Pool Chemistry 101: pH, Chlorine, Alkalinity, and Shock in Plain English
Pool Chemistry 101
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You don’t need a chemistry degree to keep a pool clear. You need to watch a handful of numbers and know which way to push each one. Start with the cheat sheet below, then use the guides when something goes wrong.
The numbers that matter
| Test | What it does | Target |
|---|---|---|
| Free chlorine (FC) | Kills germs and algae | At least 1 ppm; at least 2 ppm with CYA (CDC) |
| pH | Controls how well chlorine works and how comfortable the water feels | 7.0–7.8 (CDC) |
| Total alkalinity (TA) | Buffers pH so it doesn’t swing | 80–120 ppm ideal (Taylor) |
| Cyanuric acid (CYA) | Shields chlorine from the sun, but the more you have, the more chlorine you need | Follow your test kit’s guidance |
| Calcium hardness | Protects plaster and equipment | Follow your pool surface maker’s guidance |
| Combined chlorine (CC) | Used-up chlorine; tells you when to shock | As low as possible |
The guides
Fix a problem
- How to Lower Pool pH How much muriatic or dry acid to add, cross-checked against several published charts.
- How to Shock a Pool When to shock, which product to use, how much, and when it’s safe to swim.
- Why Is My Pool Green? The five usual causes, how to tell algae from metal, and the step-by-step fix.
Test it right
- Best Pool Test Kit Why a FAS-DPD drop kit beats strips for chlorine, and which kit to buy.
Make it a habit
- Pool Maintenance Schedule The 10-minute weekly routine, plus a free printable checklist.
Three rules that prevent most problems
- Keep chlorine matched to your CYA. Most green pools come down to too little chlorine for too long (ClearPoolCalc).
- Keep pH in range. At pH 7.5, only about half of your chlorine is in its active form (PHTA).
- Test with a kit you trust. DPD color tests can bleach out and read falsely low above about 10 ppm (CDC).
Safety
- Wear gloves and eye protection.
- Add chemicals to water, never water to chemicals.
- Never mix pool chemicals together.
- Follow every product label.
- Poison Control: 1-800-222-1222.
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