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History of Chlorine

Chlorine was first discovered in the sixteenth century, and today is one of the most produced chemicals in the world, 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.

Chlorine is produced through 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 sanitising, bleaching and production of plastics and related products.

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 micro organisms by penetrating 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 process 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.

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 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 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. This includes mixing two different types of chlorine, or chlorine and bromine. Dirt, debris or any foreign substance (algaecides, alkalis and acids, etc.) can cause spontaneous combustion when mixed with chlorine.

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 large amounts of DE Powder can choke a stream if pumped directly into it. 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.

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.

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 at the high end of the scale at 13. Liquid Chlorine is called Sodium Hypochlorite (NaOCl) and because it is already in solution, sodium hypochlorite 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. 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 stabilised form of chlorine that has achieved a great amount of use in the last ten years. "Stabilised" means that it has cyanuric acid pressed into the tablet. Cyanuric, also called stabiliser 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. 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, kilo per kilo at only 62% available chlorine. Because it contains cyanuric acid, it lasts longer than other unstabilised 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 kilogram 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 kgs of lithium to equal one kg 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. Calcium Hypochlorite 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. Some other granular forms of chlorine are more powder like, and thus dissolve more rapidly than the larger granules of Calcium hypochlorite. It's a good idea to pre dissolve calcium hypochlorite 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. calcium hypochlorite 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.

We can't tell you how many tablets or how many kilograms or litres 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. Other factors, such as the water balance, and particularly pH, have an effect on how much chlorine you'll use. 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 over chlorinating the pool, which may also irritate your swimmers.

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 kg 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 7.2 to 7.6, 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.

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 sanitising. 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 sanitise. 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.