What is the History of Chlorine?


It was in 1774 when a Swedish chemist named Carl Scheele discovered the element Chlorine when he mixed powdered pyrolusite into muriatic acid. Chlorine gained its name from the Greek word "Khloros", which means "green".

Chlorine today is one of the most widely produced chemicals in the US, finding its way into a multitude of products. Chlorine is so deeply woven into industry that finding alternatives would be a challenge.

Chlorine is also used for the production of numerous products, across all segments. Some plastics can contain over 50% chlorine by volume. Just about any manufactured product uses chlorine at one step along the way. Chlorine is also used in many industries for disinfection or sanitizing of surfaces, equipment or aqueous solutions, including pool water.

How Chlorine is 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).

When using a salt chlorine generator on a pool, the pool chlorine is created by the salt cell, a stack of electrically charged plates. When power is applied to these cells, and salty water is pumped through the cell, chlorine is created instantly as the water leaves the salt cell and is pumped into the pool.

In the manufacture of chlorine, Cl2 is isolated in its gaseous form, and used to create other chlorine compounds used for bleaching or disinfecting. The gas is passed over salts or bubbled through water to create a base product that can be packaged in granular, tablet or liquid forms.

Stabilized products, such as Tri-Chlor tablets or Di-Chlor shock, will have an addition of cyanuric acid, and some packaged products may also contain other additives to help particle coagulation or improve sanitizer effectiveness.

How much Chlorine is Needed?

Swimming pools require a constant level of chlorine in the water at all times. The moment that chlorine drops below 1.0 ppm, algae and bacteria begin to thrive. 1.0 ppm is a generally accepted minimum for public pool health standards, however recent studies have suggested that cyanuric acid in the pool water has a suppressive effect on the mobility and strength of free chlorine molecules. For this reason, some experts now recommend higher levels of free chlorine be used, from 3-6 ppm, rising with increasing levels of cyanuric acid from 30-60ppm. Most backyard pools should operate with 2-4 ppm of free chlorine, although the use of mineral purifiers or ozonators can cut the chlorine demand in half. Trichlor tablets are used to maintain the normal daily operating level, and most pools will use 2-3 tablets per 10000 gallons, per week, to achieve proper levels.

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). How much of each is created is dependent on water temperature, cyanuric acid level and especially the water pH level. Hypochlorous acid is the active, killing form of chlorine. This is what does the real sanitizing work in your pool water. The chlorine molecule or ion kills microorganisms by slashing through the cell walls and destroying inner enzymes, proteins 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. Some amount of chlorine will also gas off from an uncovered pool.

Is Chlorine Dangerous and 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 normal levels found in swimming pools pose no danger for swimmers, pets or the surrounding plants and animal life. In many cases, treated public water systems have more chlorine than your average pool (test your tap water!).

Allergic reactions to chlorine are rare, however some individuals may experience skin irritation. Chloramines, sometimes found in poorly balanced water, are often the cause however, and are also the cause of red eyes and a strong chlorine smell in the air.

Extremely high levels of chlorine in the water could release enough gas off the surface in certain conditions to render breathing difficulties, especially in poorly vented indoor pools. Swimmers who continuously breathe in air just above the water surface are subject to a higher possible uptake than others.

Another hazard is towards the person in charge of adding the chlorine to the pool. Use caution and always read the directions printed on the label. Use care when opening a container of chlorine. Breathing in chlorine gas can overwhelm, and could be fatal. Always wear protective handling gear such as eye protection and rubber gloves, and always hold your breath as you open containers, reach into containers, and apply chlorine products. 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 right away.

The label on the chlorine container will also tell you never, never, never mix chlorine with any other chemical. Use dedicated scoops used only for one chemical, and close up chemical lids tightly.

Mixing two different types of chlorine, like tablets and shock, or mixing chlorine and bromine can instantly ignite or explode. Mixing acids like ph Down, and chlorine can produce a toxic gas similar to mustard gas.

Dirt, debris or any foreign substance (like algaecide, antifreeze, alkalis, acids, clarifiers, enzymes...) can cause spontaneous combustion when mixed with chlorine. Explosions and fires are a very real possibility when Any pool chemical is mixed with pool chlorine.

