# Free chlorine

> Free chlorine is the disinfectant (hypochlorous acid and hypochlorite) added to many tap waters; it dissipates from standing or aerated water within about a day.

- Canonical: https://genetixbay.com/knowledge/glossary/free-chlorine.html
- Also written: chlorine (water treatment), residual chlorine, hypochlorite, hypochlorous acid, chlorinated water
- Glossary category: pH, EC and water · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Free chlorine is the chlorine in water present as hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻), the active forms produced when chlorine gas or sodium hypochlorite is added for disinfection. Drinking water usually carries a residual well under the 4 mg/L US limit. The balance between HOCl and OCl⁻ depends on [pH](https://genetixbay.com/knowledge/glossary/ph.md); HOCl, the stronger disinfectant, dominates below about pH 7.5.

## In practice

Free chlorine is volatile. Standing water in an open container loses much of it within 24 hours, faster with aeration or stirring. [Carbon filters](https://genetixbay.com/knowledge/glossary/carbon-filter.md) remove it efficiently, and [RO](https://genetixbay.com/knowledge/glossary/ro-water.md) pre-filters are designed to strip it to protect the membrane. Ascorbic acid also neutralises it.

Growers dechlorinate mainly to protect microbes in [living soil](https://genetixbay.com/knowledge/glossary/living-soil.md), [compost teas](https://genetixbay.com/knowledge/glossary/compost-tea.md) or [beneficial reservoirs](https://genetixbay.com/knowledge/glossary/beneficial-microbe-reservoir.md). Direct harm to cannabis at tap-water levels is not well documented. Some hydro growers deliberately add low doses of hypochlorite to [sterile reservoirs](https://genetixbay.com/knowledge/glossary/sterile-reservoir.md); that is a separate, deliberate strategy that needs commitment.

## Don't confuse with

- **Chloramine**: chlorine bound to ammonia; stable, does not evaporate.
- **Chloride (Cl⁻)**: the plant [micronutrient](https://genetixbay.com/knowledge/glossary/micronutrient.md), harmless at normal levels.
- **Chlorine (nutrient)**: the element as a plant nutrient, a separate entry.

## Related terms

[Chloramine](https://genetixbay.com/knowledge/glossary/chloramine.md), [Tap water](https://genetixbay.com/knowledge/glossary/tap-water.md), [RO water](https://genetixbay.com/knowledge/glossary/ro-water.md)

## Read more

- [Water quality for cannabis: RO vs tap, alkalinity, bicarbonates and chloramine](https://genetixbay.com/knowledge/ph-ec/water-quality.md)
- [Chlorine and chloride (Cl) in cannabis: a micronutrient you never need to add](https://genetixbay.com/knowledge/nutrients/chlorine.md)

## Sources

1. Water Quality Association. 2014. Chloramine fact sheet. https://wqa.org/wp-content/uploads/2022/09/2014_Chloramine.pdf
2. USDA Forest Service. Vitamin C dechlorination. https://www.fs.usda.gov/t-d/pubs/html/05231301/05231301.html