Chlorine will Sanitize and Oxidize
There are a number of types chlorine, bromine and permonosulfate
compounds. Each have advantages and disadvantages: cost rating and hidden
costs, safety and tips for use as a pool or spa water disinfectant and or
Chlorine products are required by law to display on the label, their
chemical name and concentration. When chlorine in its various forms is
added to the water it produces hypochlorous acid (HOCl). Hypochlorous
acid both controls bacteria and algae and oxidizes organics.
Hypochlorous acid is a weak acid and therefore not harmful to people.
Hypochlorous acid in pools and spas
- HOCl dissociates (breaks up) HOCl <===> H+ + OCl-
- HOCl reacts with bacteria and organics. HOCl is a very effective
- OCl- reacts with ammonia (NH3). OCl-
is an extremely strong oxidizer
- HOCl reacts with sunlight.
Hypochlorous acid (HOCl ) exists in a killing form (HOCl) and as a
strong oxidizer (OCl-). The pH of water determines how much
HOCl disassociates into H+ and OCl-. Chlorine as
hypochlorous acid (HOCl) is needed for sanitization. Chlorine as (OCl-)
is needed for oxidization
Chlorine is most efficient at a pH of approximately 7.4 - 7.6. At low
pH chlorine tends to use itself up too quickly. At a high pH chlorine
doesn't produce very much disinfectant or hypochlorous acid - it is mostly
in the form H+ and OCl- (plenty of oxidiser, low
disinfectant). Pools kept at higher pH values have more difficulty with
algae and bacteria counts. The level of chlorine must be raised to
compensate for this.
Hypochlorous acid is reduced to inactive chloride ion when it has done
The reaction of HOCl with ammonia produces a series of chlorine like
odorous, irritating compounds called chloramines or combined chlorine.
These chloramines irritate eyes and mucous membranes and are often
confused with chlorine. Combined chlorine has little ability to kill
Free available chlorine at levels up to 10-20 ppM has no detectable
taste or odour and causes little or no irritation to touch. This level is
not safe for swimming, however, and pools and spas should be kept below
Cyanuric Acid (CYA) - Chlorine Stabilizer
Ultra-Violet (UV) light degrades chlorine by a chemical reaction in
which two chlorine molecules combine with two molecules of water to form
four molecules of hydrochloric acid also known as hydrochloric acid plus
oxygen. Without the use of Cyanuric acid (stabilizer) on a bright sunny
day 90% of the active chlorine can be destroyed by sunlight in just 2
The theory is that hypochlorous acid (HOCl) and hypochlorite ions (OCl-)
closely attach to one of the 3 free bonding sides of the CYA molecule. As
long as they remain attached, they are not degraded by sunlight. CYA acts
like a "buffer" in that it stores chlorine and releases it to do
its work on bacteria and algae. An excess of CYA in the water (over 100 ppM)
stores up the chlorine which is then not available to sanitize. There is
some concern about the toxicity of Cyanuric acid at above 100 ppM.
It is recommended that you maintain a residual of 30-50 ppM of CYA. At
25 ppM the chlorine will last 3-5 times longer. Above 50 ppM, no marginal
stabilization benefit is observed.
To achieve 50 ppM of CYA add 2 kg of conditioner for each 40,000 litres
of water to be protected. Stabilized chlorine products like Dichlor or
Trichlor can cause CYA levels to rise to over 100 ppM.
Remove Cyanuric acid from the pool water by draining/dilution or splash
out and backwashing. It should be checked monthly, or after heavy rain has
required the draining of some pool water. If stabilized chlorine (chlorine
with CYA) is used then check the CYA more frequently.
"Liquid Chlorine" (sodium hypochlorite)
So called Liquid chlorine (sodium hypochlorite, chemical formula NaOCl
in NaOH (caustic soda)) is manufactured by bubbling chlorine gas through a
solution of caustic soda (NaOH). At l0-percent available chlorine 4 litres
of liquid chlorine contains about 0.5 kg of available chlorine. Liquid
chlorine, when added to water makes hypochlorous acid (the killing form of
chlorine) instantly. It can be used regularly or for superchlorination.
Liquid chlorine is non-flammable and is compatible with other
water-treating chemicals commonly found in a pool or hot tub.
Liquid chlorine should be stored in a cool place, shaded from sunlight.
At 26oC the available-chlorine level in
a jug of liquid chlorine will drop from more than 12 percent to 9 percent
in one day. At 34oC the available chlorine level will drop from
12 percent to 9 percent in just 2 hours.
Aeration and sunlight can destroy part of the available chlorine when
pouring it into the pool. Pour it into the pool with the jug as close to
the water surface as possible. One to two percent of the available
chlorine can be lost by pouring the liquid from only 4 feet above the
water's surface on a sunny, hot day. Common household liquid bleach is
sodium hypochlorite, at around 1.5 percent.
Adding chlorine at a return jet or in a manner that mixes it with the
water reduces the chlorine loss. If you stand in one spot while you pour
the chlorine, the sunlight can begin to destroy the chlorine before it has
had a chance to mix with and be protected by the Cyanuric
acid (conditioner). You can lose 1 to 2 percent of the available
chlorine if you don't "walk it around or mix it at a return
jet." Chlorine is more effective if added when the sun intensity has
Cyanuric acid should be added separately to keep the chlorine in the
pool from being degraded by sunlight.
Cal-hypo (calcium hypochlorite, chemical formula Ca(OCI)2)
also called bleaching powder. It is produced by passing chlorine gas over
slaked lime. Cal-hypo is stable and can be stored for long periods of time
without significant loss in available chlorine level. In water it quickly
forms hypochlorous acid (the killing form of chlorine).
