Is hardness the same thing as alkalinity?

Is hardness the same thing as alkalinity?

One of the most common confusion in water quality parameter is hardness and alkalinity. The fact that they are both expressed in mg per liter, or ppm, of calcium carbonate, is probably one of the sources of confusion itself. But are they the same thing?

Sources of hardness and alkalinity in water

Water passes through the rocks in the ground as it makes its way to rivers and lakes, and picks up minerals on the way. When water passes through limestone and dolomite, hardness and alkalinity are picked up in the water. Limestone is calcium carbonate, and dolomite is a combination of calcium carbonate and magnesium carbonate. Limestone and dolomite consist of 17% of earth’s crust.

Are hardness and alkalinity the same thing?

Hardness and alkalinity are many times related or even equal in measurement. But they are very separate measurements, and have very different significance.

Total alkalinity is the measurement of all bases in the water and can be thought of as the buffering capacity of water, or its ability to resist change in pH. Total alkalinity is expressed as milligrams per liter (mg/L) or parts per million (ppm) of calcium carbonate (CaCO3). A base is a substance that releases hydroxyl ions (OH-) when dissolved in water. In most waters these bases are principally bicarbonate (HCO3) ions and carbonate ions (CO32-). These ions are the pH buffers in water. They can do this by absorbing hydrogen ions when the water is acid and releasing them when the water becomes basic.

Total hardness is the concentration of hard ions, or more precisely the measurement of divalent cations (+2 ions) in the water and, like total alkalinity, is expressed as milligrams per liter (mg/L) or parts per million (ppm) of calcium carbonate (CaCO3).

When limestone and dolomite dissolve in water, one half of the molecule is calcium or magnesium (the "hardness") and the other half is the carbonate (the "alkalinity"), so most of the time alkalinity and hardness values are similar to magnitude. That’s another source of confusion. However, in some waters alkalinity may exceed its hardness and vice versa, in natural or treated state.

Measurement of water hardness

General guidelines for classification of waters are: 0 to 60 mg/L (milligrams per liter) as calcium carbonate is classified as soft; 61 to 120 mg/L as moderately hard; 121 to 180 mg/L as hard; and more than 180 mg/L as very hard.

Measurement of water alkalinity

Waters of low alkalinity (<20 mg/l) are poorly buffered. Waters that have moderate to high levels (50 mg/L or greater) of total alkalinity (and total hardness) usually have a neutral to slightly basic pH.

Impact of hard water

When hard water is heated, solid deposits of calcium carbonate can form. This scale can lower the efficiency of equipment, raise the cost of operation, and reduce equipment life.

When using hard water, more soap or detergent is needed to get things clean because soap easily reacts with calcium to form "soap scum".

Impact of low alkalinity

One place where total alkalinity is very important is aquatic life. Low alkalinity can mean large fluctuation in pH, which is lethal to aquatic life like fish. Alkalinity will buffer the impact from acid rain, and thus protect our environment from sudden changes.

Low alkalinity can also bring high acidity in water, which can be corrosive to equipment.

Water softener and alkalinity

Water softener works through a technology called ion exchange. It basically exchange the calcium and magnesium ion in water with sodium ion. Water softener will not do anything to the carbonate ion in water. In other words, water softener does not change alkalinity in water.

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