization
- Lots of fast growing plants
Factors that reduce carbonate hardness
- Strong input of nitrogen
- mineralization
- Lots of snails
Nitrate (NO3) is formed as a result of strong nitrogen input. NO3 displaces the carbonate hardness.
Nitrate is formed during mineralization or nitrification by bacteria. The carbonate hardness decreases due to oxidation.
During denitrification or nitrate respiration, nitrate is converted by bacteria into nitrite and further into gaseous nitrogen by consuming oxygen. In the process, nitrate is displaced by carbonate hardness.
Denitrification takes place in aquariums with a lot of sludge or filter sludge. Denitrification also takes place in special nitrate filters.
Limestone and snail shells dissolve as a result of fertilization with CO2.
The so-called assimilatory nitrate reduction takes place in plants. In the process, nitrate is converted via nitrite and ammonium (NH3) to ammonia (NH4).
Fast-growing plants absorb NO3, which ultimately increases carbonate hardness again. Like ammonium (NH3), nitrate is consumed by plants as a nutrient. Many plants can store nitrate in their sap and roots.
In the aquarium it is desirable if the nitrate value drops because more nitrate is being used than is being produced. As long as the plants are growing, they have enough ammonium and nitrate available for food. Then no nitrate has to be supplied, e.g. B. by increased feeding.
Ammonium only accumulates in the aquarium if the pH is high and there is no microflora.
If exactly as much nitrate is produced in an aquarium as is consumed or removed by plants, bacteria and water changes, the carbonate hardness will automatically increase. In addition to nitrate, the mineralization of feed primarily releases calcium, magnesium and potassium. In addition, lime can be dissolved by carbon dioxide.
All of these effects act simultaneously in an aquarium and influence the carbonate hardness. Depending on the prevailing conditions, there can be an overall increase in carbonate hardness. The carbonate hardness can then be reduced again by changing the water with soft water, by pruning or removing the plants or by feeding more heavily.
How to increase carbonate hardness
The carbonate hardness can be increased with potassium (K) or sodium bicarbonate (NaHCO3). Sodium bicarbonate is known as Kaisernatron z. B. available in pharmacies. With 3 g per 100 liters of water, the carbonate hardness is increased by 1° and the sodium content by 8.2 mg/litre.
By adding potassium and sodium bicarbonate, the standard ion ratio is changed, i. h the ratio of the salts dissolved in the water to one another changes.
Should e.g. For example, if the water values of Lake Tanganyika are set, a mixture of NaHCO3 and KHCO3 should be used for the carbonate hardness, which corresponds to the ratio between sodium (Na) and potassium (K) in the lake. The overall hardness is adjusted with a mixture of potassium chloride and magnesium sulfate or magnesium chloride. In Lake Tanganyika, the magnesium content is four times higher than the calcium content. From this…