Chemistry
Same chlorine family, different pH
A side-by-side look at the usual pH of household bleach and of HOCl-type solutions, and what that does to the HOCl / OCl⁻ split.
“Available chlorine” (often reported as free available chlorine, FAC, in ppm) is the number on a test strip or a lab report. It counts HOCl and OCl⁻ together. For what the units mean and how FAC differs from total chlorine, see ppm, FAC and TAC on disinfectant.org. For checking your own batch, hypochlorous.com has PPM and pH basics.
The strengths people actually use
| HOCl | Bleach | |
|---|---|---|
| Strength in the cited studies and protocols | Mostly tens to a few hundred ppm (for example 20–22, 35, 40–120 and 250 ppm in the studies on Sources). | CDC-cited protocols run from about 100 to 5,000 ppm. |
| A household example | — | CDC’s recipe for bottles without directions (5 tablespoons, or 1/3 cup, per gallon) made with a bleach labeled 7.1% available chlorine works out to about 1,360–1,450 ppm. |
| What the ppm reading does not show | How much of it is HOCl versus OCl⁻; that depends on pH. | The same. Alkaline bleach (above pH 8 for CDC's tap-water dilutions; 9.3 to above 11 in the cited studies) is mostly OCl⁻. |
The gap is wide. A typical bleach dilution can carry several times, sometimes tens of times, the available chlorine of a typical HOCl-type solution.
Equal-ppm tests are a different question
Several studies set HOCl-type solutions and bleach at the same available chlorine on purpose: 0.5 mg/L (Naka 2020), 35 mg/L (Quan 2010), 60 ppm (Takimoto 2013) and 40–120 mg/L (Yang 2021). Holding ppm equal isolates the effect of pH. It does not describe how either solution is used day to day, because the CDC-cited bleach protocols start at about 100 ppm, and the HOCl-type solutions in the studies cited here stay at a few hundred ppm or less.
Takimoto 2013 shows why the distinction matters. At 60 ppm the acidified solution and bleach behaved differently, but when bleach was raised to 150–200 ppm and above, the comparison changed (Takimoto 2013). The results of these tests, framed as a rated comparison, are on disinfectant.org’s rated compare table. Note that Naka 2020 has a co-author employed by Toyotomi Co., and Takimoto 2013’s acidified solution came from a commercial device.
Reading a number on either label
A ppm figure on its own tells you the amount of free chlorine, not its form. Len 2000 notes that standard titration “cannot differentiate the levels of HOCl.” Pair any ppm figure with a pH figure, and see Same chlorine family, different pH for why. Strength also drops in storage at different rates for each; see Shelf life.
Sources: CDC 2008; CDC household bleach page (checked Nov 11, 2026); Len 2000; Naka 2020; Quan 2010; Hitomi 1998; Takimoto 2013; Yang 2021; Moorman 2017. Full entries on Sources.