Pool Chemistry
CHLORINE CHEMISTRY FAQ
Q: WHAT'S THE HISTORY OF CHLORINE? ANSWER
Q: HOW IS CHLORINE MADE? ANSWER
Q: HOW DOES CHLORINE WORK TO SANITIZE? ANSWER
Q: IS CHLORINE DANGEROUS, UNSAFE OR UNHEALTHY?
ANSWER
Q: WHAT DIFFERENT TYPES OF CHLORINE ARE THERE?
ANSWER
Q: HOW MUCH CHLORINE SHOULD I ADD? ANSWER
Q: WHAT EFFECT DOES pH HAVE ON CHLORINE? ANSWER
Q: WHAT IS TOTAL, COMBINED AND FREE CHLORINE?
ANSWER
Q: WHAT IS SHOCKING OR SUPER CHLORINATING? ANSWER
Q: WHAT ABOUT STABILIZER? ANSWER
Q: WHAT DOES THE TERM "WATER BALANCE" MEAN? ANSWER
Q: WHAT IS ALGAE? ANSWER
WHAT'S THE HISTORY OF CHLORINE...
Chlorine was first discovered in the sixteenth
century, and today is one of the most produced
chemicals in the US, finding its way into a multitude
of products. Chlorine is so deeply inter-twined
into industry that finding alternatives would
indeed change our daily lives.
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HOW IS CHLORINE MADE...
Chlorine is produced by the electrolysis of salt
water. When electricity is passed through 2NaCl
(salt) and 2H20 (water), the atoms dissociate
into Cl2 (chlorine) + 2NaOH (sodium Hydroxide)
+ H2 (Hydrogen). Cl2 is isolated in its gaseous
form, and used to create other chlorine compounds
used for sanitizing, bleaching and production
of plastics and related products.
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HOW DOES CHLORINE WORK TO SANITIZE...
When chlorine is added to water, another dissociation
occurs. When we add Cl2 (chlorine) + H2O (water),
we get a reaction which leaves us with HOCl (hypochlorous
acid) + HCl (hydrochloric acid). Hypochlorous
acid is the active, killing form of chlorine.
This is what does the real sanitizing work. The
chlorine molecule or ion kills microorganisms
by slashing through the cell walls and destroying
the inner enzymes, structures and processes. When
this occurs, the cell has been deactivated, or
oxidized. The hypochlorous molecule continues
this slash & burn until it combines with a
nitrogen or ammonia compound, becoming a chloramine,
or it is broken down into its component atoms,
becoming de-activated itself.
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IS CHLORINE DANGEROUS, UNSAFE OR UNHEALTHY?
No doubt about it, chlorine certainly is a hazardous
product, with some forms of it being more dangerous
for the handler and the environment than other
forms. Chlorine in a solution of water at levels
found in swimming pools pose no danger for swimmers.
Allergic reactions to chlorine are rare, however
some individuals may experience skin irritation.
Chloramines, sometimes found in poorly balanced
water, are the cause of occasional red eyes when
swimming. Extremely high levels of chlorine in
the water could possibly release enough gas off
of the surface in certain conditions to render
breathing difficulties.
The main hazard is towards the person in charge
of adding the chlorine to the pool. Use caution
and always read the directions. Always use care
when opening a container of chlorine. Breathing
in chlorine gas can knock you right out, and could
be fatal. Always wear protective handling gear
such as eye protection and rubber gloves. If chlorine
touches your skin, you should wash it off to prevent
irritation. If chlorine splashes in the eye, irrigate
with water and contact a physician straight away.
The label on the chlorine container will also
tell you never, never, never mix chlorine with
any other chemical. You could produce something
of a bomb or even napalm. This includes mixing
two different types of chlorine, or chlorine and
bromine. KABOOM! Dirt, debris or any foreign substance
(algaecides, alkalis and acids, etc.) can cause
spontaneous combustion when mixed with chlorine.
FIRE! See our page on Chemical Safety for more
info.
As for the environment; pools have a small impact.
Being closed systems, pools just don't contact
nature very much. There could be some hazard involved
with waste water from the pool that has either
extremely high chlorine levels or extremely low
pH levels. It is unlikely that the amount of water
expelled during backwashing your filter could
pose much harm, but impacts have been seen on
drain and clean jobs. Pool water is very similar
in make up to the city water right out of the
tap. Many people water their lawns with higher
chlorine and lower pH than is found in their pool.
There are certain industrial uses of chlorine
chemistry which are detrimental to the environment,
but in a swimming pool environment, the potential
hazards are much lower, even non-existent. There
has been much publicity and controversy over chlorine
in recent years, with some organizations calling
for bans on its use. You decide if chlorine chemistry
is right for you and your pool. For more information
on chlorides, contact the NSPI.
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WHAT DIFFERENT TYPES OF CHLORINE ARE THERE...
