What Is Sodium Hypochlorite (Liquid Chlorine)?
Quick answer: Sodium hypochlorite is the active ingredient in both liquid pool chlorine and household bleach. Pool-grade liquid chlorine is typically 10-12.5% sodium hypochlorite solution. It’s the TroubleFreePool-preferred sanitizer because it adds no Cyanuric Acid and no Calcium Hardness, giving you full independent control over both parameters. See our pool chemistry guide for how it fits into the full water balance system.
If you have been using trichlor tablets for years and wondering why TFP recommends switching, this guide explains the chemistry and the practical trade-offs.
How sodium hypochlorite sanitizes
Sodium hypochlorite dissolves in pool water and dissociates to form hypochlorous acid (HOCl). HOCl is the actual killing agent: it penetrates cell membranes of bacteria and algae and oxidizes them on contact. The CDC pool chlorine guidance{:target=“_blank”} identifies HOCl as the primary disinfection compound in chlorinated pools.
The byproduct of sodium hypochlorite’s reaction in water is sodium chloride, ordinary table salt. This is why liquid chlorine is the clean option: you get chlorine plus salt, nothing else.
One practical note on pH: liquid chlorine has a pH of approximately 12-13, so it slightly raises pool water pH when added. In a liquid-chlorine-managed pool, minor pH rise is an expected, routine adjustment that muriatic acid corrects. This is different from trichlor tablets, which are strongly acidic and push pH down over time.
Dosage and strength
Sodium hypochlorite (liquid chlorine) at 10% strength raises Free Chlorine by approximately 10 ppm per gallon added to 10,000 gallons of pool water. It doesn’t add Cyanuric Acid or Calcium Hardness, the two reasons TroubleFreePool recommends it over stabilized chlorine products.
The math for common strengths:
Small detail, real impact.
| Strength | FC increase per gallon per 10,000 gal |
|---|---|
| 10% (standard pool chlorine) | ~10 ppm |
| 12.5% (commercial / “shock strength”) | ~12.5 ppm |
| 5-8.25% (household bleach) | ~5-8 ppm |
If you have a low chlorine in pool situation, knowing your pool volume and current FC deficit makes dosing straightforward with these numbers.
Shelf life is the main handling limitation: liquid chlorine degrades at roughly 0.5-1% per day in summer heat. Buy fresh stock (manufactured within 60 days), store in cool shade, and use within 3-6 months. Old product smells sharper and tests lower than the label strength.
Household bleach at 5-8.25% works chemically identical to pool chlorine. You need about twice the volume to match a given dose. The cost-per-ppm FC is similar at smaller pools. At larger pools, buying pool-grade liquid chlorine in 2.5-gallon cases is more economical.
Why TFP prefers liquid chlorine
Every chlorine source adds something besides chlorine. Liquid chlorine adds the least:
| Chlorine Source | Adds CYA? | Adds Calcium? | Notes |
|---|---|---|---|
| Liquid chlorine (sodium hypochlorite) | No | No | Byproduct is salt only |
| Trichlor tablets | Yes (~6 ppm CYA per 10 ppm FC) | No | Constant CYA accumulation |
| Dichlor powder | Yes (~9 ppm CYA per 10 ppm FC) | No | Use sparingly; CYA adds fast |
| Cal-Hypo shock | No | Yes | Raises calcium hardness each use |
With trichlor tablets as a primary sanitizer, CYA rises continuously. Once CYA exceeds 80-100 ppm, chlorine’s effectiveness drops sharply. The only fix is a partial drain and refill, which is expensive and wasteful. Using liquid chlorine with a separate pool stabilizer (cyanuric acid protector) lets you set CYA once and hold it there.
The TFP community also notes cost: liquid chlorine in 2.5-gallon cases from major retailers typically costs less per effective ppm of FC than any stabilized alternative once you account for the sunk cost of excess CYA.
