Ions Found in Hard Water
Hard water is water which contains a high concentration of a variety of ions. The origin of the term likely lies in the behavior of soaps and detergents when using this type of water; a hard scum of soap typically forms on the surface of the washwater. In fact, hard water makes it difficult to use soaps precisely because the ions combine with the detergent molecules and render them useless. Hard water can also cause other problems as minerals come out of solution to form a hard deposit known as scale.
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About Ions
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Ions are chemical compounds, either atoms or molecules, that have acquired a charge due to either losing or gaining electrons. A compound that gains an electron becomes negatively charged and is known as an anion. Losing an electron produces a positively charged species, known as a cation. The species that cause hard water are all cations. Because water is a very polar liquid, meaning its molecules have a slight distribution of charge among their atoms, it is very good at dissolving ions.
The Main Ions
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The main ions which are responsible for hard water are the doubly positive calcium cation (Ca++) and the doubly positive magnesium cation (Mg++). These ions are the most common because they frequently occur in the types of underground rock formations through which water travels. Calcium is found in the minerals calcite, gypsum and aragonite whereas both calcium and magnesium can be found in dolomite. Water can dissolve portions of these rocks, especially if it is rendered slightly acidic by dissolved carbon dioxide gas.
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Other Ions
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Hard water may also contain a variety of metallic ions, although the concentration of these ions is usually much, much less than that of calcium and magnesium and so is often not considered. These ions include iron (Fe++ and Fe+++), zinc (Zn++), manganese (Mn++) and strontium (Sr++). These elements also enter water as it flows over various rocks and minerals.
Measures of Hardness
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The total hardness of water is a measure of the overall concentration of all dissolved cations. It is reported as the equivalent of parts per million (abbreviated as "ppm" and also known as milligrams per liter) calcium carbonate. There are several methods of finding total hardness, including EDTA titration and colormetric test strips. The relation between total hardness in ppm carbonate and the concentration of the most common cations is as follows: total hardness (ppm) = 2.5 x ppm calcium + 4.1 x ppm magnesium + 1.8 x ppm iron + 1.8 x ppm manganese.
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