Härtezahl

The Härtezahl, or hardness number, is a measure used primarily in German-speaking countries to quantify the hardness of water. It indicates the concentration of dissolved calcium and magnesium ions, which are the primary contributors to water hardness. The term originates from the German word "Härte," meaning hardness, and is expressed in degrees German hardness (°dH).

The Härtezahl is determined by titrating a water sample with a standardized solution of ethylenediaminetetraacetic acid (EDTA). The amount of EDTA required to bind all calcium and magnesium ions correlates directly with the hardness value. One degree German hardness (°dH) is equivalent to 10 milligrams per liter of calcium carbonate (CaCO₃). For example, water with a Härtezahl of 10 °dH contains 100 milligrams per liter of CaCO₃.

Water hardness classifications based on Härtezahl are as follows: soft water (0–4 °dH), moderately hard water (4–8 °dH), hard water (8–12 °dH), and very hard water (above 12 °dH). Hard water can cause issues such as scale buildup in pipes, reduced soap lathering, and potential damage to appliances like water heaters and dishwashers. Softening treatments, such as ion exchange or chemical additives, are often employed to mitigate these effects.

The Härtezahl is distinct from other hardness scales, such as the American ppm (parts per million) or French degrees (°F), which may use different reference substances or units. While the Härtezahl is widely used in Germany, Austria, and Switzerland, other countries may rely on alternative hardness measurements for consistency with local standards. Regular testing of water hardness ensures proper treatment and maintenance of water systems.