Chlorine Sources: Chemical Purification Agents
Part of Water Purification
This article catalogs every practical source of chlorine compounds for water disinfection — where to find them, how to assess potency, correct dosing for each form, and long-term storage to maintain a reliable supply.
Why Chlorine is the Priority Chemical
Of all chemical disinfectants, chlorine compounds are the most practical for post-collapse water treatment. Here is why:
- Ubiquitous — chlorine products exist in nearly every home, business, and municipal building
- Effective — kills bacteria, viruses, and most protozoa at low concentrations
- Residual protection — unlike boiling or UV, chlorine continues to protect treated water for hours, preventing recontamination during storage and transport
- Shelf-stable in dry form — calcium hypochlorite powder stores for a decade or more
- Simple dosing — no special equipment needed beyond a dropper or measuring spoon
The Chemical Treatment article covers dosing across multiple disinfectants. This article goes deep on chlorine specifically: every source, every form, every concentration, and the critical details that determine whether your treated water is actually safe.
Chlorine Compound Reference
All chlorine disinfection works through the same chemistry: hypochlorous acid (HOCl) forms when chlorine compounds dissolve in water. This acid oxidizes cell membranes and destroys microbial DNA. The different products below are simply different delivery vehicles for generating HOCl in water.
Source Catalog
| Source | Active Compound | Typical Concentration | Shelf Life | Where to Find |
|---|---|---|---|---|
| Household bleach (liquid) | Sodium hypochlorite (NaOCl) | 5-8% | 6-12 months | Under sinks, laundry rooms, janitor closets |
| Concentrated bleach | Sodium hypochlorite | 8.25% | 6-12 months | Commercial cleaning supplies |
| Pool shock (granular) | Calcium hypochlorite (Ca(OCl)2) | 65-73% | 10+ years (dry) | Pool supply stores, hardware stores, rec centers |
| Dichlor pool granules | Sodium dichloro-s-triazinetrione | 56-62% | 5+ years (dry) | Pool supply, hot tub supply |
| Bleach tablets | Sodium hypochlorite (compressed) | Varies | 3-5 years | Cleaning supply stores, janitorial |
| Water purification tablets | Sodium dichloroisocyanurate (NaDCC) | Varies per tablet | 3-5 years | Outdoor/camping stores, military surplus, pharmacies |
| Bleach powder (tropical regions) | Calcium hypochlorite | 30-35% | 2-5 years | General stores in developing regions |
| Municipal treatment chemicals | Chlorine gas, sodium hypochlorite, Ca(OCl)2 | Industrial strength | Varies | Water treatment plants |
Do NOT Use These
- Scented bleach — contains fragrances and surfactants that are toxic when ingested
- Splash-less or thick bleach — contains thickening agents; lower hypochlorite concentration
- Color-safe bleach — contains hydrogen peroxide, not chlorine; does not disinfect water
- Bleach with added cleaners (Clorox Clean-Up, etc.) — contains surfactants and other chemicals
- Trichlor pool tablets (the large 3-inch pucks) — dissolve too slowly and contain cyanuric acid stabilizer that complicates dosing
Read the label. The ONLY ingredient should be sodium hypochlorite and water, or calcium hypochlorite. Nothing else.
Dosing by Source
Liquid Household Bleach
Standard unscented bleach at 5-8% sodium hypochlorite.
| Water Condition | Drops per Liter | Drops per Gallon | Contact Time |
|---|---|---|---|
| Clear, room temperature | 2 | 8 | 30 minutes |
| Cloudy or cold (< 10°C) | 4 | 16 | 60 minutes |
| Very turbid (pre-filter first!) | 4 | 16 | 60 minutes |
Verification: After the contact time, water should have a faint chlorine smell. If it does not, add 2 more drops per liter and wait 15 minutes. Repeat once. If still no chlorine smell after the second dose, the bleach has degraded — find a fresh supply.
Concentrated Bleach (8.25%)
Newer formulations sold as “concentrated” bleach.
| Water Condition | Drops per Liter | Contact Time |
|---|---|---|
| Clear, room temperature | 2 | 30 minutes |
| Cloudy or cold | 3 | 60 minutes |
The difference from standard bleach is minor. When in doubt, dose the same as standard bleach — a slight excess of chlorine is far less dangerous than underdosing.
Calcium Hypochlorite (Pool Shock)
This is the most important long-term source. A single 1-pound (454g) bag of 68% calcium hypochlorite can treat approximately 10,000 gallons (38,000 liters) of water.
You must make a stock solution first. Never add dry granules directly to drinking water — the concentration would be dangerously uneven.
Stock solution preparation:
- Dissolve 1 level teaspoon (~7g) of granular calcium hypochlorite in 2 liters of clean water
- Stir until fully dissolved; allow any sediment to settle
- This produces a stock solution of approximately 0.35% chlorine (3,500 ppm)
Treating drinking water with the stock solution:
| Water Condition | Stock Solution per Liter | Contact Time |
|---|---|---|
| Clear | 5 ml (1 teaspoon) | 30 minutes |
| Cloudy or cold | 10 ml (2 teaspoons) | 60 minutes |
Stock Solution Shelf Life
The stock solution degrades rapidly — use it within 1-2 weeks. The dry granules, however, last 10+ years in a sealed, cool, dry container. Always make stock solution fresh in small batches.
Water Purification Tablets (NaDCC)
Commercial tablets like Aquatabs, Oasis, or Puritabs contain sodium dichloroisocyanurate with precise dosing per tablet.