In its elemental state, chlorine exists as a
gas. Gas is available for swimming pool sanitation;
is very cheap, and is the purest form of chlorine,
with no binders or carriers. The % of available
chlorine is 100%. It is also extremely dangerous
and restricted in its use. It is rare to find
a pool using gas as it's sanitizer, and those
that do are usually very old, very large public
pools that have (we hope) enacted strict safety
procedures. Gas is very acidic, with a pH close
to muriatic acid, so these pools using it add
a lot of base to counteract this.
Liquid chlorine is another type which is created
by bubbling the chlorine gas through a solution
of caustic soda. The yellow liquid (stronger,
but chemically identical to bleach) has 10-15%
available chlorine, and has a pH on the other
end of the scale at 13. Liquid Chlorine is called
Sodium Hypochlorite (NaOCl) and because it is
already in solution, sodium hypo produces hypochlorous
acid instantly when it contacts water. The liquid
can be poured directly into the pool, but it is
recommended to use a diaphragm or peristaltic
pump. The use of liquid chlorine is more dominant
in larger commercial pools which have it delivered
into 55 gallon vats. For most residential pools,
its lower cost seems to be outweighed by its difficulty
in use and the amount of acid required to counteract
its pH of 13. Use care when handling as this chemical
is corrosive to just about everything.
Trichlor is a tablet form of chlorine, and is
short for Trichloro-s-triazinetrione, a stabilized
form of chlorine that has achieved a great amount
of use in the last ten years. "Stabilized"
means that it has cyanuric acid pressed into the
tablet. Cyanuric, also called stabilizer or conditioner,
is like sunscreen for the chlorine molecule; an
extender, if you will. Trichlor is created by
combining the salts of cyanuric acid and chlorine
gas into a tablet or stick, and is 90% available
chlorine. The pH is somewhat low at 3, so the
pH in your pool may gravitate downward. This form
is slow dissolving, and so it works well in floaters
or in-line erosion feeders. Using tablets in the
skimmer is not recommended, because of the corrosive
nature of the chemical contacting metal pipes
and equipment. This becomes more of a problem
when the filter pump is operated on a timer. It
has been known to strip out the copper inside
of a heater. (An effective, yet expensive means
of controlling algae). Tablets should also not
be thrown directly into the pool, they can stain
and etch plaster and bleach and deteriorate vinyl.
Another member of the chlorinated iso-cyanurate
family is Dichlor, short for Sodium Dichloro -s-triazinetrione.
Dichlor is made in roughly the same manner as
trichlor, however the product is much different.
The pH is a very acceptable 7, and it is manufactured
in the form of granules, so it dissolves rapidly
and goes right to work on contaminants. Dichlor
has less chlorine, pound per pound at only 62%
available chlorine. Because it contains cyanuric
acid, it lasts longer than other unstabilized
forms of granular chlorine. It can be used as
a shock treatment oxidizer, or for normal sanitation.
Dichlor's main drawback is it's cost per pound
of available chlorine. It is perhaps the most
expensive form of chlorine available.
There are two other types of granular chlorine
on the market - the hypochlorites. Lithium Hypochlorite,
like dichlor, is a very expensive product. At
only 35% available, it takes almost 3 lbs of lithium
to equal one lb of trichlor and its pH of 11 will
require additions of an acid to adjust the pool
water. It's main advantages are that it is calcium
free, and so it won't contribute to hardness levels;
it's dust free and non-flammable. It dissolves
extremely quickly, before it hits the floor, so
its use is safe in vinyl liner pools. Lithium
can be used for either shock treating or for regular
chlorination.
Calcium Hypochlorite is commonly available in
is granular form, but can also be purchased in
tablet form. Cal Hypo is a commonly used shock
treatment throughout the country. Although not
stabilized with cyanuric acid, it has a quick
kill rate against algae and chloramines, and has
65% available chlorine per lb. Some other granular
forms of chlorine are more powder like, and thus
dissolve more rapidly than the larger granules
of cal hypo. It's a good idea to pre dissolve
cal hypo into a bucket of water prior to adding
it to a pool. It's popularity is due mainly to
its availability and low price, despite a high
pH value of nearly 12, and the calcium binders
used which contribute to higher hardness levels.
Cal hypo is more dangerous and unstable than other
forms in that it is very dusty and becomes contaminated
easily by foreign substances which can cause combustion.
Mix only with water, don't breathe the dust, and
keep the lid tightly secure and clean.
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HOW MUCH CHLORINE SHOULD I ADD...
We can't tell you how many tablets or how many
pounds or gallons you'll need for your particular
pool, but your test kit will. Each pool has its
own chlorine demand, the amount of chlorine needed
to destroy contaminants that are present. And
each pool also has a different capacity, or gallonage.
