Strong swimming pool chemicals known
as oxidizing agents are often used to keep pools clean, especially simple
chlorine compounds such as sodium hypochlorite. Other disinfectants include
bromine compounds and ozone. Chlorine, bromine, and ozone can all be generated
on site by passing an electrical current through either the pool water
itself, in the case of chlorine or bromine, or through oxygen or air,
in the case of ozone. Chlorine may be supplied as a sodium hypochlorite
solution (bleach), powdered calcium hypochlorite ("cal hypo"),
chlorinated cyanurate compounds ("stabilized" chlorine such
as dichlor or trichlor), or by dissolving chlorine gas directly in water.
Maintaining a safe concentration of swimming pool chemical is critically
important in assuring the safety and health of swimming pool users. When
any of these swimming pool chemicals are used, it is very important to
keep the pH of the pool in the range 7.2 to 7.6; higher pH drastically
reduces the sanitizing power of the chlorine due to reduced ORP, while
lower pH causes bather discomfort, especially to the eyes. Chlorine reacting
with urea in urine from bathers can create nitrogen trichloride, which
has a teargas-type effect.
Where the water is sanitized by means of swimming pool chemicals known
as oxidizers, some suppliers of electronic monitoring equipment recommend
that the efficacy of the oxidizer be measured by the oxidation-reduction
potential of the water, a factor measured in millivolts, where the minimum
acceptable ORP level in public pools is 650 millivolts. This is supposed
to ensure a 1-second kill rate for microorganisms introduced into the
water. Unfortunately, a commonly used non-chlorine supplemental oxidizer,
potassium monopersulfate, can produce measured 650 mV levels even in the
absence of all sanitizing residuals. Cyanurated ("stabilized")
chlorinators can give falsely high chlorine readings when tested with
OTO (ortho-tolidene, a yellow indicator dye used in inexpensive test kits),
since the chlorine indicated by the dye is mostly in a combined form instead
of free, and does not contribute to ORP. ORP test cells are available
as hand-held instruments, and as probes for mounting permanently in the
pool circulation plumbing to control automatic chlorine feeders.
Swimming pool chemical test kits to make basic measurements of free chlorine
and pH from a sample of pool water, which are the most important items
to control in a swimming pool, are packaged with small dropper bottles
of reagents. These reagents are typically OTO for chlorine and phenol
red for pH. OTO has been phased out in most countries due to a connection
with the production of cancer cells in test rats. More commonly DPD tablets
replaced OTO since 1980. The kits include vials for mixing a water sample
with the test reagents, and color charts for reading the indicated levels.
These kits are termed "Comparator" kits as the test is "compared"
to a known color value. Besides chlorine and pH, which should be checked
frequently, more sophisticated reagent kits provide tests for acid demand
and base demand, total alkalinity (TA), calcium hardness, and cyanurate
("stabilizer") concentration. These additional swimming pool
chemical tests tend to vary only over weeks or months in a well-maintained
pool, and thus need not be checked as frequently as chlorine and pH.
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