Chlorine acts as both a sanitizer and oxidizer in pools. Sanitation kills microorganisms while oxidation breaks down and removes contaminants. Oxidation, or "shocking", involves oxidation-reduction reactions that change the structure of contaminants. Nitrogen compounds react with chlorine to form chloramines, which cause chlorine odor and demand. Reaching the "breakpoint" involves adding excess chlorine to break down chloramines and produce nitrogen gas, reducing odor and demand. Regular oxidation prevents buildup of contaminants and chloramines that require excessive chlorine to treat.
2. QUICK REVIEW
➤ 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. QUICK REVIEW OF OXIDATION
➤ As previously stated, oxidation is the process by which the
structure of contaminants is changed, making them easier to
remove from the system.
➤ Generally, this is referred to as “shocking” the system.
➤ Oxidation is examined as oxidation-reduction reactions.
➤ Oxidation-reduction reactions refer to the exchange of
electrons and require two items: the oxidant and the
reductant.
➤ How well two materials will react depends on an atom’s
oxidation number and ideal oxidation number (ideal
oxidation state).
4. CHLORAMINES
➤ One somewhat common pool problem experienced by pool owners is chlorine demand, the inability to hold free
chlorine.
➤ As mentioned in the Troubleshooting course, virtually everything can cause chlorine demand.
➤ At the molecular level, anything that can contribute nitrogen to the pool can contribute to a chlorine
demand.
➤ Nitrogen compounds will interact with hypochlorous acid to produce chloramines.
➤ Example: ammonia (NH3) and hypochlorous acid (HOCl)
HOCL2NH3 + 2NH2CL + 2H2O
➤ In this example, the end result NH2Cl is a monochloramine, so named because the molecule
contains one chlorine atom.
➤ In addition to monochloramines, hypochlorous acid will interact with monochloramines to form
dichloramines (NHCl2), which will in turn interact with hypochlorous acid at low pH levels (<4.5)
to form trichloramines (NCl3). Within the context of pools, trichloramines are unlikely to occur.
➤ Overcoming this reaction is the goal of breakpoint oxidation, or breakpoint chlorination.
5. THE INFAMOUS “CHLORINE ODOR”
➤ While chlorine products do have an undeniable chlorine odor to them, properly
sanitized pools do not generally have a strong, pervasive chlorine odor.
➤ Chloramines are responsible for the strong “chlorine odor” that some associate
with pools.
➤ Trichloramines have the worst odor, while monochloramines have the least-
worst odor.
➤ Chloramines can also be responsible for eye irritation, but are not the sole cause.
➤ Regular oxidation limits the amount of nitrogen compounds that chlorine is
expected to oxidize, preventing the need for the very large chemical doses needed
to achieve breakpoint within a chlorine demand.
➤ This is why shocking should be treated as a preventative measure: regular
shocking will inhibit the gradual buildup of nitrogen compounds, successfully
breaking them down and producing nitrogen gas without the need to add
excessive quantities of chlorine that can cause system degradation over time.
6. BREAKPOINT OXIDATION
➤ Reaching breakpoint oxidation requires additions of chlorine beyond what is normally required
for sanitation in order to break down dichloramines and produce nitrogen gas. In daily
problem-solving, this ratio is generally 10-to-1 in favor of hypochlorous acid.
➤ For testing purposes, meeting or exceeding breakpoint oxidation during treatment of a chlorine
demand requires frequent testing of both free and combined (or total) chlorine.
➤ Combined chlorine is a measure of chloramines in the water; when achieving breakpoint
oxidation, combined chlorine should be brought as close to 0 as possible.
➤ For routine oxidation, frequent testing to ensure breakpoint has been met is not necessary.
➤ Once breakpoint has been achieved, nitrogen gas and other byproduct gases are produced;
significant ventilation is required at this point. Pools should remain uncovered throughout the
process.
➤ For routine oxidation, 8 hours is the recommended minimum gas-off time.
➤ Reaction times for contaminants will differ; more complex molecules such as creatinine and
certain medications will take longer to oxidize than simpler compounds like ammonia from
fertilizer.
➤ Remember: the key to chloramine formation is nitrogen!