Safety and handling
Sodium hypochlorite is corrosive at pool concentrations. Follow these rules:
- Store upright in a cool, shaded location. Heat and direct sunlight accelerate degradation and can cause containers to swell.
- Never mix with muriatic acid in any container. Adding acid to sodium hypochlorite generates chlorine gas, which is toxic. If you need to add both to a pool on the same day, add one, let it circulate for at least 30 minutes, then add the other.
- Skin and eye contact: Rinse immediately with large amounts of water for 15 minutes. Seek medical attention if irritation persists or vision is affected.
- Add at the return jets, not the skimmer. Pouring directly into the skimmer concentrates undiluted chlorine briefly in the pump and filter. The broadcast method works well: pour slowly across the pool surface while the pump runs.
The EPA sodium hypochlorite safety sheet{:target=“_blank”} provides full handling guidance for concentrated hypochlorite solutions. Pool-grade liquid chlorine at 10-12.5% is more concentrated than household bleach and warrants full PPE: gloves and eye protection when handling.
Per CPO (Certified Pool Operator) standards, always add chemicals to water, never water to chemicals. We recommend pre-diluting each dose in a bucket of pool water before broadcasting. We find this single habit eliminates most surface bleaching incidents. This applies when pre-diluting liquid chlorine in a bucket: fill the bucket with pool water first, then pour in the chlorine.
For hot tub chemicals and hot tub sanitizer options, note that sodium hypochlorite is less commonly used in spas because the smaller volume makes precise dosing more critical and the higher water temperature accelerates degradation.
When NOT to use liquid chlorine
Liquid chlorine isn’t always the right tool:
- Vacation rentals where nobody adds chlorine regularly. Trichlor tablets in a floating dispenser or inline feeder provide slower, more consistent chlorine release without daily attention. Liquid chlorine requires someone to add it every 1-3 days.
- Very small pools under 5,000 gallons. Dosing precision becomes tricky; a half-gallon addition can swing FC by 10+ ppm. Tablets are easier to dial in.
- When CYA is below 30 ppm and you want to raise it. Dichlor or trichlor naturally add CYA alongside chlorine. If you need to bring CYA up, adding a small amount of stabilized chlorine is efficient. Once CYA is at target, switch to liquid chlorine.
“Liquid chlorine adds no Cyanuric Acid or Calcium Hardness, unlike Tri-chlor tablets and Cal-Hypo shock. For pools where CYA is already at target, it’s the cleanest, most controllable chlorine source available.”
FAQ
Can I use household bleach in my pool?
Household unscented bleach sans thickeners is akin to pool chlorine in its active ingredient, sodium hypochlorite. Pool chlorine usually hovers around 10-12.5% concentration, whereas household bleach ranges between 5-8.25%. Doubling the volume of bleach will approximate a similar effect to that of pool chlorine. Before dosing, ensure you check the sodium hypochlorite percentage on the label; steer clear of scented or “splash-less” versions as they contain additives that could throw off water chemistry.
Does liquid chlorine raise pH?
Yes, slightly. Sodium hypochlorite has a pH of approximately 12-13, so each addition pushes pool pH up a small amount. In a liquid-chlorine-managed pool, pH drift upward is expected. Most liquid chlorine users add muriatic acid periodically to bring pH back down into the 7.2-7.6 range. The rise is gradual and manageable; it’s not a reason to avoid liquid chlorine.
How often do I add liquid chlorine?
Liquid chlorine for outdoor pools often needs topping up every 1 to 3 days during peak season, contingent on usage and factors like sun exposure and CYA level. In a pool with 50 ppm CYA under moderate use, free chlorine (FC) might drop by about 2 to 3 ppm daily due to sunlight and bather load together. With a 10% liquid chlorine solution, this translates roughly to 0.2 to 0.3 gallons per 10,000 gallons of water approximately every other day. Always test FC levels before adding; trust your reading over a fixed schedule.