Limescale

Limescale is a hard chalky deposit, consisting mainly of calcium carbonate (CaCO3), that often builds up inside kettles, hot water boilers, and pipework, especially that for hot water. It is also often found as a similar deposit on the inner surfaces of old pipes and other surfaces where "hard water" has evaporated.

The colour varies from off-white through a range of greys and pink or reddish browns, depending on the other minerals present. Iron compounds give the reddish-browns.

In addition to being unsightly and hard to clean, limescale can seriously damage or impair the operation of various plumbing and heating components.[1] Descaling agents are commonly used to remove limescale. Prevention of fouling by scale build-up relies on the technologies of water softening.

Chemical composition

The type found deposited on the heating elements of water heaters consists mainly of calcium carbonate (CaCO3). Hard water contains calcium (and often magnesium) bicarbonate or similar ions. Calcium salts, such as calcium carbonate and calcium bicarbonate (Ca(HCO3)2), are more soluble in hot water than cold water; thus, heating water does not cause calcium carbonate to precipitate per se. However, there is an equilibrium between dissolved calcium bicarbonate and dissolved calcium carbonate as represented by the chemical equation

Ca2+ + 2HCO3 ⇋ Ca2+ + CO32− + CO2 + H2O

where the equilibrium is driven by the carbonate/bicarbonate, not the calcium. Note that the CO2 is dissolved in the water. Carbon dioxide dissolved in water (dis) also tends to equilibrate with carbon dioxide in the gaseous state (g):

CO2(dis) ⇋ CO2(g)

The equilibrium of CO2 also moves to the right towards gaseous CO2 when the water temperature rises. When water that contains dissolved calcium carbonate is warmed, CO2 leaves the water as gas, causing the equilibrium of bicarbonate and carbonate to shift to the right, increasing the concentration of dissolved carbonate. As the concentration of carbonate increases, calcium carbonate precipitates as the salt: Ca2+ + CO32− ⇋ CaCO3.

As new cold water with dissolved calcium carbonate/bicarbonate is added and heated, the process continues: CO2 gas is again removed, carbonate concentration increases, and more calcium carbonate precipitates.

Scale is often coloured because of the presence of iron-containing compounds. The three main iron compounds are wustite (FeO), hematite (Fe2O3), and magnetite (Fe3O4).

As a stone

The Roman Eifel aqueduct was completed around 80 AD and broken and largely destroyed by Germanic tribes in 260. By the Middle Ages the limestone-like limescale accretions from the inside of the aqueduct were particularly desirable as a building material, called "Eifel marble" in an area with little natural stone. In the course of operation of the aqueduct, many sections had a layer as thick as 20 centimetres (7.9 in). The material had a consistency similar to brown marble and was easily removable from the aqueduct. Upon polishing, it showed veins, and it could also be used like a stone board when cut flat. This artificial stone found use throughout the Rhineland and was very popular for columns, window frames, and even altars. Use of "Eifel marble" can be seen as far east as Paderborn and Hildesheim, where it was used in the cathedrals. Roskilde Cathedral in Denmark is the northernmost location of its use, where several gravestones are made of it.[2]

Soap scum forms when calcium cations from hard water combine with soap, which would dissolve in soft water. This precipitates out in a thin film on the interior surfaces of baths, sinks, and drainage pipes.

See also

References

  1. Hermann Weingärtner, "Water" in Ullmann's Encyclopedia of Industrial Chemistry, December 2006, Wiley–VCH, Weinheim. doi:10.1002/14356007.a28_001
  2. Tegethoff, F. Wolfgang; Rohleder, Johannes; Kroker, Evelyn. Calcium Carbonate: From the Cretaceous Period Into the 21st Century. Birkhäuser, 2001. ISBN 3-7643-6425-4
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