The Ultimate Guide to Pool Chlorine

drpool-shock-3Welcome students! Today’s topic is one that is near and dear to my heart – pool chlorine!

Chlorine has made it possible for millions of people to enjoy the benefits of swimming without concerns about pathogens that may be lurking in the water.

Not the most powerful halogen sanitizer, but the abundance of raw materials and ease of manufacture has made chlorine the most popular!

Chlorinated Isocyanurates:

trichlorThese are a group of chlorine products that contain cyanuric acid, which helps protect chlorine from degrading in sunlight. Two of the most common isocyanurates used in pools are Trichlor and Dichlor.

TriChloro-S-Triazinetrione – Cl3C3N3O3

trichlor-moleculeThe King of chlorine products, “Trichlor” is pressed into 1″ and 3″ tablets, and also in stick form. Trichlor contains 52% of cyanuric acid by weight, and contributes 0.6 ppm of cyanuric acid is added to the water, for each 1.0 ppm of Free Chlorine added.

Trichlor has a relatively low pH of just under 3 – which will require the addition of pH increaser to maintain pool pH in the 7.4-7.6 range.

Trichlor has 90% available chlorine, which means that 10% is composed of inactive ingredients, mainly binders and release agents.

Trichlor is capable of carrying other ingredients to control of algae or to improve water clarity, however this will reduce the percentage of available chlorine. For example, our Triple Action Tabs have 72% available chlorine.

Application: Trichlor tablets or sticks dissolve slowly when placed in water. 1″ tablets dissolve more rapidly due to increase surface area, which is desirable in pools or spas with higher chlorine demand.

Trichlor should be added to the pool using a chlorinator. The best type is installed into the pool equipment plumbing, or a floating chlorinator can be used. One should never place chlorine tablets in the skimmer, or toss them directly into the pool.

Precautions: Chlorine tablets are highly reactive, and can explode or ignite if exposed to contaminants or other chemicals, and will release chlorine gas when moist. For this reason, chlorine tablets should be tightly sealed, and stored in a cool, dry and well ventilated area.

Chlorine gas from pool tablets will oxidize nearby metals, and should be stored where this is not a concern. Never consolidate two buckets of chlorine products, slight compositional differences can result in a reaction, producing explosion, fire or toxic fumes.

Sodium Dichloro-S-Triazinetrione – NaCl2C3N3O3

dichlor-moleculesAnother type of stabilized chlorine, Dichlor is a granular form of chlorine. Using Dichlor will contribute to the level of cyanuric acid in the pool. By weight, Dichlor is 57% cyanuric acid; for each ppm of chlorine added, 0.9 ppm of cyanuric acid is added to the water.

The pH level of Dichlor is nearly neutral (7.0), and won’t affect the pH of the pool water noticeably, is fast dissolving and because it’s sodium based, it won’t contribute to calcium levels in pools.

Dichlor has an available chlorine level of 56% or 62%, depending on the formulation – anhydrous or dihydrate. Supplemental additives can be mixed with Dichlor, such as our Assure MultiShock with clarifiers and buffers, however this lowers the available chlorine percentage to 58%.

Application: Granular chlorine products are used by broadcasting the granules over the pool surface, or by diluting into a bucket of water, and pouring the mixture around the [inside] edge of the pool. Consult the label for dosage instructions prior to application.

Dichlor is used for shocking or superchlorinating, and can also be used for daily chlorination for small pools, spas and fountains.

Precautions: When broadcasting granular chlorine, one should mind the direction of wind that may blow the material in unintended ways.

Dichlor is incompatible with any other type of chlorine and is highly reactive to small amounts of dirt, dust or leaves. Mixing with any other pool chemical can cause a violent reaction, including explosion and fire.

Do not store open packages of Dichlor, which can spill or take on moisture or other contaminants. When using 1 lb. bags of Dichlor, use the entire bag at one time.



Another group of chlorine products is known as hypochlorites, which are ions composed of oxygen and chlorine. They are made by hydrolyzing salts of sodium or calcium with chlorine, followed by rapid drying and cooling.

Sodium Hypochlorite – NaOCl

sodium-hypochlorite-molecule-Liquid chlorine, or bleach is the most well known hypochlorite, used for a variety of cleaning and disinfecting purposes. It is not stabilized, so cyanuric acid is added to the pool separately to extend the product.

The pH level of sodium hypochlorite is very high at 13, the highest of any chlorine. This requires frequent addition of pH decreaser into pools that use liquid chlorine as the primary sanitizer.

The percentage of available chlorine is quite low for pool bleach, 12.5% is standard. Household bleach, by contrast (i.e. Clorox) is typically a 5% solution. However, due to low cost, sodium hypochlorite can be one of the cheapest chlorination methods, especially for large pools.

