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    Swimming Pool Shock 101

    May 24, 2012

    image of 5 types of pool shock

    Swimming pool shock treatments come in many formulations. With the exception of non-chlorine pool shock, most types of granular or powder pool shock begin by bubbling chlorine gas through calcium carbonate. Originally called "Bleaching Powder" in the 1800's, chemical manufacturers have refined the process to create many granular chlorine products of various strengths, and with different admixtures.

    Each type of pool shock treatment serves a specific purpose, each with it's own features and benefits. To begin, let's discuss the various types of granular pool shock available on the market today, and lead into a discussion on why and how you should use granular chlorine in your own pool.

    What Types of Pool Shock are Available?

    Cal Hypo pool shockCalcium Hypochlorite - Ca(ClO)2

    Cal-Hypo, as it's called - is the most common type of pool shock, and usually also the least expensive. It most commonly is sold in a strength of 65% available chlorine, which means that the other 35% is made up of calcium. Continual use will add to your level of Calcium Hardness in the pool, so if you have an issue with very hard water in your area, Cal-Hypo may not be the pool shock that is best for you.

    Cal-Hypo is unstabilized, which means that it has no added Cyanuric Acid (also known as Conditioner or Stabilizer), to help protect it from the sun. You can use Calcium Hypochlorite shock on any pool type, but it is not especially fast dissolving, so it's best to pre-dissolve the granules into a bucket, and then pour around the edge of the pool.

    You can find Cal-Hypo in stronger formulations, like our Super Pool Shock which is 73% available - the strongest pool shock available. Assure Multi-shock is another mixture that contains Borates, which removes carbon dioxide, an important nutrient for algae, from your pool water.  

    Lithium pool shockLithium Hypochlorite - LiCIO

    Lithium Hypochlorite is a cleaner burning pool shock alternative to Cal-Hypo, leaving behind much less residue, or "shock dust". It won't cloud the water, and there's no need to pre-dissolve Lithium shock in a bucket before adding, because it dissolves almost instantly. It has the advantage of a lower pH level than Calcium Hypochlorite - which can have a pH level as high as 12.0.

    Lithium Hypochlorite pool shock is only 35% available chlorine, and pound for pound, contains less chlorine than a 65% available Cal-Hypo. This also makes Lithium a safer granular pool shock to store and use, as it is a less reactive compound than stronger types of granular chlorine.

    Like Cal-Hypo, Lithium pool chlorine is not stabilized, or protected from the sun with the addition of Cyanuric Acid, so it will tend to burn off quickly on a sunny day. Lithium chlorine powder can be the most expensive type of pool shock, but is helpful those with hard water or vinyl pool liners.  

    non-chlorine pool shock, chlorine free shock for poolsPotassium Monopersulfate - 2KHSO5

    Often called Non-Chlorine shock or Chlorine-Free pool shock, Potassium Monopersulfate is a great alternative shock for pools using bromine, chlorine or those that are running a chlorine-free pool. Very fast releasing, with no byproducts to clean up later. It's sometimes called Oxygen Shock, because it contains about 5% active oxygen in the formula.

    Non-Chlorine pool shock treatment can be poured right into the pool, without pre-dissolving, and unlike other forms of granular pool shocks, there is no delay to swim, you can swim within minutes after shocking. It is effective at removing chloramines in addition to oxidizing all forms of bacteria that may be in the pool.

    Potassium Monopersulfate does have a low pH level - it's quite acidic at a pH of 3.0, so continual use may require some adjustment of your pool pH level. Often used by those who are sensitive to chlorine, non-chlor pool shock has no smell and you won't feel it on your skin or in your hair.

    image of dichlor pool shockDi-Chlor Granules - NaCl2C3N3O3

    Sodium dichloro-s-triazinetrione is the only readily available form of stabilized pool shock treatment. It has the addition of Cyanuric Acid to shield the molecule from depletion by the sun. With 56% available chlorine, and a nearly neutral pH, this long-lasting pool shock is preferred by many pool owners and service companies. You will pay slightly more for Di-Chlor, but it does have it's advantages.

    Di-Chlor shock is used by some as the primary method of everyday chlorination, although in most cases, using slow-dissolve Tri-Chlor tablets may be a better choice.  

    Why use Pool Shock Treatments?

