Lesson 12 - Shocking a Pool

    Everyone knows what it means to shock a pool, right? The word 'shock' actually has more than one meaning. It can mean super-chlorination (light and heavy) or it can mean oxidizing. In order to explain shocking better, I need to tell you my chlorine story. Let me preface it by saying that I am a bit apprehensive or maybe self-conscious about putting this teaching tool on paper, er... in cyber space. I've given some version of it to new pool owners for years, even once to a bio-chemist, and have gotten pretty good reviews. But to send it out to an wider audience opens me up to potentially wider ridicule. So if you are inclined to roll your eyes and proclaim this little story the stupidest thing you ever heard, then I say, "Come up with your own chlorine story". 

The Chlorine Story

    Chlorine is a molecule that lacks one electron. So it actively swims around looking for something to bond with to get that missing electron. When it comes across a bit of dirt, or fungus, or bacteria, or a germ, then the chlorine molecule attaches itself to it and chemically bonds with it. That's all chlorine does.

    As long as a chlorine molecule is swimming around waiting for something to bond to, we call it a Free Chlorine molecule. Once it attaches to something, it is called a Bound Chlorine molecule. When you first add chlorine to the water, all the chlorine is free. But after a time, some of it becomes bound up. After a long time, all of the chlorine can become bound. 

    As more and more of the individual chlorine molecules change from free to bound, then a strong odor develops. If you've ever went to a public pool and the chlorine smell was strong and acidic, then you know what I'm talking about. It's a chemical smell, not a pleasant smell. A properly chlorinated pool, with lots of free chlorine and little bound chlorine, will smell nice and clean. 

    In short, it's the ratio of free chlorine to bound chlorine that tells you when to shock your pool. If your overall chlorine reading is 3.0 ppm, then some of it is free and some is bound. If the amount of bound chlorine exceeds the amount of free chlorine, then your pool probably needs to be super-chlorinated or shocked.

    What happens when you super-chlorinate a pool is that a large dose of powdered or liquid chlorine is dissolved into the water, so that the free chlorine reading skyrockets to 10 ppm or more. When this happens, a chemical chain reaction occurs which burns off all the bound chlorine, converting it to oxygen gas which belches out of the pool, leaving only free chlorine swimming around waiting for something to latch on to. 

    Now that you know the chlorine story, you are ready to understand some of the fine points of water chemistry. For example, why is it almost impossible to shock a pool using your chlorinator? Because most residential chlorinators dissolve their tablets too slowly to reach the critical mass necessary to truly shock a pool. The whole process needs to be rather sudden, not drawn out over days or weeks. When you think shock, think Boo! That's why powders and liquids are used for shocking.

    Now here's something else really important. You can shock your pool without adding a large dose of chlorine. That's right. Let me explain why oxidizer is such an important product.

    Oxidizer is a product that when added to the water, helps to start the chemical reaction that converts bound chlorine into free chlorine. Sometimes you don't want to add a big dose of chlorine to your pool. Maybe you've got guests coming over in a few hours and you just want to freshen up the chlorine already in the pool. That's what oxidizer does. Note that because oxidizer only works on chlorine, if you add it to a pool with no chlorine then you'll get no results, and your money is wasted.

    But if a pool has chlorine in it already, then oxidizer will burn off the bound chlorine leaving just the free chlorine swimming around waiting to go to work. Another nice thing is that you don't have to add a certain minimum threshold amount to get an effect. If you add one pound it oxidizes a little, if you add four pounds it oxidizes four times more. Plus you can swim immediately after it disperses in the pool, and it doesn't cloud the water. Add to all this the fact that oxidizer is not very costly, now we're approaching the niftiness of sliced bread.

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