Using sodium hypo can elevate TDS (total dissolved solids) levels over time, due to the high level of sodium, more than solid or granular forms of chlorine.

Application: Introducing liquid chlorine into pool water is accomplished by using a chemical metering pump; one specifically designed for the purpose, which draws chlorine from a storage container, and injects it into the pool return line.

Liquid chlorine can also be hand-fed into the pool, by pouring a bucket full directly into the pool. It dissolves instantly and leaves behind no residue.

Precautions: Storage of large quantities of sodium hypochlorite is likely safer than an equal amount of tablets, and certainly much safer than using chlorine gas. Nonetheless, caution should be taken to avoid contamination of the liquid, spills or leaks in the pumping hoses.

Like other chlorinated products, strong reactions can occur when contaminated with dirt or dust, or when mixed with other pool chemicals. Accidental mixing with acids can create an extremely toxic form of chlorine gas.

Calcium Hypochlorite – Ca(OCI)2

calcium-hypochlorite-moleculeCal Hypo, as it is colloquially known, is the most common form of granular pool shock and also the least expensive. Not stabilized, and not meant for daily chlorination. Add in the evening when the sun goes down.

The pH level of calcium hypochlorite is high at 11.7, but when used for a chlorine booster or for shock treatment, one should not see any noticeable effect on pool pH levels.

The percentage of available chlorine for Cal Hypo is standard at 65%, however some formulations such as Super Shock deliver a higher potency at 73% available.

For pools with hard water or high calcium hardness levels, keep in mind that Cal Hypo contributes to your calcium levels. For every 1 ppm of free chlorine added, Cal Hypo also adds 0.7 ppm of calcium to the pool.

Application: Like all granular pool chlorine products, it can be broadcasted or sprinkled across the surface or it can be diluted by pouring into a bucket filled with water, and added to the pool.It can also be poured directly over stained areas (plaster pools only), for quick removal of organic stains.

Consult label instructions for dosage, but the standard dose is 1 lb per 10000 gallons, which should yield 7-9 ppm of free chlorine, depending on strength used.

Precautions: Cal Hypo is one of the most dangerous pool chlorine products, to use and store. It is easily contaminated and reacts strongly to any foreign substance, yielding fumes and fires. Mixing with trichlor tablets and water creates a very strong explosion.

Do not store half used bags of cal hypo; store shock in a plastic container, in a dry and cool location, out of the reach of children.

LIthium Hypochlorite – LiOCl


Lithium hypochlorite is another granulated chlorine, so fine that it looks like sugar. Fast dissolving formula is safe for any pool type, it won’t contribute to calcium levels, and is not stabilized, and adds no cyanuric acid.

Lithium is the most soluble form of hypochlorite, and is considered ‘clean burning’, because it leaves no residue behind. Unlike other hypos, the potency of Lithium is unaffected by pool pH and temperature, and it won’t cloud the pool when added to the water.

The percentage of available chlorine is low at 35%, when compared to other granular chlorine products, especially considering that the price of lithium is over twice that of other hypochlorites. This is mainly due to the cost of lithium being much greater than calcium or sodium.

However, it’s many benefits outweigh the increased cost for many users, and for others who experience water issues with other chlorine types, lithium can be the solution.

Application: Lithium can be broadcast or sprinkled into the pool. Wear safety glasses and take care not to inhale wind blown particles. The typical dosage is 1 lb per 6000 gallons, which will raise free chlorine to about 5 ppm.

Precautions: Lithium is possibly the safest of all chlorine products to have around the home. It has a low flammability and is not as reactive as other chlorine products. Nevertheless, take care in use and storage to prevent contamination, moistening or mixture with other chemicals.

Class dismissed!


Dr. Pool

Dr. Pool



The Ultimate Guide to Pool Chlorine — 24 Comments

  1. I know that mixing trichlor tablets with an acid, such as muriatic acid, can generate chlorine gas. Do you know if that gas generation will also build pressure, for example if it occurs within a chlorine feeder and the cap is placed on it? Could it lead to an explosion, or will the pressure regulate and the reaction stop?

    • Hi Todd, mixing trichlor and acid will actually create a kind of mustard gas that will quickly kill if the fumes engulf you. Gas buildup from a reaction inside of a chlorine feeder could blow the lid off and/or split the cylinder, violently, injuring or killing someone standing over it. Now mixing two types of chlorine and adding water can produce an instant explosion from the reaction, which is not a gas pressure problem but more… explosion. So never mix different types of chlorine.

  2. Hi Davy,

    My pool is cloudy and I have been using the tri- chlor tabs. My pH was low and my chlorine is too. I have added to raise the pH. Can I add liquid household bleach to bring up the free chlorine?

