Hardness of Water

Water Hardness Scale
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Water Hardness

The term “Water Hardness” is applied to the soap neutralizing power of water. Any substance that will form an insoluble curd with soap causes hardness.

Since iron, manganese, copper, barium, lead, zinc, and other trace elements are seldom present in appreciable concentrations in natural waters, hardness is attributed principally to calcium and magnesium ions.

Because most of the effect observed with soap results from the presence of calcium and magnesium, water hardness is now generally defined in terms of these two constituents alone with some rather indefinite reservations about other ionic interferences. The other ions that might precipitate soap include hydrogen ion and all polyvalent metals, but usually they are present in insignificant amounts in water of the type that is usable domestically and for which water hardness data might be obtained.

The hardness of water is conventionally expressed in all water analysis made in the U.S. in terms of an equivalent quantity of calcium carbonate. Some such convention is needed for hardness because this is a property imparted by several different cations which may be present in varying proportions. However, the actual presence of the indicated number of mg/L in the form of calcium carbonate certainly should not be assumed.

The adjectives “hard” and “soft” as applied to water are inexact and some writers have tried to improve on this situation by adding qualifying adverbs.

Hardness range mg/L calcium carbonate
0-75 soft
75-150 moderately hard
150-300 hard
300 + very hard

Hardness in water used for ordinary domestic purposes does not become particularly objectionable until it reaches a level of 100 mg/L or so. Hardness can greatly exceed this level and in many places, especially where waters have contact with limestone or gypsum, 200-300 mg/L or more of hardness will be common. Hardness in water may be caused by the natural accumulation of salts from contact with soil or geologic formations.

The significance of water hardness on the aquatic organisms can exert a considerable influence on algae. Many genera of algae occur freely both in soft, acid water and in limestone spring streams. But others seem to be primarily confined to either hard or soft waters.

Water hardness also seems to affect some higher aquatic plants, but not others. A few only occur in soft water, whereas other plants seem to require harder water and few are apparently indifferent.

Calcium not only affects the efficiency of osmoregulation, but it has also been shown for groups as far apart as flatworms and fish that both calcium and magnesium affect the rate of respiration.

Both ions raise respiratory rates at low temperatures and lower them at high temperatures. It can therefore be expected that water hardness has some important affects on the respiration of aquatic organisms.

Next Topics…
pH of Water
Relation of Acidity and Alkalinity in Water
Total Dissolved Solids in Water
Electrical Conductivity of Water
Primary Anions and Cations in Water
Dissolved Gases in Water
Primary Nutrients and Eutrophication
Toxic Constituents in Water
Pesticides in Water
Oil in Water

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Physical Water Quality
Chemical Water Quality
Biological Water Quality
Water Basics 101

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