Disinfecting Solution


WASHINGTON, DC – February 17, 2006 -- Adding white vinegar to diluted household bleach greatly increases the disinfecting power of the solution, making it strong enough to kill even bacterial spores. Researchers from MicroChem Lab, Inc. in Euless, Texas, report their findings today at the 2006 ASM Biodefense Research Meeting.

The concentrate is about 5.25 to 6 percent NaOCl, and the pH value is about 12. Sodium hypochlorite is stable for many months at this high alkaline pH value. “Laundry bleach is commonly diluted about 10 to 25-fold with tap water to about 2000 to 5000 parts per million of free available chlorine for use as an environmental surface disinfectant, without regard to the pH value of the diluted bleach. However, the pH value is very important for the antimicrobial effectiveness of bleach,” says Norman Miner, a researcher on the study.

At alkaline pH values of about 8.5 or higher, more than 90 percent of the bleach is in the form of the chlorite ion (OCl-), which is relatively ineffective antimicrobially. At acidic pH values of about 6.8 or lower, more than 80 percent of the bleach is in the form of hypochlorite (HOCl). HOCl is about 80 to 200 times more antimicrobial than OCl-. “Bleach is a much more effective antimicrobial chemical at an acidic pH value than at the alkaline Ph value at which bleach is manufactured and stored. A small amount of household vinegar is sufficient to lower the pH of bleach to an acidic range,” says Miner. Miner and his colleagues compared the ability of alkaline (pH 11) and acidified (pH 6) bleach dilutions to disinfect surfaces contaminated with dried bacterial spores, considered the most resistant to disinfectants of all microbes. The alkaline dilution was practically ineffective, killing all of the spores on only 2.5 percent of the surfaces after 20 minutes. During the same time period the acidified solution killed all of the spores on all of the surfaces. “Diluted bleach at an alkaline pH is a relatively poor disinfectant, but acidified diluted bleach will virtually kill anything in 10 to 20 minutes,” says Miner.”

February 15-18, 2006
Washington, DC

HOUSEHOLD BLEACH as a DISINFECTANT In the past, I’ve advised caution when using house hold bleach as a disinfectant for many reasons.

  • The simple 1:10 dilution has demonstrated a poor ability to kill bacteria spores.
  • Household bleach “out of the jug” is typically 5.25% - 6% and, being ~12.5 Ph means that you need eye protection and an eye wash when using/mixing “straight” sodium hypochlorite (NaOCl).
  • A typical dilution solution needs to be made fresh daily.
  • It has an ability to stain and eat through clothing and fabrics.
  • Its inability to handle high organic debris loading and still retain its disinfecting properties.
These reasons make it less than ideal for the many of the frequent needs within fire and emergency services. A 1:10 dilution of NaOCl with tap water is still alkaline (Ph ~10-11), but no longer requires an eyewash and eye protection*.

Now with the new information regarding acidified bleach, we might very well revisit using household bleach as an option in some applications. Lowering the Ph of bleach makes it much more effective at killing bacteria spores; however, its other weaknesses need evaluation when acidified.

*1:10 dilution’s Ph is <11 meaning that the 1:10 dilution technically doesn’t require eye protection or an eye wash; nonetheless, I would recommend eye protection.