    • Hi Paula, yes you can use unscented regular bleach in the pool. You would need about 3 cups, per 10000 gallons, for each 1.0 ppm that you want to add to the pool. Adding 1 gallon of 5% bleach, per 10000 gals will take free chlorine up to around 5 ppm. Cloudy water can also be symptomatic of filter problems, or phosphate contamination, or old water with high solids content.

  3. I am very confused about my pool water. I can not get the chlorine level up, I have added about 24 gallons of liquid chlorine. I have added many packets of shock (about 20). My ph is low and alkalinity is high. That confuses me. My cyanuric level is also low. My pool water is no longer cloudy green but clear but the chlorine level is still showing none.

    • Hi Valerie, if your test kit or strips are not expired and results can be trusted, the ‘chlorine demand’ must be higher than the amount being added – something is consuming the chlorine! :-/ Low pH and low cyanuric (20-40 ppm) levels, that is good for chlorine levels. Could be a phosphate/nitrate contamination, you can test for phosphates or treat with a remover if levels are testable. Is the pool clean, and the filter running long enough each day (this [hot] time of year that’s 12-18 hrs/day for most systems). You have added a lot of chlorine, not sure what size your pool is, are you also using tablets in addition to the shock treatments? A simple solution that can sometimes help is to replace a portion (20-50%) of the pool water and refill with fresh, where possible, if my other thoughts aren’t helpful.

  4. We have an in ground pool at our property that is about 18,000 gallons. Right now we are using Dichlor but it always seems that our cyanuric acid is too high (around 80-100ppm). What could we be doing wrong? What else could we use to help keep the cyanuric acid down?

    • Hi Meghan, Dichlor does contain Cyanuric acid and for each 1 ppm added to the pool from dichor, 0.9 ppm of cyanuric acid is added. Switch to Cal Hypo shock, which is not stabilized, and the problem should stop. YOu may want to drain and refill half the pool too, to drop into the 40-50 ppm range. There is a new product called BioActive that can be used to remove cyanuric acid, but results have been mixed for users, not always consistent…

  5. Hi
    I have a pool that is 15-20,000 gal. Years ago I got it fiberglass lined. I have been using liquid clorine and tri-clor tabs to clorinate. However my local pool store says I should use a shock and triclor. Does this work, or what is best for my pool or should I switch from the liquid to a diclor?

    • Hi Mark, you’re fine with liquid and tablets. Liquid does have a high pH level, and adds salt to the pool, but Dichlor adds cyanuric acid to the pool, and is much more expensive. IF you were going to use shock, I would use Cal Hypo, not Dichlor, just for the big cost savings. I like how liquid chlorine reacts (kills), and for really green pools, I like the synergistic effect (seemingly) from adding both (separately) to the pool, liquid and granular Cal Hypo shock. What is best for your pool, is kind of a non question, they are the same really. They all produce hypochlorous acid, and although all have some side effects, and different costs, they all do basically the same thing. I can’t think of a reason why Dichlor would be better than liquid, I think I would stay just where you are…

  6. Hi there, I am so confused between Di Chlor and Tri Chlor. We have a small above ground pool. Less than 5000 gallons. I read a blog that states “NO CAL HYPO” on vinyl pools. The more I read online, the more I get confused.
    Which tablet is best to use.

    • Hi – tablets are Trichlor, always and only. Well, actually you can buy cal hypo tablets, but most people use Trichlor tablets and Cal Hypo shock. The problem with cal hypo shock is that it is a granular powder that does not dissolve instantly, so it can bleach vinyl liners if poured into the pool without pre-dissolving it first, in a 5 gal bucket of water. A few stray granules are not going to instantly bleach a liner, it takes years of steady abuse actually… If you predissolve, you can use the cheap and effective cal hypo no problem. Now Dichlor, that a stabilized granular form of pool shock, also not recommended for vinyl pools, for the same reason, but no problem if you pre-dissolve. Dichlor has some cyanuric acid to protect it from the sun, but otherwise is similar to cal hypo shock. Lithium shock and non-chlorine shock are both quick dissolving and do not require predissolving, but they are much more costly. Most people use trichlor tabs and cal hypo shock…

  7. I opened my pool and had bad alge growth. I used HTH Super Green to Blue Shock System and my water is blue. I balanced my alkalinity and ph but don’t have a chlorine read. I placed chlorine tabs in my floater but still no chlorine read. Do I add more shock to get a chlorine level or granulated chlorine?

    • Hi Kara, bad test strips or reagents can produce a false negative reading, but what is likely happening is that the chlorine is getting used up, still battling the (now unseen) algae. I would shock the pool again, in the evening, with 1 lb per 10,000 gals, or a normal dose. Check the chlorine level after shocking, and again in the morning. IF still no chlorine, check that your cyanuric acid level is 20-50ppm, If very low or much higher, around 100 ppm, that could be the problem.

