Temporary Hardness

Definition - What does Temporary Hardness mean?

Temporary hardness is water hardness due to the presence of calcium and magnesium carbonates and bicarbonates, which can be precipitated by heating the water. It can be removed by processes such as boiling or lime softening, and then separation of water from the resulting precipitate.

Temporary hardness is very common and is responsible for the deposition of calcium carbonate scale in pipes and equipment. These deposit formations lead to clogged plumbing and reduced efficiency of heat exchangers.

Temporary hardness is also known as carbonate hardness.

Corrosionpedia explains Temporary Hardness

Temporary hardness is caused by a combination of calcium ions and bicarbonate ions in water. When dissolved, these ions yield calcium and magnesium cations and carbonate and bicarbonate anions. The presence of the metal cations makes the water hard. Hardness can be removed by boiling or by the addition of lime (calcium hydroxide). Boiling promotes the formation of carbonate from the bicarbonate and precipitates calcium carbonate out of solution, leaving water that is softer upon cooling. The original insoluble carbonate is reformed.

Temporary hardness is complex, because its concentration is a function of the concentration of carbonates in relation to their reaction with calcium in magnesium. Temporary hardness is easy to remove by boiling or through precipitation with lime (calcium hydroxide). In contrast, permanent hardness contains ions that cannot be eliminated by boiling.

Temporary hardness has some serious consequences, like forming deposits that clog plumbing. These deposits are called scale. Scale restricts the flow of water in pipes. In boilers, deposits impair the flow of heat into water, reducing the heating efficiency and allowing the metal boiler components to overheat. In a pressurized system, this overheating can lead to failure of the boiler. The damage caused by calcium carbonate deposits varies depending on the crystalline form, for example, calcite or aragonite.

In an electrolyte, temporary hard water can also lead to galvanic corrosion in which one metal preferentially corrodes when in contact with another type of metal, when both are in contact with an electrolyte. The softening of hard water by ion exchange does not increase its corrosivity per se. Similarly, where lead plumbing is in use, softened water does not substantially increase plumbo-solvency.

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