A2

Definition : Hardness = c(Ca 2+ ) + c(Mg 2+ ) [mol/m³] resp. [mmol/L] [ c() = Molar concentration ] The specification in degrees of German hardness (°dH) is outdated but still in use. 1 mol/m³ = 5.6 °dH The ratio of Ca to Mg in water is generally around 5:1, i.e. approx. 83% Ca and 17% Mg. However, the proportion of magnesium can also be higher or lower depending on the geology. The property that water with high hardness leads to hard and brittle laundry when washing clothes is largely no longer relevant today. In the past, soaps were used for washing, which then reacted with the hardening agents to form so-called lime soaps and deposited in the laundry, making the clothes hard and brittle. All alkaline earth ions have this property of hardness formation: Elements of the 2nd main group of the periodic table (alkaline earth metals): Beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra) However, as the other alkaline earth ions are only present in low concentrations or not at all in water, the definition of water hardness has been limited to calcium and magnesium. Today, synthetic detergents are predominantly used, mostly sulphonates, which do not have this property. However, it is still important that limescale precipitates when the water is heated and can lead to technical problems. This is why so-called hardness stabilizers are added to detergents.

However, as the soil and rock have different lime contents, the hardness of the water varies greatly from region to region and depends on the geology in the respective area. The solubility of gypsum is relatively high (CaSO 4 : L = 6.1 • 10 -5 (mol/L) 2 25 °C), so that water in contact with gypsum deposits can contain a lot of dissolved calcium, but also sulphate in higher concentrations. This applies to calcium-rich mineral waters. (L = solubility product) In carbonate-containing rock, only less calcium can dissolve due to the lower solubility product for calcite (CaCO 3 : L = 3.9 • 10 -9 (mol/L) 2 10 °C). However, the solubility is increased by the presence of CO 2 due to the resulting lower pH value. In chemical equations, the increased solubility of calcite due to CO 2 is often simplified into a single reaction equation: Summarized ( simplified !! ): CaCO 3 + CO 2 + H 2 O → Ca 2+ + 2 HCO 3 - However, the greater solubility of the calcite is caused solely by the lowering of the pH value by the CO 2 (detailed reaction mechanisms are described in the textbook ). The pH value rises again due to the dissolution of the carbonate. Further chemical conversions of CO 2 in the soil with silicate minerals (clay) also lead to the release of calcium and magnesium ions, among other things. Silicate minerals (clays): Ton + CO 2 → K, Na, Mg, Ca, Al, Silicate Exemplary degrees of hardness of natural water: 21 °dH 3,8 mmol/L Groundwater Gaienhofen (near Lake Constance) 16 °dH 2,9 mmol/L Groundwater Münsterländer Kiessandzug 9 °dH 1,6 mmol/L Lake Constance 3 °dH 0,5 mmol/L Reservoir water, Harz mountains