- Follow the package instructions — each tablet is formulated for a specific volume (typically 1L or 5L per tablet)
- Standard contact time: 30 minutes
- These are the most foolproof option when available — no measuring, no math
If the packaging is lost, a general rule for NaDCC tablets: one tablet designed for 1 liter provides approximately 3-5 mg/L of free chlorine, which is within the effective disinfection range.
Finding Chlorine in the Field
Priority Scavenge Locations
Ranked by likely yield and chlorine quality:
-
Pool supply stores and sections — Large quantities of calcium hypochlorite in sealed containers. This is your highest-value target. A single store may contain enough to treat water for a community for years.
-
Hardware stores — Pool chemicals section. Also check well treatment supplies (liquid sodium hypochlorite for well shocking).
-
Municipal water treatment plants — Industrial quantities. Requires some knowledge to handle safely, but the supply is enormous. Often sodium hypochlorite in 12.5% solution (industrial strength) or bulk calcium hypochlorite.
-
Janitorial supply closets — In any commercial building, school, hospital, hotel. Standard bleach bottles.
-
Household kitchens and laundry rooms — The most accessible source but smallest quantity. One bottle of bleach treats a few hundred liters at best.
-
Camping and outdoor stores — Water purification tablets, often iodine and chlorine types mixed on the shelves. Check labels.
-
Military surplus and emergency supply caches — Military water purification tablets (NATO standard) and bulk calcium hypochlorite.
-
Hotel and restaurant supply areas — Commercial-grade bleach, sanitizer tablets, and dishwasher chlorine supplies.
Assessing Bleach Potency in the Field
Liquid bleach degrades over time. You cannot always know when a bottle was manufactured.
The smell test: Open the bottle and sniff (at arm’s length — do not inhale directly from the neck). If it smells sharply of chlorine, it has meaningful potency remaining. If it smells faintly or not at all, it has degraded significantly.
The dose-and-check method: Treat a liter of water with the standard dose. Wait 30 minutes. Smell the treated water. If there is a faint chlorine smell, the bleach is working. If no smell, it has lost too much potency — double the dose or find a newer supply.
Approximate degradation timeline (liquid bleach, room temperature):
| Age | Remaining Potency |
|---|---|
| Fresh (< 3 months) | 95-100% |
| 6 months | ~80% |
| 1 year | ~60-70% |
| 2 years | ~40-50% |
| 3+ years | Unreliable |
For degraded bleach, simply increase the dose proportionally. If you estimate the bleach is at 50% potency, double the standard dose.
Long-Term Storage of Chlorine
Calcium Hypochlorite (Dry Granules) — Best Option
- Store in the original sealed container or a tightly sealed glass or plastic container
- Keep cool, dry, and dark — heat and moisture accelerate degradation
- Avoid contact with any organic material — calcium hypochlorite is a strong oxidizer and can ignite organic matter (wood shavings, cloth, oil) on contact
- Never store near acids, ammonia, or petroleum products
- Properly stored, 10+ years of full potency
Fire and Explosion Risk
Calcium hypochlorite is classified as an oxidizer. It can cause spontaneous combustion if it contacts organic materials (oil, grease, cloth, wood) or acids. Store it isolated from all other supplies in a non-combustible container. Never mix different pool chemicals together.
Liquid Bleach — Short-Term Only
- Store in the original opaque plastic bottle
- Keep cool and dark — UV and heat degrade sodium hypochlorite
- Useful for 6-12 months; unreliable beyond that
- Buy the plainest, cheapest bleach — no additives, no scent, no thickeners
- Rotate stock: use the oldest first, replace periodically if pre-collapse
Water Purification Tablets — Medium-Term
- Store in original sealed packaging or a waterproof container
- Shelf life 3-5 years typical
- Once a blister pack is opened, tablets degrade faster — use the strip promptly
Manufacturing Chlorine Post-Collapse
If commercial supplies are exhausted, chlorine can be produced through electrolysis of salt water. This requires:
- Salt (sodium chloride)
- Water
- A DC power source (battery, solar panel, hand-crank generator)
- Two electrodes (carbon rods from batteries, stainless steel, or graphite)
Passing direct current through salt water produces sodium hypochlorite at the anode and hydrogen gas at the cathode. The solution is dilute but functional for water treatment. This is how many developing-world systems operate today.
The process requires careful handling — hydrogen gas is flammable, and the concentration of the produced solution is unpredictable without testing. But it means that as long as you have salt and electricity, you can produce chlorine indefinitely.
A detailed electrolysis procedure is beyond the scope of this article but belongs in Chlorine Production when your community reaches that technological tier.
Key Takeaways
- Calcium hypochlorite (pool shock) is the single most important chlorine source to stockpile — it lasts 10+ years dry and treats 10,000+ gallons per pound
- Liquid bleach works but degrades to uselessness within 2-3 years; use the smell test and dose-and-check method to assess potency
- Never use scented, splash-less, color-safe, or bleach with added cleaners for water treatment
- Always make a stock solution from dry calcium hypochlorite — never add granules directly to drinking water
- After treatment, water must smell faintly of chlorine; no smell means insufficient dose or degraded bleach
- Pool supply stores are the highest-value scavenge target for long-term water security
- Store calcium hypochlorite isolated from organic materials, acids, and fuels — it is a fire/explosion risk
- Chlorine provides residual disinfection in treated water, making it superior to boiling or UV for stored/transported water