    Boosting your chlorine level in a pool that has a very low level, is a common use of pool shock treatments. Tablets can take many hours to most of a day to dissolve and bring up a chlorine level. Granular chlorine can do this in minutes. When the chlorine level in a pool drops close to zero, the water will quickly become hazy and unsanitary, and adding even a small amount of pool shock will avoid complications.

    Superchlorinating the chlorine level up after heavy pool use will help control bacteria and chloramine formation. If you have just hosted a child's birthday party for example, you may wish to superchlorinate the pool with chlorine granules. Superchlorination is less than a full-on "shock" of the pool, and refers to using an amount roughly equal to 1/2 of the amount used during shocking of the pool.

    Shocking the pool with powdered granular chlorine is a term that refers to adding enough granular chlorine to the breakpoint chlorination in poolspool to reach a threshold known as breakpoint chlorination. At this level of chlorination, molecular compounds are broken apart, necessary for removal of chloramines.

    Chloramines are chlorine molecules that have combined with ammonia or nitrogen. Chloramines (also known as Combined Chlorine) are ineffective sanitizers, they smell bad, and cause red eyes and skin irritation.

    Shocking the pool to this breakpoint threshold breaks apart chloramines, and destroys bacteria and pathogens in the water. It may seem counter-intuitive, but if your water smells strongly of chlorine and is causing eye irritation, the cure is to add a lot more chlorine to the pool, to reach the point where combined chlorine is removed. A DPD test kit can determine the level of Free Chlorine and Total Chlorine. The difference between the two is the level of combined chlorine. Taylor test kits recommend that a pool be shocked when the level of combined chlorine is 0.3 ppm or greater.

    Algae Removal is a very common reason for pool shock treatment. Nothing kills algae faster than shocking the pool. 30 ppm is the recommended level for full algae destruction. Since most test kits won't test for chlorine at that high of a level, follow the label instructions on your brand of pool shock, or keep shocking until the water turns a bluish-gray color. Chlorine pool shock will attack anything organic, and it will help immensely if you first remove as much debris from the pool floor and surface - before shocking the pool.

    Another good tip for using pool shock treatments is to first check and adjust your pool water's pH level, if necessary. Pool shocks (and chlorine in general) has much more efficacy at a lower pH level. At a pH level of 8.0, only 20% of your chlorine will convert to hypochlorous acid, the killing form of chlorine. At a pH of 7.6, about 50% is active. Lowering your pH level before shocking the pool, to 7.2, will ensure that you get the most "bang" for your buck with your pool shock treatments.

    Safety Guidelines for using Pool Shock Treatments

    Handling Pool Shock

    1. Follow label instructions carefully for shock treatment
    2. Never mix pool shock with any other chemical
    3. Dilute pool shock treatments first when adding to vinyl lined pools
    4. Do not add chlorine shock to a tablet feeder or chlorinator
    5. Never pour pool shock into the pool skimmer

    Storing Pool Shock

    1. Use entire 1 lb bags, do not store opened bags of pool shock
    2. Separate chlorine shock from other chemicals
    3. Never allow pool shock to come in contact with any other substance
    4. Keep chlorine shock clean and dry. Dirt or leaves can react violently
    5. Never mix different types of chlorine together

    Calcium Hypochlorite is particularly unstable, and when mixed with just drops of antifreeze, algaecide, alcohol, etc - it will begin to smolder, and very quickly ignite into an inferno. Water will not extinguish this type of fire, and only seems to make it angrier. Contamination from dirt, dust, leaves, insects, etc, can also cause a chemical reaction that is best avoided. Mixing chlorine with acid creates something of a mustard gas.

    Store your chlorine in a cool and dry location, out of reach of children (or teenagers). If you keep it clean and prevent contamination, you should have no problems with storing or using pool shock treatments.

    If you have questions on the best type of pool shock to use for your pool or spa, fountain or pond - give our pool shock experts a call at 800-983-7665.  

    Dorothy Ruggiero
    SPP Pool Enthusiast

    Blog Author
    The company i work for uses liquid pool shock alot. And lately, some of it has seemed almost diluted in the bottle. So i was curious if someone has been tampering with it for some reason. i know the bottles always hace a plastic cap, and alot of them have a seal you have to peel off for the first use. Does all pool shock come with those peel off seals when the bottles are new? Thanks!
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