Chlorinating Your Pool

What is Chlorine, and why is it so effective?

Chlorine was first discovered in the sixteenth century, and today is one of the most produced chemicals in North America. Chlorine is so deeply inter-twined into industry that finding alternatives would indeed change our daily lives.  One of the most practical uses of chlorine is to keep swimming water sanitized.

When chlorine is added to water, dissociation occurs.  That is, when chlorine is combined with water, we get a reaction which leaves us with hypochlorous acid and hydrochloric acid.  Hypochlorous acid is the active, killing form of chlorine; the hypochlorous molecule/ion kills microorganisms by slashing through their cell walls and destroying (oxidizing) the inner enzymes, structures and processes.  The hypochlorous molecule continues this process until it combines with a nitrogen or ammonia compound (and becomes a chloramine), or until the hypochlorous molecule is broken down into its component atoms and is de-activated.  Therefore, the active components of chlorine work together to kill microorganisms that can grow in your swimming pool.

These are three states of existence for the chlorine molecule: free, and combined. 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 (a combined molecule). The chloramine is no longer available to sanitize anything, and it floats around in the water, blocking the path of free chlorine molecules (and releasing a strong odor).  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. The total chlorine is simply the sum of combined and free levels.

Chlorine is extremely effective at low levels, because at low levels, it poses little danger to humans. 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.  However, extremely high levels of chlorine in the water can render breathing difficult.

Applying Chlorine to your pool

Always be careful and read the directions when applying chlorine to your pool. Wear protective handling gear (such as eye protection and rubber gloves), and apply extreme care when opening a container of chlorine. Breathing in chlorine gas can cause humans to pass out, and can be fatal.  If chlorine touches your skin, you should wash it off immediately to prevent irritation. If chlorine splashes in the eye, you must irrigate with water and contact a physician immediately. NEVER mix chlorine with any other chemical.

Each pool has its own chlorine demand because the amount of chlorine needed to destroy contaminants various with the type of pool and its capacity.  Many factors, such as the water balance, and particularly pH, have an effect on how much chlorine you'll use.

The efficacy of chlorine is greatly influenced by the management of the water's 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 lb of chlorine, however such a low pH would be too corrosive for any of the surfaces the water would comes in contact with, including swimmers. If you increase the pH of the water to 7.0, the efficacy of the chlorine drops to 73%, however if the pH is raised to 8.0, the efficacy of chlorine drops 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.

Most health departments will close down a public pool that does not have a minimum of 1.0 ppm of free chlorine available in the water during a health inspection. With a little experimentation, you'll be able to determine how many tablets 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, increase the dosage; if it's too high, decrease the dosage next time. Fairly soon you will develop an idea of your pool's particular chlorine demand, however, this will likely change during warmer months and periods of increased use. Chlorine is an expensive and corrosive compound.  Try to  make an active attempt to not overchlorinate the pool, because this will irritate your swimmers.

Shocking or Superchlorinating a Pool

To shock a pool means to oxidize/kill all organisms that are in the pool. By raising chlorine levels ten times the level of chloramines, a threshold is reached called breakpoint chlorination. When this is reached, a "shock" rips through the water, and kills all organisms in its path.

Shocking generally involves raising the chlorine level up to 10 ppm. If using cal hypo, you'll find that at least one bag per 10,000 gallons will do the trick. Excessive chlorine will not hurt, however, if you don't reach the crucial level of breakpoint chlorination, then your water will be saturated with chlorine, and your chloramine problem will remain unsolved.

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.   Others recommend it once every few weeks, regardless of whether or not it is needed.  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 are also very effective when algae has turned the water a green or yellow color.

The use of chlorine stabilizers

Cyanuric acid is a chlorine stabilizer.  It provides a chemical cloak around the chlorine molecule, which protects it from the sun's UV radiation, the largest killer of chlorine.  Cyanuric can save you money, 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 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, unless the level is below 30-50 ppm.