Pool Chlorine Chemistry - Pool Chlorine is Key in Swimming Pool Care

Proper pool chlorine chemistry is key in keeping your swimming pool water clean and healthy. chlorine is the most commonly used disinfectant in swimming pool and spa water treatment. Most disinfectants that are used in swimming pool care, releases chlorine as the main disinfecting agent.

The reason for this is simply due to the fact that hypochlorous acid effectively kills or inactivates swimming pool pathogens.

It is also very effective in removing swimming pool algae as well.

Another good characteristic in pool chlorine chemistry is that it causes oxidation.

This means that you chemically destroy other materials that are associated with the environment and your pool bathers. Good pool chlorine chemistry allows you to, safely maintain a proper concentration of chlorine residual in your pool water at all times. In the right conditions, this chlorine residual can be maintain for hours - even days!

Characteristic of swimming pool chlorine chemistry!

Chlorine used in swimming pool disinfecting and oxidation, are classified into two main categories. These categories are defined as unstabilized chlorines which include these pool sanitizers.

  • Sodium hypochlorite.
  • Calcium hypochlorite.
  • Lithium hypochlorite.
  • Chlorine gas.

The other category of chlorine is called stabilized chlorines. There are only two types of chlorine used for pool water treatment, that fall under this category. These two pool chemicals are also refer to as your organic chlorines. They include...

  • Trichlor-s-triazinetrione. This type of swimming pool chlorine is also referred to as trichlor or trichlor-isocyanuric acid.
  • Sodium dichlor-s-triazinetrione. We commonly refer to this type of pool chemical as dichlor or dichloroisocyanurate.

Here some interesting facts about pool chlorine chemistry that you should know! When it is added to water, it produces hypochlorous acid or HOCL. This HOCL is the active killing form of chlorine in water. Another fact about chlorine reaction in water! Hypochlorous acid is constantly dissociating into and reforming from hypochlorite ion (OCL-) and free hydrogen (H+).

Hypochlorous acid and hypochlorite ion are in equilibrium with each other. This means that a hydrogen ion attaches itself again to make up the HOCL and then detaches itself once more to form OCL-. This process occurs every second many times over. Other chlorine facts.

If by chance, some HOCL molecules are consumed, in an instant the OCL- ions will attach to H+ ions to maintain the same ratio of HOCL to OCL- ions.

Relative concentration of HOCL versus OCL- ions in pool chlorine chemistry!

The chart below shows the relative concentration of the pool chlorine chemistry clearly.

%Active HOCLpH Level%Less Active OCL
976.03
916.59
767.024
667.234
507.550
337.867
248.076
98.591

Chemical reactions in swimming pool chlorine chemistry

Part of chlorine reactions in water, is that as you use HOCL, the OCL will immediately convert back to HOCL. This is necessary because it maintains the equilibrium in accordance with your pH level. An example of this process is as follows...if your swimming pool maintains 3 ppm of free chlorine (FC) at a pH of 7.5, you would end up with about 1.5 ppm of HOCL and 1.5 OCL-.

If 1 ppm is consumed, your free chlorine content would drop down to 2 ppm; thus, your HOCL and OCL- ratio will be 1 ppm each. Another fact about chlorine is that if your pH level rises to 7.8 and your free chlorine remains at 2 ppm, your HOCL would be 0.66 and your OCL- would be 1.34 ppm.

Remember though, as your pH level drops, your pool water will become more acidic.

Free chlorine disinfectant in swimming pools

Swimming pool sanitation occurs in the form of free chlorine being present in your pool water. This free chlorine can be produced organically, in-organically or it can be generated. The free chlorine in your swimming pool water is the sum of the HOCL and OCL-.

This is the chlorine that you will determine with a DPD test. The pool chlorine chemical formula that is used, looks like the one below...

FC - (Free Chlorine) = HOCL (Hypochlorous Acid) + OCL- (Hypochlorite Ion)

For good pool chlorine chemistry, you should maintain your FC between 2.0 to 4 ppm. This is the required standard for good swimming pool care. The minimum accepted level for free chlorine level is 1.0 ppm - with the maximum accepted level being 5.0 ppm.

Combined pool chlorine chemistry!

Part of your swimming pool chlorine chemistry is knowing your combined chlorine (CC)Level. It develops in your pool or spa water, as result of your free chlorine (FC) reacting with ammonia or organic (carbon-containing) nitrogen compounds.

This combined chlorine is commonly referred to as chloramines in the swimming pool and spa industry. Sources of ammonia in your pool or spa water are connected to these elements which include...

  • Urine.
  • Sweat.
  • The environment.

As a pool sanitizer, free chlorine also reacts with nitrogen compounds. The nitrogen compounds are part of the organic molecules that come from the environment and your swimmers.

Other factors that can affect your pool chlorine chemistry!

There are other factors that react with the free chlorine in your swimming pool water. These elements can seriously affect the pool chlorine chemistry of your pool water; thus, resulting in the production of chloramines. These elements include

  • Proteins.
  • Urea.
  • Amino acids.
  • Other organic molecules that contain nitrogen.

You will encounter difficulties in removing this organic chlorine from your pool. Sadly, break-point chlorination will not do the trick. Therefore you have to seek alternative methods to removing this problem from your pool system.

In fact, because your pool chlorine chemistry level is stable and in excess, break-point chlorination may cause an increase organic chloramines.

