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What you need to know about Chlorine

Chlorine is a very efficient killer. It not only kills bacteria it will break down organic material. Like most killers though it is unstable. In its natural state pure chlorine is a gas and while big public pools can put it in their water in gas form this option is not available to home pool owners. We have to use chlorine that is bonded to another chemical to make it solid and easier to handle. When the solid dissolves in the water the chlorine breaks free and goes on its killing spree. The trouble is that we want the chlorine in the pool but not necessarily the other chemicals that it was bonded to. These leave behind what the techies call 'residuals'. Too many residuals is a bad thing. The chlorine gets used up as it kills stuff but it can also be burnt off by sunlight, so if you want it to stay longer in your pool this unstable chemical needs to be stabilized.

There are 3 main types of chlorine you can put in your home pool. The two most most common types have chlorine bonded with cyanuric acid. These are "Granular Chlorine" and "Trichlor Tablets". As the name suggests the former is granules and the latter comes in big tablets. The granules go straight in your water, the tablets go in a feeder or your skimmer basket.

The cyanuric acid stabilizes the chlorine helping it stay in the pool and not be burnt off by the sun. But the chlorine will eventually go but the cyanuric acid stays behind and each time you add these to the pool you add more and more cyanuric acid. So what? If it stabilizes the chlorine it must be good? Well, no, too much cyanuric acid will 'over stabilize' your pool and the chlorine will never be allowed out to play. To lower the cyanuric acid level you have to 'dilute' your water. Silly as the concept of diluting water may seem it is quite logical. You must remove some over stabilized water to rid yourself of the cyanuric acid and top up with tap water. A well maintained pool rarely has this problem because its owner regularly gives the filter a good backwash thus taking out pool water and replacing it with tap water.

The other type of chlorine is unstabilized. What good is that? Well there are times when you need to boost up your chlorine levels to make sure every thing is well and truly dead in there (e.g. if you have a bad algae attack). This is called shock treatment. But you do not want the high levels of chlorine to remain because you want to swim in the water later. Unstabilized chlorine soon burns off leaving you with a clean pool and normal chlorine levels. The chlorine used in shock treatment is usually bonded to calcium and this does not cause excessive problems as a residue.

It is a good idea to 'shock' your pool every two weeks or so even if you may think it does not need it. The technicalities of this for are delved into further on but for now we'll just say that it finishes off the smashing up of the bugs that the stabilized chlorine hasn't got around to yet.

Never use stabilized chlorine to 'shock' your pool - it is a waste of good chlorine, it will take much longer to 'calm' down to normal levels and will add loads of unwanted cyanuric acid to your water.

We are going to get a bit more technical now with more technical knowledge than is strictly necessary for the domestic pool owner to know. Here goes.

The chlorine mentioned above when disolved in water forms Hypchlorous Acid (HOCl) and it is that which does all the killing. At the same time the hypochlorite ion (OCl-) is formed. The two together are referred to as the 'free chlorine'. The DPD No. 1 tablets test for this. The hypochlorite ion is in equillibrium with hypochlorous acid and when the pH of the water is at 7.5 the balance is about 50:50. This is very important because it is only the hypochlorous acid that kills bacteria, and the lower the pH the more acid is available. At pH 7.0 is about 70% available and at pH 8.0 it is about 20%. One would think that you should keep the pH at 7.0 or lower but at that pH level the water would start to sting your eyes so the optimum level for pH is 7.2.

Still there? We going even further now.

You can get quite a bit of nitrogenous material in your pool from urine or sweat and other sources. When hypochlorous acid reacts with nitrogenous material it forms chloramines, also called combined chlorine. There are 3 types of chloramine and it is chloramines that cause the 'chlorine' smell to the air around a pool, not the chlorine you added. Chloramines can be ridded from your pool by what is called 'shocking' it. You add more chlorine to take the level up to about 10 ppm. The increase in concentration of hypochlorous acid leads to the breakdown of the chloramines in to water and harmless nitrogen gas. It also kills off any algae. The free chlorine plus the chloramines are refered to as 'total chlorine' DPD tablets Nos. 2,3 and 4 test for chloramine content. So too does the fifth test on the 5 way test strips. It in nice to know the level of total chlorine but domestic pool owners do not really need this test.