Hypochlorous acid vs bleach
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Hypochlorous acid (HOCl) and bleach (sodium hypochlorite, NaOCl) carry the same active chlorine, and pH decides which form dominates1,2. Diluted bleach is alkaline, so most of its chlorine is hypochlorite ion, the better cleaner; HOCl dominates below about pH 7.5 and is the stronger germ killer, but it breaks down faster in light, air and heat1,2,3.
What is the difference between hypochlorous acid and sodium hypochlorite?
Hypochlorous acid is an oxyacid of chlorine: a weak, unstable acid that exists only in solution1. Sodium hypochlorite, the active ingredient in household chlorine bleach, is a salt of hypochlorous acid, as is calcium hypochlorite1,4. When either salt or chlorine gas is added to water, the result is a mixture of hypochlorous acid, hypochlorite ion and chlorine, together called free available chlorine1,2.
The balance between these forms is set by pH. Hypochlorous acid is a weak acid that splits into hypochlorite ion (OCl-) and a proton depending on the solution pH2. Sodium hypochlorite diluted to the common working range of 50 to 200 mg of available chlorine per liter has a pH of 8.5 to 10, where most of the chlorine is hypochlorite ion2. Hypochlorous acid is the dominant form from pH 2 to 7.5, and at pH 6.5 about 95% of the chlorine is hypochlorous acid1. So a hypochlorous acid product and a bleach dilution differ mainly in pH and concentration, not in the chlorine they contain.
| Solution pH | Main chlorine form | What it means in practice |
|---|---|---|
| Below 4 | Chlorine gas (Cl2) | Chlorine escapes from the solution, efficacy drops and the acidity corrodes equipment1,2 |
| 4 to 6 | Hypochlorous acid | Highest germicidal activity2 |
| 6 to 7 | Mostly hypochlorous acid (95% at pH 6.5) | The range typically used to limit metal corrosion while keeping good efficacy1,2 |
| 8.5 to 10 | Mostly hypochlorite ion | Household bleach diluted to 50 to 200 mg per liter2 |
| Above 10 | Hypochlorite ion | Very stable: available chlorine stays almost constant over 6 months of storage2 |
Which kills germs better, hypochlorous acid or bleach?
Hypochlorous acid is generally regarded as the active germ-killing species in chlorine solutions, while the hypochlorite ion is the key to cleaning power2. Measured as concentration multiplied by contact time, hypochlorous acid has been reported to be 80 times more effective than hypochlorite ion against Escherichia coli bacteria, and about 100 to 1,000 times more effective against Bacillus spores2. Its electrical neutrality and small, water-like size let it penetrate microbial cells and attack them from the inside, while the charged hypochlorite ion acts mainly from the outside2.
This is why the same chlorine kills faster when the pH drops. Electrolyzed water shows its maximum bactericidal activity between pH 4 and 6, where the hypochlorous acid concentration is highest2. In practice, weakly acidic hypochlorite solutions are commonly used at 50 to 200 mg per liter with 1 to 2 minutes of contact to disinfect produce and processing surfaces2.
Bleach is still a strong disinfectant at the right dilution. A Washington State Department of Health chart mixes bleach to about 1,000 ppm for disinfecting hard surfaces and about 100 ppm for sanitizing food-contact surfaces, with a contact time of at least 2 minutes5.
Which is better on dirty surfaces?
Bleach. The cleaning action of sodium hypochlorite comes from the oxidizing power of the hypochlorite ion combined with hydroxide ions, which dissolve proteins, polysaccharides and fats2. Manufacturers often add sodium hydroxide to sodium hypochlorite to strengthen its cleaning power, stabilize the chlorine and reduce corrosion of metal2. Weakly acidic electrolyzed water and hypochlorite solutions, by contrast, have a poor ability to remove microbial cells and organic soils2.
Both lose power in dirt. Organic matter such as debris, soil and microorganisms reduces sanitizer efficacy, so working solutions need to be monitored and refreshed1. In a hypochlorous acid solution, excess proteins and carbohydrates rapidly consume the hypochlorous acid and significantly decrease its microbicidal activity3. Bacteria in biofilms are 150 to 3,000 times more resistant to hypochlorous acid at pH 7 than free-floating cells, and alternating an alkaline hypochlorite solution with a weakly acidic one is expected to work against biofilm2.
The practical rule is the same for both: clean with soap and water and rinse before you disinfect or sanitize5.
Is hypochlorous acid gentler than bleach on skin and equipment?
At neutral pH, yes. With neutral electrolyzed water, corrosion of processing equipment and hand irritation are lesser concerns, stainless steel is very resistant, and chlorine loss is significantly reduced at pH 6 to 81. Hypochlorous acid is also the oxidant that the body's own neutrophils produce to fight infection1.
