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Recommended Free Chlorine Levels for Pools and Spas:

bulletPools - maintain a 1-3 ppm residual at all times
bulletSpas - Maintain a 2-3 ppm residual at all times.

Chlorine is the least expensive, widest used and most effective sanitizer for pools and spas.  Chlorine is also one of the most misunderstood chemicals you will encounter.  We will explain the mechanics of chlorine in relation to pool and spa sanitization and also explain the proper method for using chlorine as a sanitizer.

bulletTo learn more about a particular type of chlorine, select it from the list at the left.

Chlorine - The Analogy:

We can compare Chlorine to Gasoline in our car.  If we have Chlorine in our pool it is like having gasoline in our car.  The chlorine is available to kill bacteria and the car is ready to be driven.  As Chlorine kills bacteria and is used, the available level decreases just as the gas in our car decreases as we drive around.  In our car, when we burn gas, the used gas is turned into exhaust and goes out the tail pipe.  When Chlorine is used it turns into a "Chloramine" or used chlorine but stays in the water.  We must "Shock Treat" the pool or spa water to allow the Chloramine to turn into a gas and escape the water.  If we were to put a potato in the tail pipe of our car, it would cause the car to run poorly and we may even smell the exhaust inside the car.  This is similar to what happens if we do not properly shock treat the pool/spa water.  Chloramines will build up in the water and not allow the "Free Chlorine" to work effectively and we will begin to smell a chlorine odor.  Shock Treating the pool/spa is like removing the potato out of the tail pipe on our car!

Chlorine - The Basics:

bulletMaintain the proper residual of Free Available Chlorine
bulletShock Treat on a regular basis to prevent Chloramine (used chlorine) buildup and odor
bulletIf there is a difference in the Free Chlorine and Total Chlorine test results, shock treatment is needed.
bulletPools - Shock treat every two weeks, every week when it is 85 or above, or after a rain or heavy bather load.
bulletSpas - Use Oxy-Brite and shock treat on a weekly basis for residential spas and daily for commercial spas.
bulletTest water after heavy use and add extra chlorine if needed
bulletAutomatic Chlorinators can be used to dispense chlorine to the water automatically on both pools and spas.  Call or Email for details.
bulletOzonators can reduce chlorine use on spas.

Chemistry of Chlorine:

A chlorine molecule or ion kills bacteria, algae, and disease-causing organisms by moving through the cell membranes and deactivating the cell's essential enzymes and structures.  All chemical forms of chlorine provide oxidation, but different forms vary in effectiveness depending on how easily they move through the cell membranes.  The most effective form of chlorine for disinfection is Hypochlorous Acid; the Hypochlorite Ion form is next most effective; Chloramines are less effective.

Free Available Chlorine:

(Hypochlorous Acid + Hypochlorite Ion) Free Available Chlorine or free chlorine (FAC or FC) is the measure of chlorine's active disinfecting power.  The free chlorine reading is the sum of the hydrochlorous acid and the hypochlorite ion residual concentration levels in the pool/spa water in parts per million (ppm).

Hypochlorous Acid is the most active disinfectant form of chlorine in the water.  Hypochlorous acid is formed when chlorine or a chlorine compound is dissolved in water.

At a pH of 6.0, some of the hypochlorous acid begins turning into the hypochlorite ion, then reforms again into the hypochlorous acid in a chemical equilibrium.  The hypochlorite ion is also an active disinfectant form of chlorine, but it is less powerful than hypochlorous acid.  The breaking down of hypochlorous acid into the weaker hypochlorite ion is called dissociation.  As pH increases, more and more of the hypochlorous acid dissociates and stays in the form of the hypochlorite ion.  To keep chlorine most active, it is important to keep the pH in the range of 7.2 to 7.8

Combined Chlorine:

When chlorine reacts with ammonia and nitrogen compounds in the water (urine and other organic wastes) it forms chloramines and nitrogen.  Chloramines cause an unpleasant chlorine odor and eye irritation to swimmers.

Chloramines are otherwise known as combined chlorine (CC).  Combined Chlorine is considerably less active in killing bacteria than the forms of uncombined chlorine (hypochlorous acid and hypochlorite ion).  Combined chlorine can be destroyed and converted into nitrogen and water by the process known as breakpoint chlorination.

Shocking and Superchlorination:

Superchlorinate the pool or spa if there is any difference between the Free Chlorine Residual (FAC) and the Total Chlorine Residual (TC) test results.

