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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 oxidizer.

Chlorine Compounds

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.

Hypchlorous acid in pools and spas

  1. HOCl dissociates (breaks up) HOCl < == > (H+) and the hypochlorite ion (OCl-).
  2. HOCl reacts with bacteria and organics. HOCl is a very effective sanitizer
  3. OCl- reacts with ammonia (NH3). OCl- is an extremely strong oxidizer
  4. 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 hypoclorous 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 (salt) (chloride ion) when it is done its job.

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 bacteria.

Free available chlorine at levels up to 10-20 ppm has no detectable taste or odor and causes no irritation. [Go to top]

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 hours

.The theory is that hypochlorus 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 100ppm.

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 liters 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 at least twice a year. If stabilized chlorine (chlorine with CYA) is used then check the CYA more frequently. [Go to top]

Gas Chlorine

Gas chlorine (liquified chlorine gas, chemical formula Cl2 is very dangerous. It is 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. It is therefore necessary to add about 1.5 pounds of soda ash or 2 pounds of sodium sesquicarbonate for each pound of gas used. Chlorine gas Cl2 with water H2O or H+ and OH- < == > HCl (hydrochloric acid) and HOCl hypochlorous acid. The HCl (hydrochloric acid) has to be neutralized with an alkali ( eg soda ash)or alkaline buffer (eg sodium bicarbonate).

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. [Go to top]

"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 liters of liquid chlorine contains about .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 decreased.

The pH of liquid chlorine is 13, so it causes a very alkaline condition to occur in the pool by drastically raising the pH. It is necessary to add about one liter of hydrochloric (HCl) acid per 8 liters of liquid chlorine.

Cyanuric acid should be added separately to keep the chlorine in the pool from being degraded by sunlight.

The total dissolved solids (TDS) of an average pool is increased by roughly 2.5 ppm for each liter of liquid chlorine added, due to the salts(NaCl). This means the TDS will increase about 650 ppm annually, based on 250 liters used over the year. This will make it necessary to drain the pool more often than with gas chlorine. There are additional costs for chemicals to rebalance the water and additional cyanuric acid.

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Calcium Hypochlorite

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 650-700 gms of available chlorine.

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 color out of it and weaken the vinyl. You must use a feeder or pre-dissolve it in water and then add it to a vinyl-liner pool.

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 .65 kg of available chlorine, you will need 50kg of cal-hypo to get the same as 250 liters of liquid chlorine or 30kg of gas chlorine.

The pH of cal-hypo is 11.8. It raises pH causeing an alkaline condition to occur. Keeping pH at 7.5 requires the addition of 250gms of hydrochloric acid per Kg of cal-hypo.

Cyanuric acid must also be added separately to keep the chlorine in the pool from being degraded by sunlight. [Go to top]

Lithium Hypochlorite

Lithium hypochlorite (chemical formula LiOCl) is produced by bubbling chlorine gas through a solution of lithium, sodium and potassium sulfates. It is supplied as a free-flowing powder that provides 35 percent available chlorine.

Lithium hypochlorite is:-

Because it dissolves so rapidly, it cannot be used in a dry-chlorine feeder. It can be predissolved and dispensed in a liquid feeder.)

One kg of lithium hypochlorite provides about 350 gms of available chlorine.

90kg of lithium-hypo produces 30kg of available chlorine.

The pH of lithium hypochlorite is 10.7, so it raises the pH causing an alkaline condition to occur in the pool. Keeping pH at 7.5 requires the addition of 250gms of hydrochloric acid per Kg of lithium hypochlorite.

Cyanuric acid must be added separately to keep the chlorine in the pool from being degraded by sunlight. [Go to top]

Sodium Dichlor

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 buildup 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 gms of stabilizer.

Sodium dichlor is the second most expensive per kg of available chlorine but it does not have any "hidden costs" associated with it.

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 gms of available chlorine. [Go to top]

Trichlor (Tabs)

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 slowly.

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 discoloration 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 kgs of available chlorine necessary to sanitize a "typical" pool of 5 x 10 meters for 1 year.[Go to top]

Prepared by Trevor Croll,

42 Pearse Street, Keperra, Queensland Australia, 4054.
Phone 61 7 3855 1115