The pH of a solution describes its hydrogen-ion activity. The pH scale ranges between 0 and 14.
Water with a pH of 7 is neutral and less than 7 is acidic; a pH greater than 7 denotes alkaline
solution. Dissolved gases, such as carbon dioxide and hydrogen sulphide and ammonia strongly
influence the pH of a solution.
The pH of a water sample can change drastically and rapidly with time, especially when the sample
is exposed to air, changes in temperature, biologic activity and other phenomena. For this reason,
the pH of a water sample should be determined as soon as it is collected.
The pH of some surface waters may be characterized by annual or even diurnal fluctuations. For
example, a stream may have a higher pH in summer than winter because of active summer time
photosynthesis of aquatic plants. This process depletes the dissolved carbon-dioxide content and
raises the pH. Similarly, on a daily basis, plants use carbon dioxide during daylight hours, thus
raising the pH; but at night the increase in carbon dioxide lowers it.
The pH of drinking water, in itself, has no effect on health. On the other hand, corrosion is
associated with pH levels less than 6.5. Corrosion releases metals, such as lead, zinc, copper and
cadmium from pipes and plumbing fixtures, and these substances can be toxic. Furthermore; if the
pH is less than 4, the water may have a sour taste.
Other problems can occur in water with a higher pH. For example, when the pH exceeds 8.5, water
may have an alkali taste, scale may form in pipes and equipment, the germicidal activity of chlorine
is reduced.
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