That does not make either product benign. Chlorine disinfectants have been shown to cause occupational dermatitis and skin irritation1. Very acidic chlorine solutions below pH 4 release chlorine gas, creating health hazards, and can corrode equipment and instrument surfaces1. Accidental swallowing of household bleach has caused burning of the mouth and throat, stomach irritation and vomiting1. When diluting bleach, wear gloves and eye protection, mix it into cool water in a well-ventilated area, and never mix bleach with other chemicals5.
| Feature | Hypochlorous acid solution | Household bleach |
|---|---|---|
| Main chlorine form | Hypochlorous acid, dominant from pH 2 to 7.51 | Hypochlorite ion at pH 8.5 to 10 when diluted2 |
| Strength as sold or used | Not over 200 ppm available chlorine as a food-contact sanitizer6 | 2.75% to 8.25% sodium hypochlorite as sold; read the label5 |
| Germ killing per unit of chlorine | Stronger2 | Weaker2 |
| Removing organic soil | Poor2 | Good2 |
| Storage stability | Breaks down in UV light, sunshine, air contact and heat of 25 degrees C or more3 | Stable above pH 102; working dilutions are mixed fresh each day5 |
| Corrosion and irritation | Lesser concern at neutral pH1 | Gloves and eye protection when mixing5 |
Can you make hypochlorous acid from bleach at home?
Industrially, yes: a weakly acidic hypochlorite solution made by adding hydrochloric acid to dilute sodium hypochlorite is chemically identical to weakly acidic electrolyzed water2. Electrolyzed water itself is made by passing a current through a dilute salt solution, which forms hypochlorous acid at the anode1.
At home, no. When chlorine bleach is mixed with an acid, chlorine gas is given off, and common acids include vinegar, some glass cleaners, toilet bowl cleaners, drain cleaners and rust removers4. Chlorine gas, even at low levels for short periods, almost always irritates the eyes, throat and nose, and higher levels can cause pneumonia and fluid in the lungs4. Overshooting the pH is easy, and below pH 4 hypochlorous acid converts to chlorine gas that escapes from the solution2. Mixing bleach with ammonia is just as hazardous, because it produces toxic chloramine gases4.
Commercial routes are regulated. The US EPA registers sanitizers and disinfectants as pesticides, while on-site electrolyzed water generators that use only salt and water are not registered as pesticides but must comply with 40 CFR 152.5001. Whichever product you use, follow the product label.
How long do hypochlorous acid and bleach solutions last?
Hypochlorous acid is the more fragile of the two. A 200 ppm, pH 6 solution was unstable against ultraviolet light, sunshine, contact with air and temperatures of 25 degrees C (77 degrees F) or higher3. Inorganic ions such as nitrite, sulfite, iron and copper also consume it quickly3. Stable hypochlorous acid needs very pure water and storage in dark, cool conditions below 10 degrees C (50 degrees F)3. Even without contaminants, undissociated hypochlorous acid in the neutral pH range of 6.5 to 7.5 decomposes at a relatively high rate during storage2.
Concentrated bleach holds up better because its chlorine sits as hypochlorite ion in alkaline water: above pH 10 the available chlorine stays almost constant after 6 months2. Diluted bleach is different. Health guidance calls for mixing a new bleach solution each day, labeling it with the amount of bleach and the date, and checking strength with chlorine test strips5. Bleach is sold at several strengths, so the percentage of sodium hypochlorite on the label decides how much goes into each gallon5.
Key takeaways
- Hypochlorous acid and bleach contain the same active chlorine; pH decides whether it sits as hypochlorous acid or as hypochlorite ion.
- Hypochlorous acid kills microbes faster per unit of chlorine, while alkaline bleach cleans organic soil better.
- Both lose power in dirt, so clean and rinse before you disinfect.
- Never acidify bleach at home: acids such as vinegar release chlorine gas, and ammonia releases chloramines.
- Hypochlorous acid breaks down in light, air and heat, and diluted bleach is mixed fresh daily.
References
- Hypochlorous Acid, Handling: Technical Evaluation Report
- Mechanisms of Actions of Sodium Hypochlorite in Cleaning and Disinfection Processes
- Stability of Weakly Acidic Hypochlorous Acid Solution with Microbicidal Activity
- Dangers of Mixing Bleach with Cleaners
- Disinfecting and Sanitizing with Bleach: How to Mix Bleach for Child Cares (DOH Publication 970-216)
- 40 CFR 180.940 Tolerance exemptions for active and inert ingredients for use in antimicrobial formulations (Food-contact surface sanitizing solutions)
- Application of slightly acidic electrolyzed water for inactivating microbes in a layer breeding house
Frequently asked questions
Can hypochlorous acid be used in a poultry house?
Yes, research has tested it there. In a layer breeding house, slightly acidic electrolyzed water (pH 5.0 to 6.5) at 250 mg per liter of available chlorine inactivated the bacteria and fungi in dust, feces, feathers and feed more efficiently than benzalkonium chloride or povidone-iodine at 1:1,000, and it significantly reduced microbes on floor, wall, feed trough and water pipe surfaces7. Spraying surfaces with tap water first played an important role in the result7.
Is hypochlorous acid allowed on food-contact surfaces?
Yes, within limits. US rules exempt hypochlorous acid from a residue tolerance in food-contact surface sanitizing solutions when the ready-to-use concentration does not exceed 200 ppm total available chlorine and the surface drains before contact with food6. FDA regulations also permit sodium hypochlorite sanitizing solutions on food processing equipment and food-contact surfaces1.