Strong spa 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 spa pool chemical is critically important
in assuring the safety and health of swimming pool users. When any of
these spa 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 spa 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 spa pool circulation plumbing
to control automatic chlorine feeders.
Spa 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 spa 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 spa pool chemical tests tend to vary only
over weeks or months in a well-maintained spa pool, and thus need not
be checked as frequently as chlorine and pH.
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