The chemistry of chlorine in pools: Keeping water safe and clear

by Dave Flaherty | December 9, 2020 9:44 am

Chlorine in pool water is effective at the removal of bacteria and viruses and the prevention of algae growth.
Chlorine in pool water is effective at the removal of bacteria and viruses and the prevention of algae growth.

By Terry Arko

Water has long been considered one of the most vital molecules on earth. As a strong polar solvent, water can dissolve almost anything from sugar to solid rock.

Likewise, chlorine has a long history of effectively killing a wide range of dangerous germs. In fact, it has been used for well over 100 years to make the water in many communities safe, clean, and clear.

The reactive power of chlorine as a disinfectant is a result of its ability to bond with and destroy bacteria and viruses. Today, more than ever, chlorine has been called into duty during natural disasters, and against the COVID-19 virus, having widespread use both in water and on surfaces.
The amazing properties of water and chlorine work together to create reactions that lead to the destruction of a myriad of harmful germs. Water acts as an ideal carrier for chlorine in certain forms such as sodium hypochlorite. Chlorine undergoes many rapid changes in the process of oxidation, sanitization, and disinfection.

Hypochlorous acid: A powerful destroyer  of germs since the beginning of humanity

The first and most immediate reaction that occurs when chlorine meets water is the formation of hypochlorous acid (HOCl) in solution with the water. HOCl has existed as a germ-killer since the beginning of humanity; it is produced by one’s white blood cells to fight off invasive bacteria that can enter the body through cuts and wounds. The existence of HOCl outside the human body was first observed by Michael Faraday in 1823 when he was experimenting with the electrolysis of brine solutions.
Faraday’s technology is what led to the manufacturing of liquid chlorinating compounds and salt chlorine generators in pools.

HOCl is the workhorse created from the addition of a chlorinating compound to destroy bacteria and prevent algae in pools. A secondary occurrence of mixing water with chlorine is the reaction from hydrolysis of the HOCl to create hypochlorite ion OCl-.

In comparison to HOCl, the OCl- is a very weak sanitizer. It represents only one per cent destructive ability of bacteria and viruses. HOCl is the strong killing agent of chlorination with 99 per cent destructive ability.

Knowing this, it is obvious that when adding chlorine to sanitize or disinfect a majority of HOCl over OCl- would be preferred. This is an important point, as the transition from HOCl to OCl- ions goes back and forth in a water system depending upon the pH. Higher pH leads to a higher percentage of OCl- and weaker disinfection. Lower pH leads to higher percentages of HOCl and greater destruction of germs in pool water.

In a pool with no cyanuric acid (CYA) and a pH of 7.5, HOCl and OCl- are both at 50 per cent. With water chemistry everything is about balance, and ideally in pools the pH is 7.5 to keep the water more neutral. If the pH is too low, the water could become corrosive; if it is too high, it could lead to scaling and staining from metals. Conversely, in a pool with no CYA and a slightly higher pH of 8, only 20 per cent of the better killing agent HOCl is available.

 Cyanuric acid and chlorine

Chlorine in pool water is effective at the removal of bacteria and viruses and the prevention of algae growth. However, in sunlight HOCl is very unstable. Fifty per cent of chlorine in pool water can be destroyed by ultraviolet (UV) sunlight in less than one hour. HOCl and OCl- make up much of free chlorine (FC). UV sunlight could quickly cause a drop in the concentration of FC below the Environmental Protection Agency (EPA) recommended levels of one-to-four parts per million (ppm). This can lead to the risk of disease to swimmers.

CYA is a stabilizer; when used at proper levels, the FC in pool water will last three to 10 times longer than without it. Therefore, it is important to keep CYA at the proper concentration in the pool.

Graph from Sodium Hypochlorite Handbook 2014 Oxychem
Graph from Sodium Hypochlorite Handbook 2014 Oxychem.

Emerging research is showing that high levels of CYA lead to a decrease in the effectiveness of HOCl to kill bacteria and prevent algae. Additional research has recently revealed the presence of CYA in pool water supersedes the effect of the pH on the percentage of HOCl to destroy germs. In other words, the CYA level in the pool water is the more prevalent determiner of HOCl levels rather than the pH. Also, it is believed that in the presence of CYA a pH of 8 will not have the same effect in reducing the HOCl.

It should be noted, however, there are many other reasons for keeping water balanced according to standards. For instance, high levels of CYA will bind to most of the free chlorine and reduce the effectiveness of HOCl to destroy bacteria, while running a pH at 8 or above will lower the oxidation reduction potential (ORP) of the HOCl as well.