Other factors, such as the water balance, and
particularly pH, have an effect on how much chlorine
you'll use. Most health departments will close
down a public pool that does not have a minimum
of 1.0 ppm of free chlorine available in the water
during a health inspection. With a little experimentation,
you'll be able to determine how many tablets,
for instance, it takes to achieve a consistent
minimum level of 1.0 ppm. For example, test the
water, then add three tablets into your feeder.
Check the residual in 12-24 hours. If it's too
low, add more; too high, remove some. Fairly soon
you will develop an idea of your pool's particular
chlorine demand, however, this will change during
warmer months and periods of increased use, among
other things. Chlorine is an expensive and corrosive
compound, so make the attempt to avoid consistently
overchlorinating the pool, which may also irritate
your swimmers.
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WHAT AFFECT DOES pH HAVE ON MY CHLORINE...
The efficacy of chlorine, that is, the power
of it to have an effect, is greatly influenced
by the care with which you manage your pH levels.
As the pH of your pool increases, the killing
power of your chlorine decreases. At a pH of 6.0,
we'll get 96% or so of the potential out of each
lb of chlorine, but at what cost? Such a low pH
would wreck havoc on all of the surfaces the water
comes in contact with, including swimmers. It's
just too corrosive. Move the pH up to 7.0 and
the efficacy of the chlorine drops to 73%, but
raise it up to 8.0, where many a pool seems to
drift to, and it drops dramatically...down to
21%! At a perfect pH level of 7.5, we can expect
to have about 50% of our chlorine in the molecular
structure of hypochlorous acid, the active, killing
form. The remaining half is in the form of a hypochlorite
ion, which is also an active form of chlorine,
but very weak and slow to kill.
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WHAT IS TOTAL, COMBINED AND FREE CHLORINE...
These are states of existence for the chlorine
molecule. If a molecule is free, it has not bonded
with or combined with another compound. It is
therefore available for sanitizing. When free
chlorine molecules encounter and destroy a nitrogen
or ammonia containing compound, they combine with
them to create a combined chlorine compound, or
a chloramine. The chloramine is no longer available
to sanitize anything, and it floats around in
the water, blocking the path of those do-gooder
free chlorine molecules, and stinkin' the place
up! If you smell a strong aroma of chlorine in
and around a pool, chances are it has high combined
chlorine levels. This level can also be tested
with a DPD test kit which measures total and free
levels separately and allows the tester to determine
combined levels by subtracting the two. Total
chlorine is simply the sum of combined and free
levels.
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WHAT IS SHOCKING OR SUPERCHLORINATING...
These are synonymous terms for oxidizing everything
in the pool. By raising chlorine levels ten times
the level of chloramines, a threshold is reached
called breakpoint chlorination. When this is reached,
something of a shock, or perhaps more akin to
a lightning bolt, rips through the water, slashing
and burning everything in its path.
When to shock? Some recommend shocking the pool
when combined chlorine levels reach .3 ppm, while
others suggest shocking after a party full of
kids get out of the pool :-) (the theory here
is that kids=urine=nitrogen+chlorine=chloramines).
Others recommend it once every few weeks, whether
it needs it or not. You may use your senses to
determine the need for shocking. If the pool is
hazy, because somebody left the filter off or
forgot to add chlorine, your eyes may tell you
it's time to shock. If you notice a strong chlorine
smell to the water, and the eyes are burning,
you may sense the need for shocking. Large doses
of chlorine, in the way of shocking, are also
very effective when algae has turned the water
or walls a yellow or green color.
How much chlorine is required to shock? Generally,
we want to raise the chlorine level up to around
10 ppm. If using cal hypo, you'll find that at
least one bag per 10,000 gallons will do the trick.
A little more wouldn't hurt, because if you don't
reach the crucial level of breakpoint chlorination,
not only is the chloramine problem not solved,
but matters have been made potentially worse.
Follow instructions on the package of granular
chlorine or non-chlorine shock, which may be potassium
peroxymonosulfate. Liquid chlorine can also be
used for superchlorination. Whatever chemical,
we must introduce 10 times the potential of the
chloramines. For example, if combined chlorine
levels are at 1.0 ppm, we need 10 ppm of free
chlorine levels to reach breakpoint.
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WHAT ABOUT STABILIZER...
Cyanuric acid is a chlorine stabilizer, providing
a chemical cloak around the chlorine molecule
which protects it from the sun's UV radiation,
the largest killer of your chlorine. It can be
a real money saver, that cyanuric, especially
during the hottest times of the year. A test kit
can measure how much cyanuric acid is present
in the water. Recommended levels from the National
Spa & Pool Institute are at 30-50 ppm. The
stabilizer (also called conditioner) is fed directly
into the skimmer at a rate of 4 lbs per 10,000
gallons. It dissolves inside the filter and you
will immediately see a reduction in your chlorine
demand. If you are using chlorine from the iso-cyanurate
family, the cyanuric acid is already present in
the tablet and you shouldn't need to add any additional,
unless the level is below 30-50 ppm.
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