1. The quantitative capacity of water to neutralize an acid; that is, the measure of how much acid can be added to a liquid without causing a significant change in pH. Alkalinity is not the same as pH because water does not have to be strongly basic (high pH) to have high alkalinity. In the water industry, alkalinity is expressed in mg/L of equivalent calcium carbonate. The following chemical equilibrium equations show the relationships among the three kinds of alkalinity: carbonate (CO32-), bicarbonate (HCO3-), and hydroxide alkalinity (OH). Total alkalinity is the sum of all three kinds of alkalinity.
CO2 + H2O <===> H2CO3 <===> H+ + HCO3- <===> pH 4.5 pH 8.3 2H+ + CO32-
Above pH 9.5 (usually well above pH 10), OH alkalinity can exist or CO32- and OH alkalinities can coexist together. Different tests are used to determine the quantity of the different kinds of alkalinities present in water.
2. A property of water soluble substances (or mixtures) causing the concentration of hydroxyl ions (OH) in water solutions to be higher than the concentration of hydrogen ions (H+). Alkalinity is exhibited in solution by alkalies such as sodium hydroxide and by alkaline salts such as sodium carbonate.

Soap and soap-based products are alkaline, since soap is a moderately alkaline salt and performs well only in an alkaline medium. Detergent products can be formulated with any desired level of alkalinity as dictated by the needs of the cleaning tasks to be performed.

Since the alkalinity is useful in removing acidic, fatty, and oily soils, most detergents are more effective on laundry soils when on the alkaline side. Generally, alkalinity is supplied to laundry detergents by builders.

All automatic dishwasher detergents utilize alkalinity, as do most cleansers and hard surface cleaners. In contrast, most hand dishwashing detergents are close to neutrality, performing efficiently without alkalinity because of the mechanical action of hand rubbing with sponge or dishcloth. (See alkali, base.)

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