1 2 3 World Health Organization Hardness in Drinking-Water, 2003 1 2 Hermann Weingärtner, "Water" in Ullmann's Encyclopedia of Industrial Chemistry, 2006[December], Wiley–VCH, Weinheim. doi:10.1002/14356007.a28_001 DOI 1 10.1002/14356007.a28_001 ↑ Christian Nitsch, Hans-Joachim Heitland, Horst Marsen, Hans-Joachim Schlüussler, "Cleansing Agents" in Ullmann's Encyclopedia of Industrial Chemistry 2005, Wiley–VCH, Weinheim. doi:10.1002/14356007.a07_137 ↑ "Lime Softening". Retrieved 4 November 2011. ↑ "Lenntech Water hardness calculator". ↑ Wisconisin DNR - Carbonate chemistry ↑ Stephen Lower (July 2007). "Hard water and water softening".