Is Hydrogen Peroxide an Acid or a Base?

Darron Birgenheier/CC-BY-2.0

Hydrogen peroxide is slightly acidic pH of 6.2 and is, therefore, similar to milk or rain water. That is the pH of hydrogen peroxide at full concentration; all hydrogen peroxide solutions sold commercially are diluted and are more acidic because they actually have a lower pH.

The hydrogen peroxide used to bleach hair is a solution containing between 6 and 10 percent hydrogen peroxide. At that level of concentration, the solution has a pH of around 5.3, making it as acidic as black coffee. The most acidic hydrogen peroxide solutions are those whose concentrations are between 50 and 70 percent. These have a pH of 4.5. The process for commercially manufacturing hydrogen peroxide solutions also leaves behind acidic byproducts which can lower the pH by two to four points; this makes these solutions significantly more acidic. The extent of this decrease in pH depends on the subsequent method of purification. The stabilizers added to commercial solutions also tend to be acids, though some are bases or neutral. Sometimes phosphoric or nitric acids are added as additional stabilizers because hydrogen peroxide decomposes less readily at lower pH.