BIO201: Chlorine Generation

1. BIO201: CHLORINE GENERATION
Pool & Spa University

2. 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.
➤ Remember that the primary sanitizer/oxidizer for chlorine is hypochlorous acid
(HOCl).

3. CHLORINE GENERATION
➤ Hypochlorous acid can be produced through the electrolysis of salt (sodium chloride
or NaCl). Electrolysis is a technique used to drive chemical reactions through the
application of an electric current.
2H20 + 2NACL(AQ) CL2- (G) + H2(G) +2NAOH
➤ In the above reaction, sodium chloride is dissolved into water. When a current is applied,
the aqueous sodium chloride reacts with water to produce chlorine gas, hydrogen gas, and
sodium hydroxide (NaOH). The chlorine gas will immediately react with water to
produce hypochlorous acid and hydrochloric acid, the latter of which will react with
sodium hydroxide to form salt (NaCl) and water.
CL2- (G) + H2O HOCL(AQ) + HCL(AQ)
HCL + NAOH NACL(AQ) + H2O

4. CHLORINE GENERATION
➤ The final byproducts of chlorine generation:
➤ Hypochlorous acid (the desired byproduct)
➤ Hydrogen gas (will gas off with proper ventilation)
➤ Sodium hydroxide (will raise pH of water)
➤ While not technically a “byproduct,” salt is eventually
“reintroduced” through chemical reactions; it is not consumed
during the process of electrolysis.
➤ While some of the sodium hydroxide will react with
hydrochloric acid to produce salt and water, there will still be
excess sodium hydroxide in the water, causing the pH to drift
up over time.

5. SALT IS CHLORINE!
➤ As shown through the
balanced equations, the
eventual desired byproduct of
electrolysis of salt in pools is
hypochlorous acid, i.e.
chlorine. Therefore, salt pools
are chlorine pools.
➤ What sets saltwater pools
apart is the way they acquire
chlorine: the system produces
its own chlorine with the
electrolytic cell.

6. ELECTROLYTIC CELL
➤ The electrolytic cell (salt cell) is a piece of pool equipment
composed of many plates—usually made of coated titanium—
through which a direct current is passed. As water flows
through the cell, the chemical reactions detailed previously
will occur.
➤ The environment inside of the cell is high-chlorine, high-pH.
Scale formation is very likely to occur inside of the salt cell,
reducing its efficiency. Cleanings should be performed
annually with an approved cell cleaning solution.
➤ Due to the high chlorine content produced inside the cell, the
salt cell should always be the last installed piece of
equipment and several feet away from any installed heater(s).

7. ELECTROLYTIC CELL
➤ Salt cells are generally programmed to operate for a specific
amount of time in a 24-day period, oftentimes expressed as a
percentage. A cell set to 60%, for example, will run for
approximately 14 hours.
➤ Cell operation times will vary based on gallonage, pool usage, and
the amount of stress placed on the swimming pool environment
by microorganisms and contaminants.
➤ Frequent testing and adjustment of pH are necessary to offset the
increase in pH as a result of the electrolytic process inside the cell.
➤ Because the chlorine produced by the salt cell is unstabilized, the
use of a stabilizer in outdoor pools is recommended to prevent the
breakdown of chlorine due to sunlight.

8. CONGRATULATIONS!
You have completed the activity!