Chlorine | Total, Combined and Free Chorine | Shocking | Stabilizer | Alternatives
Chlorine is the chemical most often used to keep swimming pools and Jacuzzis free of bacteria that can be hazardous to humans.
Chlorine kills bacteria though a fairly simple chemical reaction. The chlorine solution you pour into the water breaks down into many different chemicals, including hypochlorous acid (HOCl) and hypochlorite ion (OCl-). Both kill microorganisms and bacteria by attacking the lipids in the cell walls and destroying the enzymes and structures inside the cell, rendering them oxidized and harmless. The difference between HOCl and OCl- is the speed at which they oxidize. Hypochlorous acid is able to oxidize the organisms in several seconds, while the hypochlorite ion may take up to 30 minutes.The levels of HOCl and OCl- vary with the pool's pH level. If the pH is too high, not enough HOCl is present and pool cleaning can take much longer than normal. Ideally, the level of pH in the pool should be between 7 and 8; 7.4 is ideal -- this is the pH of human tears. Once the HOCl and OCl- are done cleaning the pool, they either combine with another chemical, such as ammonia, or are broken down into single atoms. Both of these processes render the chlorine harmless. Sunlight speeds these processes up. You have to keep adding chlorine to the pool as it breaks down.
While the bacteria-killing properties of chlorine are very useful, chlorine also has some side effects that can be annoying to humans, and possibly even hazardous. Chlorine has a very distinctive smell that most find unpleasant, and some find overwhelming. There is also the "itch factor" -- chlorine can cause certain skin types to become itchy and irritated. The hypochlorite ion causes many fabrics to fade quickly when not rinsed off immediately after exiting the pool. This is why your swimsuit looks faded and worn so early in the summer.
Extremely high amounts of chlorine gas hovering above your pool can be hazardous to your breathing. Some companies have developed alternatives to chlorine, including other chemicals and ion generators. Some of these are good alternatives, but they don't achieve the cleanliness, oxidation levels or low price that chlorine provides.
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.
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.
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.
