3.6.3 Total water hardness Total hardness is the sum of the temporary hardness (caused by calcium and magnesium bicarbonates) and the permanent hardness (caused by sulphates, chlorides and calcium/magnesium nitrates). ѓ Hardness expressed in French degrees (°F) represents the quantity of calcium carbonate, in grammes, contained in 100 litres of water. ѓ Hardness expressed in German degrees (°D) represents the quantity of calcium oxide, again in grammes per 100 litres of water. Conversion of °F - °D: 1 °D = 1,79 °F 1 °F = 0,56 °D Classification of total water hardness expressed in °F and °D Negative effects of hard water: ѓ Reduction in the effectiveness of anionic surface-active agents.
ѓ Increase in the quantity of dirt re-deposited. ѓ Increase of residue deposited on the fabrics. ѓ Increase of scale formation on the heating elements. Hard water may cause the formation of incrustation on mechanical parts, due to the presence of carbonates, sulphates and alkaline-terrous metal silicates. This may also lead to a considerable reduction in the efficiency of the appliance, since the reaction is endothermic; in other words, the increase in temperature favours the formation of carbonates. The most important of the soluble salts present in water are calcium bicarbonate and magnesium bicarbonate.
When the water is heated to over 60°C, these react and precipitate to form limescale (which deposits on the fabrics and causes stiffness). Water softening mechanisms Hard water can be softened in three ways: SEQUESTRATION: soluble compounds such as TPF, polycarboxylics, citrates PRECIPITATION: insoluble compounds such as soaps, sodium carbonate ION EXCHANGE: insoluble compounds such as zeolites, lamellar silicates. Water softeners are used only for washing temperatures in excess of 60°C, i.e. when the high temperature causes the calcium to precipitate. SOI/DT 2006-03 dmm 12/70 599 37 67-82 °F °D SOFT 0 – 14 0 - 7 AVERAGE HARDNESS 15 – 26 7 - 14 HARD 27 – 39 14 - 21 VERY HARD > 40 > 21