  8. Hello, I have been using Dichlor bags to shock my inground 10k gallon pool. Can I use regular chlorine tabs in my feeder with this? or do I have to buy dichlor tablets? I bought Walmarts version of Clorox extra blue. Was in a bind when opening pool. Had massive issues getting the water blue. When it kept reading at 0 for chlorine (used liquid shock to open) switched to dichlor , and now water is turning blue again. Can i put the chlorine tabs in the tab feeder and use dichlor to shock once a week?

    • Hi, yes put the chlorine tablets (Trichlor) into the feeder, and apply dichlor chlorine by hand, broadcasting over the pool. You can use them together… but don’t let them touch each other, dont put the granular in the chlorinator, or… boom! could be a big problem…

  9. Hello. Is DI-ZAP safe to use regularly in a vinyl in-ground pool?
    We used it last year and i found that it kept our pool sparkling
    clean without having to use a lot of other chemicals, but now I’m confused reading about di-chlor, tri-chlor and lithium chlor comments
    on the web. Can you clarify (no pun intended)? Thank you.

    • Hi Diane, as long as you predissolve the Di-Zap in a bucket of water, stirring to dissolve the granules, it is safe to use on vinyl liners. Lithium is a much more powdery form of shock that releases from the binders very fast, so there is no need to predissolve, the same with non-chlorine shock. But Cal Hypo or Di-chlor shocks should be predissolved. trichlor is the tablet form of chlorine, so that’s different… For vinyl pools, you want to avoid undissolved granules laying on the surface of the vinyl, which can weaken or fade the colors. Once or twice won’t hurt, but if you repeatedly broadcast the shock over the pool, it can affect the vinyl over time.

  10. I understand it is very dangerous to mix dry forms of both Calcium Hypochlorite and Trichlor together in a small amounts of water. But Is it ok to use the slow dissolving Tricolor tablets inside ones automatic feeder and occasionally add Calcium Hypochlorite powder to super chlorinate or shock ones pool as needed ?(not adding it to another feeder but just broadcasting the granules over the pool’s surface.)The Tricolor is already dissolved in a large amount of pool water when the dry Calcium Hypochlorite is added. I have read the instructions on the 3 in.Jumbo slow dissolving Trichlor chlorinating tabs container from Leslie Pool Supply and they recommend one add your tabs as stated above and super chlorinate with powder power plus (active ingredient Calcium Hypochlorite)as explained above.I have also asked their store personnel and they say one can do this safely ,do you agree?
    I have been doing this for a few wks and have not seen any explosions.

    • Hi Donna, yes no problem at all to use Cal Hypo in a pool treated with Trichlor tablets. You are right that the two chemicals are highly incompatible, and can explode if they contact each other with water added, in a small space like a chlorinator. But adding Cal Hypo to the pool water will not cause a problem. Just don’t add shock the chlorinator, or allow it to contact tablets (or any other chemical). Keep your chemicals separated, dry, cool and always tightly covered. Thanks, good question!

    • Hi Bob, well that’s a lot of CYA from Latham – millions of vinyl pool owners use Trichlor without problems, or without running the pump 24/hrs per day. However, problems could occur to vinyl if people put tablets in the skimmer basket and don’t run the pump 24/7. When the pump shuts off, the tablets continue to leach in the skimmer basket, and start to overflow the skimmer. over time (a few years), this starts to fade and pucker the vinyl under the skimmer faceplate. Secondly, if using a chlorine floater, and not a chlorinator, the floater could become stuck on a step, or behind a ladder, or tip over and spill it’s contents, all of which could cause damage or fading to vinyl liners. And if you don’t run the pump all the time with a floater, the floater could sit in one spot for a long time, and bleach the vinyl on the wall.

      To avoid all of this, vinyl pools should use a chlorinator installed by the equipment. I like the Pentair 300 model, or the offline 320. Sani-King also makes a nice tablet feeder, and of course there’s Hayward chlorinators.

  11. Thank you for this great info! I was given a Hayward inline chlorine dispenser that says “use only tri-chlor”. I have not installed this yet. My fear is that if I only use tri-chlor in the dispenser, that my CYA level will become too high. FYI,I just installed a new liner, so the pool water is presently city water with one dose of cal-hypo shock. Thanks.

    • If your pool gets a normal amount of rain, or if you backwash frequently, or if you winterize the pool – these things help cyanuric acid levels from getting too high. Using trichlor tablets raises cyanuric acid levels, because there is a small amount in each tablet, but it can take years before the level rises to a level where it needs to be lowered, and mostly only in dry areas with little rain, and maybe a cartridge filter that is not backwashed, and a pool that does not have water lowered each fall for winterizing. All of these things dilute the water enough to keep it from rising too fast. But if it does get too high, you just drain and refill a portion of the pool, which is good to do every few years anyway – or now we have Bio-active, a product that reduces cyanuric acid in pools. So, I wouldnt worry about it getting too high, if I was you…

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