A pool's filter system does the heavy lifting in keeping the water
clean, but it takes chemistry to do the fine-tuning. It's important to
carefully manipulate the chemical balance in pools for several reasons:
Dangerous pathogens, such as bacteria, thrive in water. A pool filled
with untreated water would be a perfect place for disease-carrying
microorganisms to move from one person to another.
Water with the wrong chemical balance can damage the various parts of
the pool.
Improperly balanced water can irritate the skin and eyes.
Improperly balanced water can get very cloudy.
To take care of pathogens in the water, you have to introduce a
disinfecting agent that will get rid of them. The most popular pool
disinfectant is the element chlorine, in the form of a chemical compound such as
calcium hypochlorite (a solid) or sodium hypochlorite (a liquid). When
the compound is added to the water, the chlorine reacts with the water
to form various chemicals, most notably hypochlorous acid. Hypochlorous
acid kills bacteria and other pathogens by attacking the lipids in the
cell walls and destroying the enzymes and structures inside the cell
through an oxidation reaction. Alternative sanitizers, such as bromide,
do basically the same thing with slightly different results.
Chlorine is typically prepared in liquid, powder or tablet form (though
some professionals use gaseous chlorine), and it can be added to the
water anywhere in the cycle. Pool experts generally recommend adding it
just after the filtering process, using a chemical feeder. If it's added
directly into the pool, using tablets in the skimmer boxes, for
example, the chlorine tends to be too concentrated in those areas.
One problem with hypochlorous acid is that it's not particularly
stable. It can degrade when exposed to ultraviolet light from the sun, and it
may combine with other chemicals to form new compounds. Pool
chlorinators often include a stabilizing agent, such as cyanuric acid, that
reacts with the chlorine to form a more stable compound that does not
degrade as easily when exposed to ultraviolet light.
Even with a stabilizing agent, hypochlorous acid may combine with other
chemicals, forming compounds that are not very effective sanitizers.
For example, hypochlorous acid may combine with ammonia, found in urine,
among other things, to produce various chloramines. Not only are
chloramines poor sanitizers, but they can actually irritate the skin and eyes
and have an unpleasant odor. The distinctive smell and eye irritation
associated with swimming pools are actually due to chloramines, not
ordinary hypochlorous acid -- a strong smell usually means there is too
little free chlorine (hypochlorous acid), rather than too much. To get rid
of chloramines, pool custodians have to shock treat the pool -- add an
unusually strong dose of chemicals to clear out organic matter and
unhelpful chemical compounds.
Chloramine formation is related to the second major element in pool
chemistry, maintaining the right pH in the pool. The water's pH is a
measure of its total acid-alkalinity balance -- the relative proportion of
acids and alkalis in the water (check out this site for a description of
acids and alkalis). Simply put, water that is either too acidic or too
alkaline will cause undesirable chemical reactions. If the water is too
acidic, it will corrode metal equipment, cause etching on the surface
materials and cause skin irritation. If the water is too alkaline, it
can cause scaling on the pool surface and plumbing equipment and can
cloud the water. Additionally, both high acidity and high alkalinity alters
the effectiveness of the chlorine. The chlorine won't destroy pathogens
as well if the water is too alkaline, and it will dissipate much more
quickly if the water is too acidic.
On the pH scale, zero indicates extreme acidity, 14 indicates extreme
alkalinity and 7 indicates a neutral state. Most pool experts recommend
a pool pH between 7.2 and 7.8. To raise or lower pH, a pool custodian
simply adds acids or alkalis into the water. For example, adding sodium
carbonate (soda ash) or sodium bicarbonate (baking soda) will generally
raise the pH, and adding muriatic acid or sodium bisulfate will lower
the pH.
Maintaining the proper balance of chemicals in the pool is a continual
process, because any new element -- oils from a swimmer's body, a shot
of chlorine, stuff that falls in the water -- shifts the water's total
chemical makeup. In addition to pH, pool custodians also monitor total
alkalinity, calcium hardness and total dissolved solids. Check out this
site for information on these related water chemistry measures, as well
as remedies for various water imbalances.
When you consider all the chemistry and machinery involved in swimming
pools, it's clear that they really are remarkable pieces of technology.
It takes a surprising amount of work and ingenuity to build and
maintain these summertime staples.
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