Fire or explosion can result if chlorine becomes contaminated with small amounts of dirt or bits of leaves. Chlorine products react with water, and water or moisture inside of a chlorine bucket produces a strong and potentially hazardous yellow gas. Keep all chlorine products tightly closed, clean and dry.

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 or high algaecide levels. It is unlikely that normal levels in water expelled during backwashing your filter could pose much harm. However, large amounts of D.E. Powder can choke a small stream, and if pumped directly into it. Even small levels of chlorine and algaecide could damage the ecology if you pump downhill into the woods.

An impact to the environment can occur when acid washing, if acidic water is pumped into nearby streams. Before discharging acid waste water, balance the pH of the water, by adding enough base to neutralize the acid used. In other cases, allow sanitizer and algaecide levels to drop to low levels before discharge or draining. Discharge hoses should be moved frequently to avoid erosion and saturation and placed in areas that won’t drain directly into a small stream, which always leads to a larger stream.

Salt pools should also be conscious of the salt level in their pool, and be especially cautious not to pump too much, too fast toward a neighborhood watershed. If distributed about large, flat areas, and not allowed to rush down hillsides and enter estuaries, the effect should be minimal. Many freshwater organisms cannot tolerate the saline levels over 1000 ppm, so please be careful with 3000 ppm in pool salt water. Cartridge filters or DE filters with separation tanks can be used to eliminate backwash waste water from a pool.

Chlorine Test Kits

Pool water can be 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 (test it!). There are certain industrial uses of chlorine chemistry which are detrimental to the environment, such as paper or plastic manufacturing, but in a swimming pool environment, the potential hazards are much lower, even non-existent. There has been some who call for reductions in our chlorine reliance in recent years, even some organizations calling for a ban on its use. You decide if chlorine is right for you and your pool.

What different types of Chlorine are there?

In its elemental state chlorine exists as a gas. Gas is available for swimming pool sanitation. It is very cheap, and is the purest form of chlorine with no binders or carriers. The percentage 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 a sanitizer, and those that do are usually very old, very large public pools that have (we hope) enacted strict safety procedures. Chlorine 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 very high pH, on the other end of the scale at 13. Pools using liquid chlorine for sanitation will add a lot of acid to the pool, to counteract this, and lower the pH down into proper range.

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 pump or a peristaltic pump to inject it into the plumbing. The use of liquid chlorine is more dominant in larger commercial pools which have it delivered by trucked and pumped into 55 gallon vats. For most residential pools, the lower cost may 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.

Chlorine and Bromine

Tri-Chlor Tablets are tablet or stick chlorine and the term is short for Trichloro-s-triazinetrione, a stabilized form of chlorine that has achieved a great amount of use in the last thirty years. "Stabilized" means that it has cyanuric acid pressed into the tablets. Cyanuric, also called stabilizer or conditioner, is like sunscreen for the chlorine molecule; used as an extender. Tri-chlor is created by combining the salts of cyanuric acid and chlorine gas into a pressing machine to form a tablet or stick. Testing of cyanuric acid levels at least annually is required, to maintain it in the range of 30-50 ppm. Most Trichlor is 90% available chlorine, which is a high percentage, as compared to other forms of chlorine. The pH of pool tablets is somewhat low at 3.0, so the pH in your pool may gravitate downward, and require periodic adjustment with ph Increaser. Tablets or sticks are slow dissolving when placed in Floaters or in-line Chlorinators. Using tablets in the skimmer is not recommended because of the corrosive nature of the chemical contacting metal pipes and equipment, and soft Flex PVC pipe. This becomes more of a problem when the filter pump is operated on a timer. Tablets should also not be thrown directly into the pool, they can stain and etch plaster and bleach and deteriorate vinyl. Tablets have been known to strip out the copper inside of a heater, when a chlorinator is installed incorrectly. Despite these restrictions, chlorine tablets are an effective, yet expensive, means of controlling algae and bacteria, and the most common method used by pool owners.