ORP is a system used for measuring the oxidative potential of chemicals such as chlorine in water. It is measured in millivolts (mV). Reducing chlorine’s ability to oxidize increases the threat of disease-causing bacteria. Chlorine in pool water serves a primary purpose to sanitize by reducing the numbers of threatening micro-organisms to a safe level. Chlorine also acts as an oxidizer of organic contaminants such as those from bather waste—mainly sweat, saliva, and urine. Chlorine is primary and preferred because it can do these three things in water:

There are differing opinions; however, following standards and using balance in water management is still the best way to keep the pool water safe.

Types of chlorine

Service professionals and do-it-yourself pool owners have some choices when it comes to chlorinating water. For many, the type of chlorine being used may be based on anything from the makeup of regional source water, storage and safety, or cost. Liquid sodium hypochlorite (chlorine) or calcium hypochlorite (cal-hypo) may be preferred in regions with softer water.

Soft water is generally low in calcium and alkalinity, so the liquid or cal-hypo naturally add to what the water needs. In areas where calcium is high, and water is hard, trichlor tablets may be preferred. Trichlor tablets contain CYA and have a very low pH. Because they are acidic, they will lower the pH and total alkalinity.

When choosing a type of chlorine, it is important to understand the byproduct that is left behind after chlorination and how this will affect the water and type of pool surface. When it comes to cost many may reach for the bargain brand; however, that may not always be the best choice. Certain types of chlorine can lead to additional cost due to the byproducts, additional chemicals or increased water draining that may be necessary. The following are the main types of chlorine available and information on the effectiveness and byproducts of each. Chlorine is available in three categories: elemental, inorganic, and organic.

Elemental chlorine: Chlorine gas

With an available strength of 100 per cent, chlorine gas is the purest form of chlorine. It is also the most economical. That said, there are additional costs involved as elemental chlorine is by far the most hazardous form used for pool water sanitization and, as a result, has strict handling regulations.

Gas chlorine can only be applied by a technician who has been registered by the federal EPA as an applicator of chlorine. It has a pH of less than 1 and it is highly acidic. Due to the very low pH, 0.9 kg (2 lb) of soda ash is needed for every 0.45 kg (1 lb) of gas used. It is also two-and-a-half times heavier than air. Chlorine gas is very irritating to mucous membranes and can cause serious health problems if inhaled. As a result of increased regulations, handling, storage, and safety concerns gas chlorine has become much less prevalent today. However, it is the standard by which all other forms of chlorine are compared in terms of efficacy and killing power against germs.

Inorganic chlorine: Cal-hypo (65 per cent)

Cal-hypo is produced by passing chlorine gas over slaked (hydrated) lime. It is a white granular material or tablets. The most common cal-hypo contains 65 per cent available chlorine although there are other higher strengths. The pH of cal-hypo is 11.7. It is preferred in soft water regions because of its calcium content.

Although an effective shock, the drawbacks of cal-hypo use show themselves in hard water environments, where it will significantly increase the calcium hardness and, in combination with alkalinity, will tend to cloud the water. Increased calcium can lead to the formation of calcium carbonate scale as well.

Because of its inherently high pH, additional acid is needed when using cal-hypo to compensate for the rise that occurs as soon as it is added. It is also classified as an ‘extreme oxidizer;’ (class 3) it is highly volatile if it comes in contact with a broad variety of organic matter such as carbonated drinks, sweat, oil, or dust. There are newer cal-hypo tablets that are classified as class 2 oxidizers and are less reactive than the original form. When cal-hypo combines with any of these materials it can cause an explosive fire. One should never mix any other chemicals with cal-hypo.

 Sodium hypochlorite 12.5 per cent

Trichlor tablets contain CYA and have a very low pH. Because they are acidic, they will lower the pH and total alkalinity.
Trichlor tablets contain CYA and have a very low pH. Because they are acidic, they will lower the pH and total alkalinity.

Made by passing chlorine gas through a solution of caustic soda, it is the most widely used chlorinating compound in the pool industry. It is not only cost effective, but also highly efficient.

Sodium hypochlorite has available chlorine of 12.5 per cent and a pH of 13.

The most notable drawback of sodium hypochlorite is its decomposition profile. It decomposes readily over time, and that decomposition is exacerbated at high temperatures. It should be stored in a cool dark area and used more readily to prevent degradation.

