Chlorine is effective as both a sanitizer and oxidizer in pools. As a sanitizer, chlorine kills bacteria and microorganisms. As an oxidizer, it chemically breaks down and removes contaminants from the water. The primary sanitizing agent resulting from chlorine products is hypochlorous acid. Hypochlorous acid exists in equilibrium with hypochlorite ions, and the ratio depends on pH. At a pH of 7.5 the ratio is equal, favoring HOCl at lower pH and OCl- at higher pH. Hypochlorous acid is the most effective form of chlorine sanitizer. Stabilized chlorine products contain isocyanurates that bond chlorine

1. BIO201: CHLORINE CHEMISTRY
Pool & Spa University

2. SANITATION VS. OXIDATION
➤ Chlorine is effective as both a sanitizer and an oxidizer. However, these two
functions are distinct.
➤ Sanitation is defined as the process by which organisms are destroyed. In pools,
this generally refers to killing bacteria and other microorganisms.
➤ Oxidation is defined as the process by which contaminants are chemically changed
(broken down) and removed from the system.
➤ All pools require continuous sanitation and routine oxidation.
Regardless of sanitizer choice, there must always be sanitizer present to
prevent the spread of waterborne illnesses. Oxidation should be performed
on a regular and as-needed basis to reduce contaminant load in the system.
➤ Only products registered with the EPA are appropriate for use as
sanitizers and oxidizers. BioGuard® products are EPA-registered for use
in swimming pools and spas.

3. CHLORINE
➤ Despite all the different types of chlorine-based compounds available,
the primary sanitizer that results from the introduction of these
products to water is hypochlorous acid (HOCl).
➤ Hypochlorous acid is not available to add directly to water for
sanitation; instead, a variety of products, when added to water, result
in hypochlorous acid:
➤ Chlorine gas
➤ Hypochlorite granules (calcium hypochlorite)
➤ Isocyanurates (dichlor and tricolor compounds)
➤ Salt, when exposed to an electric current, will also result in
hypochlorous acid by a process known as electrolysis. This process
will be covered later in the course.

4. HYPOCHLOROUS ACID
➤ Hypochlorous acid exists in the pool in equilibrium alongside
two ions (particles/molecules with a charge): hydrogen ions
and hypochlorite ions.
HOCL H+ OCL-+
➤ As pH increases, the concentration of HOCl molecules decreases and OCl-
ions increases. As pH decreases, the concentration of OCl- ions decreases and
HOCl molecules increases.
➤ At a pH of 7.5, the concentration of HOCl and OCl- is equal (50/50). At 7.2,
the concentration shifts to 2:1 in favor of HOCl; at 7.8, 2:1 in favor of OCl-.
➤ This is why chlorine is ineffective at high pH; the HOCl is the actual
chemical doing work, but at high pH levels there is not enough of it to
effectively sanitize.

5. HYPOCHLOROUS ACID
➤ Hypochlorous acid will react
with a number of different
products in the water to
produce a variety of byproducts:
➤ HOCl + nitrogen-based
compounds (like ammonia)
= combined chlorine
(chloramines)
➤ HOCl + contaminants =
chloride ion (Cl-), known as
the “salt form”
➤ HOCl + sunlight = chloride
ion (Cl-)

6. WHY HYPOCHLOROUS ACID?
➤ Hypochlorous acid is a more effective form of sanitizer than any
other type found in pools.
➤ Chloramines (combined chlorine) are a sanitizer, but very weak
when compared to hypochlorous acid.
➤ Hypochlorous acid is neutral (lacking a charge), making it more
capable of penetrating the cell wall of negatively charged bacteria
than OCl-.
➤ Hypochlorous acid is a smaller molecule than those of the various
chloramine compounds, making it easier to diffuse through cell
membranes.
➤ Monochloramine, in particular, is used to sanitize drinking water
due to its higher stability, but is not as fast as hypochlorous acid.

