CHLORINE CHEMISTRY IN WATER

WHEN WAS CHLORINE FIRST USED?
What we know as "Chlorine" was first discovered in the 16th century, and today it is used in a variety of industrial products, as well as many household products we need & use every day. In fact, chlorine is one of the most produced chemicals in the US!

WHAT EXACTLY IS CHLORINE?
Chlorine is a by-product of the electrolysis of salt water. When electricity is passed through salt & water (2NaCl & 2H20) the atoms dissociate into (chlorine, sodium Hydroxide & Hydrogen (Cl2 + 2NaOH+ H2). The Cl2 is isolated in a gaseous state, and is used to create other chlorine compounds used for sanitizing, bleaching and in the production of plastics and related industrial products.

HOW DOES CHLORINE SANITIZE WATER?
When chlorine is added to water, it causes a reaction which produces HOCl (hypochlorous acid) + HCl (hydrochloric acid). Hypochlorous acid is the active (bacteria-killing) form of chlorine, which sanitizes water. Chlorine kills microorganisms by breaking through the cell walls and destroying the inner enzymes, which in turn deactivates, or oxidizes the cells. The hypochlorous molecule continues this process until it either combines with nitrogen or ammonia compounds to become a chloramine, or it is broken down into its component atoms, becoming de-activated itself.

IS CHLORINE DANGEROUS, UNSAFE OR UNHEALTHY?
Chlorine can certainly be a caustic & dangerous chemical to work with, with some forms being more dangerous for the user and the environment than others. At levels found in swimming pool or spa water, chlorine poses no danger to swimmers or pets. Allergic reactions to chlorine are rare. However some people may experience skin irritation. Additionally, chloramines (common in poorly balanced pool & spa water) are typically the cause of occasional red eyes when swimming. Extremely high levels of chlorine in the water could possibly release gases under certain conditions that can cause breathing difficulties.

The main hazard in working with chlorine is to the person adding the chlorine to the pool or spa. Care should always be taken, and manufacturer's label instructions always followed. When opening a container of chlorine, always be aware that breathing in chlorine vapors can cause unconsciousness and can even be fatal. You should always wear protective gloves & goggles when handling chlorine. If your skin comes in contact with chlorine, wash it off with plenty of warm water to prevent irritation. If chlorine splashes into your eye, irrigate the eye with water and contact a physician immediately. Most importantly – never mix chlorine with ANY other chemical (not even different types of chlorine) as this can cause a toxic gas or even a powerful explosion! Dirt, debris and foreign substance such as algaecides, alkalis, acids, etc. can cause spontaneous combustion when mixed with chlorine.

Pools have little to no impact on the outside environment. As a closed system, your pool or spa doesn't contact nature very much. There could be some potential hazard involved with waste water from a pool that has either extremely high chlorine levels or extremely low pH levels. But it is unlikely that the amount of water expelled during backwashing your filter could pose much harm. Pool water is similar in make-up to the water that flows out of your tap. Many people who water their lawns are exposing the environment to higher chlorine and lower pH than is found in their pool water! While there are certain industrial uses of chlorine that are potentially harmful, in a swimming pool environment the potential hazards are much lower, even non-existent. There has been much publicity and controversy over the use of chlorine in recent years. Some organizations have even called for bans on its use.


WHAT DIFFERENT TYPES OF CHLORINE ARE THERE?
In its elemental state, chlorine exists in a gaseous form. This type of chlorine is available for swimming pool sanitation; is very cheap, and is the purest form of chlorine, with no binders or carriers. This type of chlorine features 100% available chlorine. It is also extremely dangerous and is therefore restricted in its use. It is rare to find a pool that uses chlorine gas as a sanitizer, and those that do are usually older, very large public pools that have enacted strict safety procedures. Gas is also very acidic, with a pH close to muriatic acid, so pools that do it must also add a lot of base to counteract the acidity.

Liquid chlorine is created by bubbling the chlorine gas through a solution of caustic soda. This yellow liquid is stronger, extremely corrosive & chemically identical to bleach. It has 10-15% available chlorine, and 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 we recommend the use of a diaphragm or peristaltic pump. The use of liquid chlorine is more common in larger commercial pools which have it delivered in large vats. For most residential pools, its lower cost seems to be outweighed by difficulty in use, as well as the amount of acid that is required to counteract its pH of 13.

