Alkalinity of Drinking Water Explained

The alkalinity refers to the measure of water’s capability to neutralize the acids. Alkalinity of water may be due to the presence of one or more of a number of ions. These include hydroxides, carbonates, and bicarbonates. As discussed in previous articles, hydroxide ions are always present in water, even if the concentration is extremely low. However, significant concentrations of hydroxides are unusual in natural water supplies but may be present after certain types of treatment. Small amounts of carbonates are found in natural water supplies in certain sections of the country, rarely exceeding 3 or 4 gpg(grain per gallon). They may also be found in the water after lime soda has been used to soften the water. Bicarbonates are the most common sources of alkalinity. Almost all natural supplies have a measurable amount of this ion, ranging from 0 to about 50 gpg.

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The alkalinity of water may be defined as its capacity to neutralize the acid. Alkali substances in water include hydroxides or bases. They can be detected by their acrid taste and by the fact that they cause red litmus paper to turn blue.

Phosphates and silicates are rarely found in natural supplies in concentrations significant in the home. Compounds containing these ions may be used in a variety of water treatment processes. Moderate concentrations of alkalinity are desirable in most water supplies to balance the corrosive effects of acidity. However, excessive quantities cause a number of problems. These ions are, of course, free in the water, but have their counterpart in cations such as acidity, magnesium and sodium or potassium.

You probably will not notice an alkaline condition due to bicarbonate ions except when present in large amounts. In contrast, you should readily detect alkalinity due even to fairly small amounts of carbonate and hydroxide ions.

Strong alkaline water has an objectionable "soda" taste. The EPA Secondary Drinking Water Regulations limit alkalinity only in terms of total dissolved solids (500 ppm) and to some extent by the limitation on pH value.

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Highly mineralized alkaline waters also cause excessive drying of the skin due to the fact that they tend to remove normal skin oils.

Troublesome amounts of alkalinity can be removed by reverse osmosis technology, along with other total dissolved solids. Other methods of water treatment also remove total dissolved solids, but they are somewhat less suitable for household use compared to reverse osmosis. These methods are distillation and deionization (demineralization). Distillation and deionization equipment are quite expensive and use large amount of space; whereas reverse osmosis water filtration systems could be installed undersink in kitchen or even on countertops.

Several other methods of water treatment will remove alkalinity, but these methods are not satisfactory for household use. These include:

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1. Lime softening removes hardness. At the same time, this process will precipitate an equivalent amount of alkalinity. Lime softening is usually restricted to industrial and municipal installations.

Lime softening. While reducing total alkalinity, lime softening does convert HC03– to CO3--, a stronger alkalinity ion.

2. An anion resin regenerated with sodium chloride removes substantially all the anions (carbonates, bicarbonates, and sulfates, as well as nitrates). It replaces these anions with a chemically equivalent amount of chloride ions. The disadvantage of this process is that in almost all cases, it results in a high chloride ion concentration. At the point of exhaustion, the resin has the tendency to unload high concentrations of the anions it carries including the nitrates. For household purposes, such results are almost as undesirable as the original alkalinity.

3. The feed of a mineral acid will neutralize the alkalinity of water. Hydrochloric acid, sulfuric acid or a combination of these can be used. This process converts the bicarbonates and carbonates present into carbonic acid. At this point, it is advisable to provide some method to permit the resulting carbon dioxide gas to escape into the atmosphere. The disadvantages of this acid feed technique are obvious. It is important to have precise control of the process and caution in handling the strong acid.

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