What is the pH of H2O2 solutions?

It depends upon a number of factors discussed below. But first, a few points on what pH means: pH is a logarithmic measure — pH = log10 1/[H+]. Hence, if you mix equal parts of a pH 2 solution and pH 4 solution, you will not necessarily end up with a pH 3 solution –

Reason No. 1: According to the definition of pH, the pH 2 solution has 100 times the concentration of hydrogen ion [H+], not twice the concentration.

Reason No. 2: Both the pH 2 solution and pH 4 solution may contain “buffering agents” which dampen shifts in pH despite the addition of acids or bases.

Hence, to anticipate the resulting pH, it is useful to measure the relative buffering capacities of the two solutions in addition to their pH. The measures used to define buffering capacity (acidity and alkalinity) are derived from the amount of acid or base needed to bring the solutions to neutrality (pH 7).

With this background, the following factors influence the pH of commercial solutions of H2O2:

1. Industrial strength solutions of H₂O₂ (30-70%) depress the pH readings obtained when using a combination glass electrode. The difference between this “apparent pH” and “real pH” varies from about 1.3 pH units for 35% H₂O₂ to about 2.7 pH units for 70% H₂O₂.
2. Correcting for apparent pH deviations, solutions of pure H2O2 and water exhibit a pH which varies with concentration of H2O2 as follows:
   

   % H2O2 Conc.	0	10	20	30	40	50	60	70	80	90	100
   pH @ 25-deg C	7.0	5.3	4.9	4.7	4.6	4.5	4.5	4.5	4.6	4.9	6.2

3. Virtually all commercial production of H2O2 now utilizes a process based on the auto-oxidation of anthraquinones. The degradation by-products of this reaction are typically acidic and, depending on subsequent purification steps, will typically result in a more acidic product than suggested in the above table (by 2-4 pH units), and add significantly to the acidity of the product.
4. Because H2O2 solutions are generally more stable at low pH, some producers may add mineral acids (e.g., phosphoric or nitric acids) to further lower the pH – either in the production process or afterwards.
5. The water used to prepare commercial solutions of H2O2 is generally of very high quality (i.e., deionized, with low acidity), and so does not significantly affect the (real) pH of the product.
6. Most commercial solutions of H2O2 contain stabilizers (chelating and sequestering agents) which have been added to minimize decomposition of the product through transport and storage. While some stabilizers (such as stannate) are alkaline, most (such as phosphonic acids) are acidic and exhibit buffering properties which add acidity to the product. The amount and type of stabilizers varies between producers, product grades, and H2O2 concentration. Electronic and Reagent grades are more pure (less stabilizers, less acidity) while Dilution and Cosmetic grades have among the highest levels of stabilizers – more on this below.

Consequently, it is not possible to state with any certainty the pH of commercial H2O2 solutions. However, it is likely that the apparent pH will be pH 4-5 for the more dilute products (3-10% H2O2) and pH 1-4 for the more concentrated products (35-70%). In terms of buffering capacities, one would expect to find an inverse correlation with product purity. Thus, a general ranking of H2O2 grades might be as follows:

Buffering Capacity