Free Chlorine
The active, unsaturated chlorine available to sanitize pool water.
Definition
Free chlorine (FC) is the portion of chlorine in pool water that has not yet reacted with contaminants. It exists primarily as hypochlorous acid (HOCl) and hypochlorite ion (OCl−). HOCl is the active sanitizing form and is 40–80× more effective than OCl−. The ratio of HOCl to OCl− is determined by pH — lower pH means more HOCl. Free chlorine must be maintained above the minimum level dictated by the cyanuric acid concentration.
Quick Facts
- Ideal Range
- 1–3 ppm
- Units
- ppm
- Also Known As
- FC, free chlorine, available chlorine, FAC, residual chlorine, pool chlorine
Relationships
Outgoing Relationships
- affected by → pH
- affected by → Cyanuric Acid
- affected by → Temperature
- measured by → Water Testing
- calculated by → formula-02
- explained by → glossary/free-chlorine
- solves → Low Chlorine
- solves → Algae
- solves → Green Water
- recommended for → Swimming Pool
- recommended for → Hot Tub
- compared with → Bromine
- compared with → Combined Chlorine
- part of → Total Chlorine
- related to → ORP
- uses → Liquid Chlorine
- uses → Calcium Hypochlorite
- uses → Trichlor Tablets
Incoming Relationships
- related to ← pH
- related to ← Cyanuric Acid
- requires ← Breakpoint Chlorination
- requires ← Water Balancing
- uses ← Sanitizing
- references ← PHTA