7. TYPES OF CHLORINE
➤ Free available chlorine (FAC/FC/FCL)
➤ Measured as the combination of
HOCl and OCl- ions
➤ Little to no odor or taste and no
irritation for bathers
➤ Combined chlorine (CC)
➤ Measurement of chlorine compounds
containing nitrogen, i.e. chloramines
➤ Infamous “chlorine” odor
➤ Not as effective as HOCl as a
sanitizer
➤ Total chlorine (TC/TCL)
➤ Sum of free chlorine and combined
chlorine

8. CHLORAMINES
➤ Chloramines are formed when hypochlorous acid comes into contact with ammonia-
based compounds. Two types of chloramines exist: inorganic and organic.
➤ Inorganic chloramines are generally a result of ammonia combining with
hypochlorous acid. Future discussion of chloramines will use this type as an
example.
➤ Organic chloramines are generally a result of proteins and amino acids.
➤ Inorganic chloramines break down more quickly than organic chloramines;
additionally, organic chloramines are ineffective as sanitizers.
➤ When chloramines are present in the water, undesirable effects (skin/eye irritation,
chlorine smell) can result.
➤ Chloramines can be removed from the water through breakpoint sanitation or
oxidation.
➤ Note that this method differs when dealing with chlorine demand; identification
and treatment of chlorine demand will be covered in another course.

9. TYPES OF CHLORINE - NOT RECOMMENDED FOR USE
➤ Chlorine gas (Cl-)
➤ Produces HOCl and hydrochloric acid (HCl)
➤ Lowers pH of water due to HCl
➤ Difficult to handle and store; impractical for most pool applications
➤ Sodium hypochlorite (NaClO)
➤ Also known as bleach
➤ Produces HOCl and sodium hydroxide (NaOH)
➤ Raises pH of water due to NaOH
➤ Unstable; will break down in container after bottling, reducing effectiveness
by as much as half
➤ High level of by-products and fillers raises TDS quickly
➤ Weakest available chlorine on the market

10. TYPES OF CHLORINE - UNSTABILIZED CHLORINE
➤ Lithium hypochlorite (LiClO)
➤ High pH
➤ Not as strong as other granular/pressed products, but no additional by-
products added to water
➤ Primarily sold as a shock (oxidizer)
➤ Calcium hypochlorite (Ca(ClO)2)
➤ High pH
➤ Slower-dissolving than lithium hypochlorite but delivers more chlorine
(second only to trichlor products)
➤ Will raise calcium hardness by 0.8ppm for every 1ppm of HOCl delivered
➤ Sold as both a sanitizer and as an oxidizer
➤ Active ingredient in CLC®3, CLC® Classic, BurnOut® 3, BurnOut® 73

11. TYPES OF CHLORINE - STABILIZED CHLORINE
➤ Stabilized chlorine products are made up of products
containing isocyanurates, which are composed of cyanuric
acid (C3H3N3O3 or (CNOH)₃) and chlorine.
➤ The bond formed is not the same as the bonds within
chloramines; the chlorine in the isocyanurate molecule is
still an effective sanitizer, whereas the chlorine in the
chloramine molecule is not. Chlorine that is bonded to
cyanuric acid will show up as free chlorine on tests.
➤ When bonded to cyanuric acid, chlorine is less susceptible to
breakdown due to sunlight.

12. TYPES OF CHLORINE - STABILIZED CHLORINE
➤ Sodium dichloro-s-triazinetrione (C3Cl2N3NaO3)
➤ Also known as sodium dichlor or just dichlor
➤ Near neutral pH
➤ Fast-dissolving
➤ Contributes 0.9ppm cyanuric acid (CYA) per 1ppm of free chlorine
➤ Sold as both a sanitizer and an oxidizer
➤ Active ingredient in Super Soluble™, Smart Shock®
➤ Trichloroisocyanuric acid ((CClNO)3)
➤ Also known as trichlor
➤ Low pH
➤ Highest concentration of chlorine available (unless blended with other materials)
➤ Slow-dissolving
➤ Contributes 0.6ppm CYA per 1ppm of free chlorine
➤ Sold as a sanitizer
➤ Active ingredient in Smart SilkGuard® Sticks, SilkGuard® Sticks, SilkGuard® Tabs, Power Chlor™,
BioGuard® Basic Tabs

13. ➤ Overuse of dichlor and trichlor products can result in high CYA
levels in pools with very little water turnover.
➤ High CYA levels will slow down chlorine’s ability to kill
bacteria and oxidize contaminants. For 99% of bacteria, this is
not an issue, even at 100ppm of CYA, provided that there is
at least 1ppm free chlorine in the water.
➤ For certain water incidents (the primary one being
Cryptosporidium parvum), additional sources of chlorine and
additional time will be needed when CYA is present in the
water.
➤ Regular applications of fresh water will keep CYA levels from
increasing rapidly while using dichlor and trichlor products.

14. CONGRATULATIONS!
You have completed the activity!