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Calcium
©2011 Inorganic Ventures, Inc. |
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Magnesium
©2011 Inorganic Ventures, Inc. |
Who among us that live somewhere the water is hard haven’t asked that before ourselves? I certainly have, and I know why the water is hard. It’s usually more a rhetorical cry of frustration than it is a cry for answers, but none the less, I thought someone out there might want to know a bit more about water hardness, both actual and perceived, so hold on to your hats, because here we go. Water hardness is defined as the concentration of polyvalent cations, and is expressed as the equivalent concentration of calcium carbonate (CaCO3) in mg/L. Huh? A cation is just an atom or molecule that has a positive electric charge and polyvalent means the positive charge is greater than +1. So, for example, an atom of calcium dissolved in water is a polyvalent cation because it has a charge of +2, and is shown like this: Ca+2. Simple! The most common polyvalent cations in drinking water that make up hardness are calcium (Ca+2) and magnesium (Mg+2), but can also include a whole host of ions such as iron, copper, zinc, nickel and others. These latter elements are usually present in small enough quantities that they do not appreciably add to the hardness, however, so for the most part we can just consider calcium and magnesium. The quantities for all these polyvalent cations that are included in hardness get converted to equivalent concentrations of calcium carbonate for simplicity and ease of comparing one sample to another. Consumers like you and I for the most part don’t care about polyvalent/polyshmalent, cat-ions/dog-ions – all we care about is the spots on our glasses and that crusty stuff on our faucet. Although hardness is a large part of that, it’s also affected by another measurement known as Total Dissolved Solids, or TDS. That measurement is a bit easier to understand and is pretty much just as its name implies; it’s a measurement of all the stuff dissolved in the water. It includes hardness, because the calcium and magnesium cations are a dissolved solid and therefore get included in TDS. But it also includes things like sodium (Na+1), sulfate (SO-4), potassium (K+1), chloride (Cl-1) and others that can lead to those dreaded spots and crustiness when water is allowed to dry on glasses or fixtures, leaving the dissolved solids behind. Many people also think of these compounds collectively as “salt”, and indeed when they dry, they do form salts such as the one we all know, sodium chloride or table salt, but also salts such as calcium sulfate or potassium chloride. But fixture deposits and spotted dishes aren’t the only problems excessive hardness and TDS cause. Over the years, calcium, magnesium, and TDS compounds can precipitate out of the water and form a coating on the insides of pipes in the distribution system or in home plumbing systems reducing the flow of water. These precipitates can also collect in water heaters, reducing their efficiency, or plug up faucet aerators. On the plus side, there have been some medical studies showing that the calcium and magnesium in hard water can be a significant part of the dietary intake necessary for good health, and that it can even be effective in lowering the incidence of cardiovascular disease. Other medical studies have been inconclusive on this last point, so the jury is still out. General hardness ratings are shown in the Table. Where does all this stuff in the water come from? For groundwater, the minerals in the water are those that have been dissolved from the local rocks and soils as the water moves through the aquifers beneath our feet. Calcium, magnesium, and all the rest are very common in the local geologic structures, and water, being such a good solvent, is very effective at removing them. Surface water, such as that in the California State Water Project, does not come in contact with rocks and soil for nearly as long as groundwater, so it has fewer of these elements dissolved in it, and therefore isn’t as hard.
