What is total hardness?
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As water seeps into the ground, it absorbs minerals and salts. Salts consist of 2 so-called ions. In table salt, these two ions are sodium and chloride.
The most important salts in water are calcium and magnesium. They are dissolved in the water together with other so-called alkaline earths or alkaline earth metals.
The total hardness describes the concentration of ions of alkaline earth metals that are dissolved in the water. It does not matter in which compound the alkaline earth metals are present.
Alkaline earth metals are the chemical elements barium, beryllium, calcium, magnesium, radium and strontium. The total hardness is mainly determined by calcium and magnesium. The other elements only occur in traces in water. One degree of German total hardness corresponds to 10 mg of calcium or magnesium oxide in one liter of water.
Alkali metals are hydrogen, lithium, sodium, potassium, rubidium, cesium and francium.
Ions are electrically charged particles. Unlike atoms, which are electrically neutral, ions have different numbers of protons and electrons. Cations have an excess of protons and are therefore positively electrically charged ions. In table salt, Na+ is the cation. Anions have an excess of electrons and are negatively charged ions. The anion in table salt is Cl-.
In chemical compounds, alkaline earth metals usually have a +2 charge (Ca+2, Mg+2). Calcium and magnesium ions make up over 80% of the cations in natural fresh water.
Dissolved salts such as calcium hydrogen carbonate (Ca(HCO3)2) and calcium sulphate CaSO4 are the main causes of the overall hardness. A high amount of calcium and magnesium salts means hard water, a low amount of calcium and magnesium salts means soft water.
The alkaline earth metals are also called hardeners. In Central Europe and in most natural waters, the total hardness usually corresponds to the calcium and magnesium content in the water.
Although the concentration of dissolved salts can vary greatly in different freshwater areas, the ratio of the concentrations of the salts to one another is very similar. The total hardness is therefore a good indicator of other water properties such as dissolved salts, conductivity and osmotic pressure.
Malfunctions due to a lack of trace elements are therefore less likely in hard water than in soft water. Because of this, most plants grow better in hard water.
The importance of the total hardness for aquaristics:
- Fish need calcium to build their skeleton and cell walls.
- Calcium is necessary for the functioning of the nervous system.
- Magnesium is needed in small amounts by fish to break down sugar and to activate muscles and nerves.
- Plants require trace amounts of calcium.
- Plants need magnesium in photosynthesis and in the production of chlorophyll.
- Because the water hardness is related to the concentration of dissolved salts, it directly influences the processes in the cells of the fish through the osmotic pressure.
- The metabolism of sensitive fish species, e.g. B. in red neon, can be disturbed if kept in too hard water.
- Heavy metals are less toxic in hard water than in soft water.
- Some soft water fish need very soft water for rearing, otherwise sperm or eggs will die due to osmotic pressure.
- Fish should not be exposed to rapid changes in osmotic pressure.
- Water with a higher total hardness usually also has a higher carbonate hardness. The pH of harder water is therefore more stable than the pH of softer water.
- Deficiency symptoms due to a lack of trace elements are less likely in harder water than in softer water.
- Most plants grow better in harder water than in softer water.
For the reasons mentioned, harder water is preferable if the fish and plants that are being cared for can tolerate this water.
Change the overall hardness
The addition of calcium or magnesium salts can increase the overall hardness without increasing the carbonate hardness. If the corresponding carbonates are added, the total hardness and the carbonate hardness increase.
The carbonate hardness can be increased by adding caustic soda (sodium hydroxide, NaOH), sodium carbonate (soda, Na2CO3) and sodium bicarbonate (NaHCO3) without increasing the overall hardness. Sodium hydrogen carbonate is the least dangerous of the three substances and is contained in products such as KH-Plus.
The carbonate hardness can be reduced with mineral acids such as hydrochloric or sulfuric acid without changing the overall hardness.
With osmosis systems, total and carbonate hardness can be reduced at the same time.