Di-Zap Dichlor Pool Shock

Granular chlorine. Another member of the chlorinated iso-cyanurate family is Di-chlor, or Sodium Dichloro-s-triazinetrione. Di-chlor is made in roughly the same manner as Tri-chlor; however, the product is very different (and incompatible). The pH is a very acceptable 7.0, and it is formulated as a granular, so it dissolves rapidly and activates quickly. Di-chlor has less chlorine, pound per pound, at only 62% available chlorine. Because it contains cyanuric acid, it lasts longer than other un-stabilized forms of granular chlorine, which can deplete in a single sunny day. It can be used as a shock treatment oxidizer or for normal sanitation. Di-chlor has the drawback of cost per pound of available chlorine, being one of the more expensive types of chlorine.

There are two other types of granular chlorine on the market - the hypo-chlorites. Lithium Hypochlorite, like di-chlor, is another more expensive product. Lithium has 35% available chlorine, and a higher pH level of near 11. Its main advantages are that it is calcium free, and will not contribute to hardness levels; it is also dust free and safer to store and use. It dissolves extremely fast, before it hits the floor of the pool, so its use is safe in vinyl liner pools. Lithium Hypochlorite can be used for either shock treating or for regular chlorination.

Calcium Hypochlorite Granules

Calcium Hypochlorite is the most commonly used granular chlorine, or pool shock, and is available in 1 lb bags, or sold loose in 25 & 50 lb. buckets. ‘Cal-hypo’ usually refers to the commonly used shock treatment, a granular product. Although not stabilized with cyanuric acid, it has a quick kill rate against algae and chloramines and has 65% available chlorine per pound (Super Shock has 73%). Cal Hypo can also be purchased in tablet form for pools with calcium or cyanuric concerns. Do not use cal hypo tablets in a chlorinator that feeder meant for Trichlor tablets, they must be used only with a cal hypo tablet feeder. Some other granular forms of chlorine are finer, and thus dissolve more rapidly than the larger granules of cal-hypo. For vinyl pools, pre-dissolve cal-hypo into a (clean) bucket of water prior to adding it to a pool. The popularity of Cal-Hypo is due mainly to its availability and low price, despite a high pH value of nearly 12, and the calcium binders used which can contribute to higher hardness levels. Mix this product with water only and be careful not to breathe the dust. The granules and the solution will also bleach clothes, so handle carefully. Store all chlorine types in a dedicated plastic bucket with the lid tightly secured, clean and dry. Never let any other chemicals or even small bits of leaves or other debris contaminate your chlorine. In the presence of foreign chemicals, Cal Hypo and other types of chlorine will ignite - rapidly and violently.

Pool and Spa Brominating Tablets

Bromine is a sanitizer that has similar properties to chlorine. Bromine however, cannot be stabilized or protected from the sun like chlorine can with cyanuric acid, and for this reason it’s often not used in sunny outdoor pools. Bromine tablets are preferred for use in spas and hot tubs, due to its power across a broad pH spectrum, and stability in hot water. Bromine is also popular for indoor pools, with reduced odor and fewer disinfection byproducts.  Bromine is sold in 1” tabs, which dissolve more rapidly than larger tablets, and also more suited to smaller bodies of water. Bromine pools or spas need to build up a residual in the water, which some call a ‘bromide bank’. Bromine works by having a buildup of bromide ions in the water, which are continuously reactivated by oxidation, with chlorine. Bromide booster chemicals are used to bring up levels rapidly after draining the water, or add additional bromides. Once the bromides have built up, you will now be able to test for a residual of bromine with a water test kit. Bromine is twice as dense as chlorine, so the levels used in a pool or spa are twice that of chlorine. Most users of bromine maintain a level of 3-5 ppm of bromine, to ensure sanitation of the water. Like chlorine, minerals or ozone can also be used to supplement bromine, and reduce bromine demand.

Related Product Pages:

Chlorine and Bromine Pool Shock Treatments Lithium Shock Salt Chlorine Generators Pool Stabilizer / Cyanuric Acid

Chlorine & Bromine
Chlorine Test Kits
Granular Chlorine
Cyanuric Acid
DE Powder
Pool pH Chemicals  
Salt Chlorinators

Related Blog Posts:

Chlorine Buyer's Guide
Fresh Thoughts on Cyanuric Acid
So You Want a Chlorine Free Pool?
Swimming Pool Shock User Guide
The Many Types of Pool Shock