It is important to note, there is no CYA or calcium byproduct from the use of liquid sodium hypochlorite. It makes for a very good shock, especially for pools using chlorine generators. It can also be used as a primary sanitizer, especially in cases where reduced CYA levels are needed. The addition of muriatic acid may be needed depending on usage.

Organic chlorine

These forms are blended with CYA and are also known as stabilized chlorine because they contribute additional CYA to the pool water, which provides an inherent stabilizing effect and lessens the reduction of FC due to UV and high temperature exposure in the pool.

Trichlor

Trichlor has a pH of 2.9 (very acidic) and, therefore, requires diligent pH and total alkalinity (TA) testing when it is used.
Trichlor has a pH of 2.9 (very acidic) and, therefore, requires diligent pH and total alkalinity (TA) testing when it is used.

Trichloro-s-triazine-trione is made by drying and cooling the sodium salt of CYA in the presence of chlorine gas. This compound has an available chlorine percentage of 90. It is available in 25-mm (1-in.) and 76-mm (3-in.) tablets, as well as in sticks and granular formats.

The granular form is effective against black algae; however, it is not recommended for use on coloured plasters, fibreglass, or vinyl-lined pools. Trichlor has a pH of 2.9 (very acidic) and, therefore, requires diligent pH and TA testing when it is used. Additional soda ash or sodium bicarb may be needed to control decreases in pH or alkalinity.

 Dichlor

There are two forms of dichlor available.

  1. Sodium dichloro-s-triazinetrione dihydrate is one form, which has 56 per cent available chlorine. A white granular material, which is lightly soluble in water, dichlor has a pH of 6 to 6.8. Because the pH is very close to neutral, this form is ideal for hot tub use.
  1. Sodium dichloro-s-triazinetrione anhydrous 62 per cent is different from dihydrate mainly because all the bound water molecules are removed. It has a higher AC of 62 per cent and, as a result, it is used primarily for shocking.

The true strength of chlorine in the pool environment

Chlorine labels can be confusing in determining the actual percentage of pure chlorine in pool water. In fact, it is a subject that would take another article to fully explore. As previously stated, chlorine gas was established as the standard in determining the amount of pure chlorine in other chlorine compounds since it is at a strength of 100 per cent. Another aspect is to compare the parts per million of FC that is provided by either a 3.8 L (1 gal) of liquid or 0.45 kg (1 lb) of dry chlorine.

In sodium hypochlorite, trade per cent is often used and is technically the volume (rather than weight) per cent of available chlorine. It is the only type of chlorine where the per cent shown on the label exactly matches the available parts per million in pool water from 3.8 L (1 gal) in 37,855 L (10,000 gal). In other words, 3.8 L (1 gal) of 12.5 per cent liquid sodium hypochlorite equals 12.5 ppm of free chlorine in 37,855 L (10,000 gal) pool water.

Dry forms of chlorine such as cal-hypo and trichlor are listed in weight per cent and not volume per cent. The available weight per cent of chlorine on the label does not match the actual percentage of free chlorine in the pool water once it is used. The chart below shows the free chlorine created in 37,855 L (10,000 gal) from 3.8 L (1 gal) of liquid 12.5 per cent compared to 0.45 kg (1 lb) of 65 per cent cal-hypo and 0.45 kg (1 lb) of 90 per cent trichlor.

Differing types of chlorine offer their advantages based on the type of facility, regional source water, availability, and ease of use. Overall, chlorine still offers the most time-proven way to inactivate disease-causing bacteria and viruses, reduce moulds and fungus, and prevent algae. It is no wonder the American Chemistry Council calls chlorine ‘The Element of Surprise’ for all the benefits it brings in keeping water clean, clear, and safe.

References

Terry Arko is a product training and content manager for HASA Pool Inc., a manufacturer and distributor of pool and spa water treatment products in Saugus, Calif. He has more than 40 years’ experience in the pool and spa/hot tub industry, working in service, repair, retail sales, chemical manufacturing, technical service, commercial sales, and product development. He has written more than 100 published articles on water chemistry and has been an instructor of water chemistry courses for more than 25 years. Arko serves as voting member on the board of the Recreational Water Quality Committee (RWQC). He is a Commercial Pool Operator (CPO) course instructor, a teacher of the Pool Chemistry Certified Residential course for the Pool Chemistry Training Institute (PCTI), and a member of Pool & Spa Marketing’s Editorial Advisory Committee. Arko can be reached via email at terryarko@hasapool.com.

Endnotes:
  1. https://www.chlorine.org/: https://www.chlorine.org/

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