What is Chlorine Shock?

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  • Written By: M.R. Anglin
  • Edited By: Kristen Osborne
  • Last Modified Date: 04 October 2019
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A swimming pool or hot tub has the potential to harbor a swarm of bacteria, algae, and other organic materials. In order to keep the pool clean and sanitary, chlorine is often used to get rid of these unwanted substances. In the course of sanitizing, chlorine can be broken down by the sun, bind with unwanted substances, or deteriorate, bringing the free chlorine level down. A chlorine shock is a high dose of chlorine usually administered to a pool once a week. The purpose of this shock is to bring chlorine levels back up to their proper sanitizing power.

Chlorine works to sanitize a pool by killing microbes or binding with unwanted substances. When it binds to a substance, it becomes inactive. The sun also can contribute to the deterioration of chlorine. A chlorine shock raises a pool’s chlorine level so that the bound chlorine is burned off. Since chlorine levels after a shock are so high, a person should not swim in the pool until the chlorine has returned to normal levels. Some people suggest adding a shock to a pool in the evening so that the chlorine can do its work all through the night without the sun deteriorating it.


Adding a super dose of chlorine should usually be administered after certain periods. A chlorine shock is often recommended after a heavy rain, after heavy use, and when algae have started to appear. It is also recommended for use when the water quality has deteriorated, or if the water starts to get cloudy. Weekly chlorine shocks are also suggested as a part of ongoing maintenance. To apply the shock, many products may have to be dissolved in a bucket of pool water before being poured in front of the return line.

One of the most common types of chlorine shock is calcium hypochlorite, which is available in granular form. The calcium in this product may contribute to hard water and calcium deposits on the sides of the pool. Lithium shock is lithium hypochlorite. It is usually more expensive than calcium hypochlorite but has the advantage of dissolving faster. Since it has no calcium, it also does not usually contribute to hard water.

Liquid chlorine, which is the same chemically as bleach, but has a higher concentration of chlorine, also can be used in a pool. However, but some people believe it is not a good value. Since the chlorine content is lower than other forms of chlorine, many bottles have to be used.

Each of these different types of chlorine shock has different chlorine concentrations. Liquid chlorine has as little as 10% chlorine, while granular shocks can have as much as 75%. The pH level in the product can also vary and affect the pH of the pool, so a person may have to add an acid or a base to balance the pool’s pH.


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