The proportion of free chlorine in its active form (hypochlorous acid, HOCl) is determined entirely by pH. This table shows the effective fraction at each pH level.
HOCl Percentage at Different pH (at 77°F / 25°C)
| pH | % as HOCl (Active) | % as OCl- (Inactive) | Effective FC at 2 ppm Total FC |
|---|---|---|---|
| 6.8 | 90% | 10% | 1.80 ppm |
| 7.0 | 83% | 17% | 1.66 ppm |
| 7.2 | 67% | 33% | 1.34 ppm |
| 7.4 | 55% | 45% | 1.10 ppm |
| 7.6 | 40% | 60% | 0.80 ppm |
| 7.8 | 28% | 72% | 0.56 ppm |
| 8.0 | 22% | 78% | 0.44 ppm |
| 8.2 | 15% | 85% | 0.30 ppm |
| 8.4 | 10% | 90% | 0.20 ppm |
Notes
- HOCl is 40–80x more effective than OCl- as a disinfectant.
- Keep pH at 7.2–7.6 to maintain chlorine at maximum effectiveness.
- At pH 8.0, you would need 3x the free chlorine to achieve the same active HOCl level as at pH 7.2.
Sources:
- White, G.C. — Handbook of Chlorination and Alternative Disinfectants
- Taylor Technologies — Pool/Spa Water Chemistry Reference