Some suggested methods that are commonly used are as follows:

  • Draining and replacing some of your pool or spa water.
  • Adding ozone treatment to your pool filtration system.
  • Use water clarifiers.

  • Use a filtration enhancer in your treatment system.

The effect of pH on chlorine in water!

When your swimming pool consist of inorganic chloramines, monochloramine is usually the predominant form of combined chlorine. This is usually the case when your pH level of the water falls between 7.2 and 7.8. The chlorine to ammonia ratio in your pool water at this point is 5:1 or less. As the level of your pool chlorine chemistry increases, your chlorine to ammonia ratio will also increase.

Furthermore, you will have dichloramine and nitrogen trichloride forming as well. Dichloramine and nitrogen trichloride are far more irritating than monochloramines. This means that your pool bathers will have more serious burns to their eyes and mucus membranes. These two entities are also responsible for the chlorine smell, that is produced in an indoor water facility with a poor ventilation system.

Chloramines are considered effective pool disinfectants. Unfortunately they act more slowly than HOCL. Ideally, the amount of combined chlorine in your pool should be zero. According to local and state regulations, the maximum ppm for combined chlorine in pools is 0.2 and 0.5 ppm for your spas (hot tubs).

Facts about pool chlorine chemistry and DPD testing!

Unfortunately a DPD test does not measure the combined chlorine level directly. This test does in fact, measure the level of free chlorine (FC) during the first step. In the second step, you will be measuring your total chlorine (TC) level. Your total chlorine is the sum of your combined chlorine + your free chlorine.

Another way of stating this fact is this - your combined chlorine is the difference between total chlorine and free chlorine. The formula for this pool chlorine chemistry equation looks like the one below.

CC (Combined chlorine) = TC (Total Chlorine) - FC (Free Chlorine)

Break-point Chlorination for swimming pool care!

Whenever there is sufficient free chlorine added to your pool water, the inorganic chloramines are converted to dichloramine. It is then converted to nitrogen trichloride, and finally into nitrogen gas. The conversion of your combined chlorine compounds to nitrogen gas is what we call break point chlorination (BPC).

In order for you to achieve break point chlorination in your pool chlorine chemistry, you have to add more free chlorine to your water. However, the addition of new chlorine has to be ten times the amount of your combined chlorine (CC). This has to be calculated. The pool chlorine chemistry formula for this calculation is

BPC = 10 X CC

BPC = 10 X (TC - FC)

Let us look at an example using this formula.

A 65,000 gallon swimming pool with 1.6 ppm of free chlorine, 2.4 ppm of total chlorine, a pH level of 7.3, causes eye irritations. How many ppm of additional chlorine do you have to add to achieve break-point chlorination? Plugging our figures into the formula give us this pool chlorine chemistry equation!

TC = 2-4 -FC = 1.6

CC = 0.8

Our pool chlorine formula now becomes...

BPC = 10 X CC

BPC = 10 X 0.8

BPC = 8.0ppm

Your break-point chlorine is always the additional chlorine that you have added to your pool water. This is also referred to as the desired change. Any amount of FC that is less than what you have calculated, will not achieve break-point chlorination for you.

Thus the problems that you are experiencing due to combined chlorine will not be eliminated. If you have an indoor swimming pool, it is important that you have a proper ventilation system. This is important because some chloramines can evaporate and then dissolve right back into your pool water.

Pool chlorine chemistry and oxidation

The process of removing body wastes and organic matter from your pool water is accomplish through the oxidizing properties of your pool chlorine, bromine or other oxidizing products that may or may not be pool disinfectants.

Chlorine and bromine in the form of HOCL (Hypochlorous acid) and HOBr (Hypobromous acid) respectively, are classified as oxidizers.

Consequently, without proper chlorine chemistry - keeping your swimming pool water clean and healthy is not an easy task.

Generally, there are two main cleaning methods in swimming pool care.

The first of these two methods is conducted through your mechanical system.

The process is called the mechanical cleansing method. Cleaning is achieved through the filtration system for your swimming pool system. The second cleansing method is achieved through your pool chemicals. This method is done through oxidation.

Oxidation is the process of changing the chemical structure of contaminants - so that they are more readily removed from your swimming pool water. In some cases, oxidation may convert organic material into single gases, such as nitrogen and carbon dioxide. On the other hand, it adds oxygen to the contaminant.

This process will also permit your organic materials to be more easily removed through the filtration process.

Super Chlorination of your pool water!

If your swimming pool or spa water has a dull appearance, or if it looks hazy, you may need to treat it with an extra dose of chlorine. This treatment is usually between 8 - 10 ppm. In the case of pool chlorine chemistry - We call this process super-chlorination of your swimming pool and spa.

In achieving proper swimming pool care, we must remember that some pool chemicals are used for both disinfection and oxidation. This is why it is important that you review the product labels of all pool chemicals used. From the labels, you can learn about your pool chlorine chemistry.

It does not matter if your chlorine is in powder or your chlorine tablets form. The information printed on the labels, will give you the MSDS (Material Safety Data Sheets) on the pool chlorine chemistry make up. This will help in avoiding any accidents - such as chlorine poisoning.

You will know exactly what the product intended use should be. This should always be done with any pool chemical that you will be using in your pool chlorine chemistry.

Good pool chlorine chemistry kills pool water contaminants!

Swimming pool care home.

Back To Top

New! Comments

Have your say about what you just read! Leave me a comment in the box below.