Trichlor is a solid form of chlorine available in stick or tablet form. "Trichlor" is short for Trichloro-s-triazinetrione (a stabilized form of chlorine that has become very common in residential pools over the last ten years). "Stabilized" means that it contains cyanuric acid pressed into the tablet itself. It features 90% available chlorine & a somewhat low pH level at 3, so the pH in your pool may gravitate downward. This form of chlorine is slow dissolving, 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 that would come in contact with metal pipes and equipment. This becomes more of an issue you're your filter pump operates on a timer, as Trichlor can strip out the internal copper components of a pool heater. It remains however, an effective & expensive method of controlling algae. Tablets should also not be thrown directly into the pool, as they can stain and etch plaster & bleach, as well as deteriorate your vinyl liner.

Another form of chlorine is Dichlor granules, (short for Sodium Dichloro-s-triazinetrione). Dichlor is made in the same manner as Trichlor, but the two products are very different. The pH of Dichlor is a very acceptable 7, and it dissolves rapidly and goes right to work on contaminants. Dichlor has less chlorine(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 everyday sanitation. Dichlor's main drawback is cost, as it is perhaps the most expensive form of chlorine available.

There are two other types of granular chlorine, both known as hypochlorites. Lithium Hypochlorite is a very expensive product that features only 35% available chlorine. It takes almost 3 pounds of lithium to equal just one pound of Trichlor, and with a pH of 11, it will require an acid to adjust the pool water. The main advantages are that it is (1) calcium-free, and so it won't contribute to hardness levels, (2) dust- free, and (3) non-flammable. It also dissolves extremely quickly, so it is safe in vinyl liner pools. Lithium can be used for both 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 features 65% available chlorine and a high pH level of nearly 12. It does not dissolve as rapidly as more powder-like forms of granules, so it's a good idea to pre-dissolve cal hypo into a bucket of water before broadcasting it into a pool. Its popularity is due mainly to availability and low price. 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 always keep the lid tightly secured & clean.

HOW MUCH CHLORINE SHOULD I ADD?
Each pool has its own chlorine demand, and we can't tell you how many tablets or how many pounds or gallons you'll need for your particular pool. A good starting place is a quality water testing kit. Factors such as size, gallonage, amount & frequency of use, water balance and pH can all have an effect on how much chlorine you'll need to use. With a little experimentation, you'll be able to determine how much chlorine it takes to achieve a consistent minimum level of 1.0 ppm (parts per million). As an example, first test the water, then add three tablets into your feeder. Check the residual again in 12-24 hours. If it's too low, add more, if it's too high, remove some. Fairly soon you will develop an idea of your pool's particular chlorine demand. Remember also that demand can change during warmer months, among other things. As chlorine is an expensive and corrosive compound, make every attempt to avoid consistently over-chlorinating the pool, as this can irritate swimmers skin & eyes.

WHAT AFFECT DOES pH HAVE ON MY CHLORINE?
The power of chlorine is greatly influenced by how well you manage your water's pH level. As the pH of your pool water increases, the killing power of your chlorine decreases. At a pH of 6.0, you will achieve approximately 96% of the potential from each pound of chlorine. But at what cost? Such a low pH would also wreck havoc on all surfaces the water comes in contact with (equipment, pool surface, liner, swimmers themselves). It's just too corrosive. By pushing the pH up to 7.0, the efficacy of the chlorine drops to 73%. 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.

WHAT ARE TOTAL, COMBINED AND FREE CHLORINE?
These are states of existence for the chlorine molecule. If a molecule is "free", it has not bonded with another compound, and 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 (also called a chloramines). The chloramine is no longer available to sanitize anything, and it blocks the path of free chlorine molecules, and creates the strong odor associated with chlorine. Combined chlorine level can be tested with a test kit that measures total & free levels separately, and allows the tester to determine combined levels by subtracting the two values. "Total" chlorine is simply the sum of combined and free levels.

WHAT IS MEANT BY "SHOCKING" OR "SUPER-CHLORINATING"?
These are synonymous terms for oxidizing the pool water. By raising chlorine levels ten times the level of chloramines, a threshold is reached called "breakpoint chlorination". When this occurs, something of a shock rips through the water, killing everything in its path.

When should you shock your pool water? Some recommend shocking the pool when combined chlorine levels reach 0.3 ppm. Others recommend it once every few weeks, whether it needs it or not. Use your senses to determine the need for shocking. If the pool water is cloudy or 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 odor in the water and notice eye irritation during swimming, you may sense the need for shocking. Large doses of chlorine are also very effective when algae has turned the water or pool walls a yellow or green color.

How much chlorine is required to effectively shock? Generally, you want to raise the chlorine level up to around 10 ppm. When using cal hypo, you'll find that at least one bag per 10,000 gallons will be sufficient. 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. Always follow instructions on the package of granular chlorine or non-chlorine shock (which may be potassium peroxymonosulfate). Liquid chlorine can also be used for super-chlorination. Whatever chemical you choose, the goal is to introduce 10 times the level 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.