Shock Treatment:

Free available chlorine is used up when it destroys organic contaminants (algae and other wastes) in a pool or spa.  When excess algae and organic matter are present, we shock the pool - that is, we treat this excess burden with a shock treatment product.  The shock treatment is an oxidizer which oxidizes and thus destroys the excess organic contaminants, without allowing them to use up all of the residual free disinfectant.

To shock, you add a large amount of oxidizer/disinfectant.  Usually, you use enough product to add 10 ppm free chlorine.

The current trend in pool/spa maintenance is to shock treat on a routine basis to kill algae and organic wastes.

Calcium hypochlorite, sodium hypochlorite, and lithium hypochlorite are used for shock treatment.  Dichlor can be used if you wish to build a residual of cyanuric acid (stabilizer).  Trichlor is not normally used for shocking.  Non-chlorine shock treatments are also used.  The chemical process of oxidation of chloramines is similar.  Non-chlorine shock treatments include products such as potassium monopersulfate, potassium dipersulfate, and hydrogen peroxide.  These products are oxidizers, not disinfectants.

A residential pool or spa should be shocked every other week, or weekly when the temperature is over 85.  When a pool or spa is heavily used it may be necessary to shock more often.  This need can be determined by regular water testing.

After shocking, the pool or spa should not be used until the free available chlorine residual drops below 3.0 ppm in pools and 5.0 ppm in spas.  As a result, shocking is often done in the evening.  For spas, shocking is done after the last use of the day and it may be done as frequently as daily or twice daily on a commercial spa. Oxy-Brite is a non chlorine shock treatment which is recommended for use in pools or spas and it does not raise the chlorine level.  Commercial pools and spas can use this product which allows the pool to remain open due to maintained chlorine levels.


When chlorine is the disinfectant used, free chlorine turns into combined chlorine as it destroys nitrogen-bearing contaminants, and becomes much less effective.  When combined chlorine becomes too high - over 0.2 ppm - we have to destroy it, converting it to nitrogen, water, and chlorine salt.  This is done by achieving breakpoint chlorination (Super Chlorinating) the pool/spa water.  The purpose of super chlorination is to destroy combined chlorine (Chloramines).  It takes 10 - 12 ppm of free chlorine to destroy 1 ppm of combined chlorine.

Superchlorination as here defined is the equivalent of breakpoint chlorination.  To achieve breakpoint chlorination you must add enough chlorine shock product to raise the FAC to 10 times (or more) the combined chlorine level.  Superchlorination is done by adding an oxidizer - usually a shock product.

For instance, to super chlorinate a pool whose combined chlorine reading is 0.2 ppm, you must add an amount of FAC (in the form of chlorine compound or shock product) that would yield a FAC reading of 2 ppm.  If FAC is already 1 ppm, you need to add 1 ppm shock product FAC.  A second example:  To super chlorinate a pool whose combined chlorine reading is 2 ppm and whose FAC is 0.5 ppm, you must add 19.5 ppm FAC (in the form of oxidizer or shock product) to achieve the needed total of 20 ppm.

The first additions of shock product cause the total residual chlorine level to climb (TC = FAC + CC).  However, with continued additions, the total chlorine residual suddenly drops at a certain point called the breakpoint.  The breakpoint will vary from pool to pool and will even fluctuate in a particular pool or spa depending upon the amount of organic matter present.

Pool super chlorination - careful measuring to achieve breakpoint - may be done once a month, but depends very much on the needs of the particular pool.  Spas are not super chlorinated because they are shocked daily.

Testing Chlorine Residuals:

Water test kits and strips are used to measure the amount of active disinfectant in the pool/spa water.  Test strips work fine for routine use and Expert Pool & Spa recommends the use of test strips for most applications.

Total Chlorine (TC):

One common test measures the total available chlorine residual (TAC) or total chlorine residual (TC) - the total amount of chlorine compounds in the water, whether they are active disinfectants or not.  Thus, TC is a measure of free chlorine plus combined chlorine (chloramines).

Importance of Free Available Chlorine (FAC):

To maintain water of ideal quality, the free available chlorine (FAC) residual should be between 1.0 and 3.0 ppm.  Test strips and DPD test kits will test for FAC.  OTO kits will not.

ORP Testing:

ORP = Oxidization Reduction Potential

ORP testing is usually performed by a computer controlled chlorination system like the Watermatic Systems.

The ORP test measures the oxidation potential of chlorine, indicating the activity level of the disinfectant.  Hypochlorous acid, the most effective oxidizing form of chlorine, has the highest ORP reading by far.  An ORP reading of 650 mV or higher indicates a sufficient concentration of free available chlorine.

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This website was last updated on
Tuesday May 24, 2011