It is supplied in granular form or as tablets. It can be used for
regular chlorination as well as superchlorination. It should be
pre-dissolved in water and then added as a liquid to the pool when needed.
One kg of calcium hypochlorite provides about 650-700g of available
Cal-hypo is classified as an extreme oxidizer which is why it bleaches
so well. All the other types of chlorine are classified as oxidizers with
the exception of gas chlorine. Avoid mixing cal-hypo with acids, ammonia,
soda pop, oil, trichlor or just about anything but water. Mixing with
organics will cause a fire. Be careful when sweeping around
chemical-storage areas. Mixing the dust and spillage of various products
together and then putting them into a trash can or dumpster may cause a
fire. Clean up product spills separately.
Cal-hypo granules and powder will temporarily cloud the water because
the calcium takes a long time to dissolve completely. Cal-hypo granules
sitting on vinyl will bleach the colour out of it and weaken the vinyl.
You must use a feeder or pre-dissolve it in water and then add it to a
Depending on local water conditions, using cal-hypo will increase the hardness
level by an average of 3 to 10 ppM per month or about 1 to 5 ppM for each
kg you add to an average 5 x 10 meter pool. This may make it necessary to
test the hardness level in the pool more often.
Because one kg of cal-hypo provides 0.65 kg of available chlorine, you
will need 50kg of cal-hypo to get the same as 250 litres of liquid
chlorine or 30kg of gas chlorine.
If the form of cal-hypo being used does not contain Cyanuric acid, this
must also be added separately to keep the chlorine in the pool from being
degraded by sunlight.
Lithium hypochlorite (chemical formula LiOCl) is produced by bubbling
chlorine gas through a solution of lithium, sodium and potassium sulphates.
It is supplied as a free-flowing powder that provides 35 percent available
Lithium hypochlorite is:-
Because it dissolves so rapidly, it cannot be used in a dry-chlorine
feeder. It can be pre-dissolved and dispensed in a liquid feeder.
One kg of lithium hypochlorite provides about 350g of available
90kg of lithium-hypo produces 30kg of available chlorine.
Cyanuric acid must be added separately to keep the chlorine in the pool
from being degraded by sunlight.
Sodium dichlor (sodium dichloro-s-triazinetrione, chemical formula NaCl2C3N3O3)
is the only popular type of chlorine that does not require the addition of
either a neutralizing chemical or Cyanuric acid.
Sodium dichlor is produced by adding soda ash and Cyanuric acid to a
solution of trichlor. When dried the result is a granule that may provide
56 percent or 62 percent available chlorine, depending on the method of
manufacture. The 56-percent formulation is by far the most readily
available of the two.
Sodium dichlor is fast dissolving, will not cloud the water and has a
long shelf life. It can be used for regular and for superchlorination.
Because it is fast dissolving, it cannot be used in a dry chemical feeder.
It should not be pre-dissolved and dispensed in a liquid chlorinator.
Sodium dichlor can cause a build-up of Cyanuric acid in the pool water.
It is 57% stabilizer (Cyanuric acid) by weight. Cyanuric acid levels
should be more frequently checked. Partial drain and refill the pool is
required if the Cyanuric acid level exceeds 100 ppM. One kg of sodium
dichlor contains slightly more than 560gms of available chlorine and 570 g
Sodium dichlor is the second most expensive per kg of available
chlorine but it does not have any "hidden costs" associated with
Sodium Dichlor has a pH close to 7 (neutral) and so does not require
any neutralizing chemicals to be added to the water. It already contains Cyanuric
acid saving on this cost also.
One kg of sodium dichlor provides 560 g of available chlorine.
Trichor (trichloro-s-triazinetrione, chemical formula Cl3C3N3O3)
contains 90-percent available chlorine - the highest of all. Trichlor is
produced by drying and cooling the sodium salt of Cyanuric acid in the
presence of chlorine gas.
Trichlor is mostly available as tablets, sticks or a cartridge. It has
a long shelf life, and it is very slow dissolving, so it works extremely
well in floaters and erosion-type feeders. It can be used for regular
chlorination but not for superchlorination because it dissolves too
The granular form can be used as a "spot algicide." Trichlor
does not require the addition of Cyanuric acid to
the pool water. Trichlor is highly acidic (pH 2.8 - 3). It will corrode
equipment and pool plaster if improperly used. Corroded metal, usually
copper, will deposit on the pool walls as a turquoise discolouration and
can cause blue fingernails and green hair for swimmers. It is necessary to
add about 350gms of soda ash for each kg of trichlor used
Trichlor increases the Cyanuric acid in the pool water. Cyanuric acid
levels need checking more frequently. Pools should be partially drained
and refilled the pool if the level exceeds 100 ppM.
One kg of trichlor provides 900gms of available chlorine.
35kg of trichlor will supply the 30 kg of available chlorine necessary
to sanitize a "typical" pool of 5 x 10 meters for 1 year.
Gas chlorine (liquefied chlorine gas, chemical formula Cl2 is
very dangerous. It is the cheapest and the most dangerous to use and
store. Gas chlorine is the purest form of chlorine you can buy. There are
no fillers or carriers, so half the chlorine you add to the pool water is
used for disinfecting, sanitizing and oxidizing.
Gas chlorine causes a very acidic condition
to occur in the pool as it drastically lowers the pH
by the formation of Hydrochloric acid (HCl) as a by-product. It is
therefore necessary to add about 6kg of soda ash or 10kg of sodium
sesquicarbonate for each 1kg of gas used.
Because gas chlorine does not have Cyanuric acid (conditioner) in it,
you must use Cyanuric acid separately to keep the chlorine in the pool
from being degraded by sunlight.
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