Swimming Pool Chemicals Part Three-Shock

by Pool Builders on 11-26-2006 in Articles

Anytime a pool is getting heavy use chlorine levels will drop as free chlorine combines with contaminants and forms chloramines. Chloramines cause that heavy "chlorine" smell any swimmer knows and loves. Though attributed to too much chlorine it's actually the best indicator that chlorine is way too low! The pool needs a big dose of chlorine right away or bad things will happen. This is where pool shock comes to the rescue. Designed to add lots of chlorine in just minutes shock products are often the same stuff you use day to day but in a different form. There are two kinds of shocks; chlorine based and non-chlorine. Each is useful but in different situations.

Chlorine shock

Chlorine shock comes in three forms; calcium hypchlorite (Cal Hypo), sodium dichlor ()Dichlor) or lithium hypochlorite(Lithium). Cal hypo and dichlor were discussed in Part One of this series as regular sanitizers. Lithium hypochlorite is only used as a shocking agent. In each of these products the chlorine molecule is carried or bound to a different substance; lithium, sodium or calcium. Lithium and sodium dissolve nicely in pool water, calcium doesn't so lithium and dichlor can be added directly to a pool while calcium should be dissolved in a bucket of water before adding. If not pre-dissolved Cal Hypo will settle to the bottom and bleach lots of nice little white spots on a vinyl liner.

A shocking agent is meant to be a quick fix. The chlorine level is raised to a level as high as 10 ppm (parts per million), too high for swimming. Since unstabilized chlorine is quickly burned out by sunlight, cal hypo and lithium are nice shock agents as they are both unstabilized. Dichlor, however, is stabilized and will hang around for up to six hours. Dichlor is fine but don't plan on using the pool for a while.

Dichlor and Cal Hypo both pack a powerful punch with 56 and 65 percent available chlorine respectively. Lithium usually provides 35 percent and is a relatively weak sister requiring a heavier dose for the same results.

All three shocks are available in convenient 1 lb. pouches or in pails up to 50 lbs. Cal hypo is the cheapest and most popular, lithium the most expensive and least used. Dichlor is mid priced and widely available. All three types require the pool to be closed until chlorine levels drop below 3 ppm.

Non Chlorine shock

A relatively new development, non-chlorine shock has proved to be a popular choice with many pool owners primarily because the pool can stay open for use even after shock is added. Pretty cool, huh?

Sort of. Non-chlorine shock, as one might expect, contains no chlorine. If chlorine is what a pool needs how does this stuff do any good? The active ingredient in non-chlorine shock is potassium monopersulfate or PMP. It has little if any oxidizing potential but it does do something pretty neat. PMP actually frees chlorine bound up in chloramine compounds and makes it available to sanitize again. And the pool can stay open while this miraculous process takes place. There is a catch. Often after prolonged summer use pool water has little or no chlorine or chloramines. Sunlight and bather load has used up almost everything. Adding non-chlorine shock at this point will do nothing because the PMP has nothing to work with. Make sure to take a total chlorine reading before using non-chlorine shock. If total chlorine is low use a shock containing chlorine.

Using Shock Properly

-Regardless of the type used it's best to add shock when people are done using the pool for the day. The overnight hours are best because unstabilized chlorine will last longer. Remember to run that filter.

-Always try to verify that shocking your pool has pushed chlorine levels to at least 5 ppm. Lower concentrations won't do a proper job and could lead to trouble.

-It's good practice to pre-dissolve shock before adding to the pool even if it's lithium or dichlor. Always add shock to water NOT water to shock.

-Remember, non-chlorine shock WILL NOT WORK if chlorine levels are too low.

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