you are absolutely right from a purely net stoichiometry point of view. However, as recognized by several chemical companies, the pH of H2O2 (Hydrogen-Peroxid)
solutions is certainly not 7.0. For a 30% solution ("food grade" H2O2) has a pH of 3.3, or a hydronium ion concentration [H3O+] of 10^-3.3. Here are some of the very basics for everyone to know about hydrogen peroxide:
hydrogen peroxide, chemical compound, H2O2, a colorless, syrupy liquid that is a strong oxidizing agent and, in water solution, a weak acid. It is miscible with cold water and is soluble in alcohol and ether. Although pure hydrogen peroxide is fairly stable, it decomposes into water and oxygen when heated above about 80°C; it also decomposes in the presence of numerous catalysts, e.g., most metals, acids, or oxidizable organic materials. A small amount of stabilizer, usually acetanilide, is often added to it. Hydrogen peroxide has many uses. It is available for household use as a 3% (by weight) water solution; it is used as a mild bleaching agent and medicinally as an antiseptic. The 3% solution is sometimes called ten volume strength, since one volume of it releases ten volumes of oxygen when it decomposes. Hydrogen peroxide is available for commercial use in several concentrations. Highly concentrated solutions were first used in World War II by the military, e.g., in fuels for rockets and torpedoes. It is used as a bleaching agent for textiles, e.g., wool and silk, and in paper manufacture. It is also used in chemical manufacture. Hydrogen peroxide is prepared commercially by oxidation of alkylhydroanthraquinones and by electrolysis of ammonium bisulfate. It can also be prepared by reaction of barium peroxide with sulfuric acid and is prepared (with acetone) by oxidation of isopropanol. Hydrogen peroxide was discovered (1818) by L. J. Thenard.
HYDROGEN PEROXIDE SOLUTION, 30%
Physical and Chemical Properties
Clear, colorless liquid.
Slight acrid odor.
% Volatiles by volume @ 21C (70F):
Vapor Density (Air=1):
Vapor Pressure (mm Hg):
25 @ 30C (86F)
Evaporation Rate (BuAc=1):
- Physical properties
(Molecular formula : H2O2 (Hydrogen-Peroxid)
/ Molecular Weight : 34.822)
Category / Type 35% H2O2 (Hydrogen-Peroxid)
Appearance Colorless, transparent liquid Colorless, transparent liquid
Odor Weak peculiar odor
Active oxygen wt % 16.5
Specific gravity (20, g/ml) 1.133
Apparent pH 2.5
Melting point 108
Freezing point -33
Vapor pressure mmHg 30 - -
Total Pressure(H2O2 + H2O) 23
Partial pressure (H2O2 ) 0.36
Viscosity 20, cP 1.11
15. 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:
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.
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 10
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
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.
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.
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.
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.
22. Is H2O2 really a "natural" substance?
Yes. H2O2 is produced by both animal and plant cells -- being associated with the mitochondrial respiratory chain, as well as various hydroxylation and oxygenation reactions. For example, the popular free-radical dietary supplement, superoxide dismutase (SOD), serves to convert the toxic superoxide radical (O2-) into H2O2, as do the flavin-linked oxidases. A number of other enzymes such as the heme-containing catalase (located in mitochondrial peroxisomes) then decompose the H2O2 to oxygen in order to protect the cells from subsequent damage. You can see these enzymes in action for yourself when you apply H2O2 topically to a cut or wound and witness the intense foaming. Also, the whitening of skin when exposed to more concentrated H2O2 is due to trapped oxygen beneath the epidermis caused as a result of H2O2 permeating through the skin and being decomposed by epidural enzymes. Saliva is another source for H2O2-decomposing enzymes -- hence the foaming when brushing with H2O2-containing tooth pastes. H2O2 is also an active intermediate in the bioluminescence reaction involving the luciferase enzyme (from fireflies).
It is interesting to note the wide variety of substances which catalyze the breakdown of hydrogen peroxide into water and oxygen gas, H2O2 ---> 1/2 O2(g) + H2O
Not only by various metal salts but also the catalase enzyme which is widespread throughout the living kingdom. There is some logic to this in that hydrogen peroxides strong oxidizing nature tends to make it a toxic substance. Interestingly the action of hydrogen peroxide on open cuts is due to the catalase in our blood, not due to bacteria. The hydrogen peroxide acts as a disinfectant because bacteria lack the enzyme to render it harmless. thus the bacteria are killed, while our catalase protects us. Not every bacteria is killed in this fashion. Bacteria can be categorized as "catalase positive" or "catalase negative" using a procedure much like this lab. The body can also produce its own hydrogen peroxide which can be used to attach foreign cells at the molecular level. The hydrogen peroxide is particularly effective in reacting with the biomolecules with carbon carbon double bonds.
I think it is important to note that essencially all the decomposition reactions of H2O2 that take place in the body are catalyzed reactions. Here is an example of a chemically catalyzed reaction of H2O2:
Br2 + H2O2 → 2 Br - + 2 H + + O2
2 Br - + H2O2 + 2 H + → Br2 + 2 H2O
net reaction is still:
2 H2O2 → 2 H2O + O2
The significance is that catalized reactions are done in steps. Durring part of the reaction there are 2 H+ ions, and an O2 molecule.
So my point is that I agree the overall decomposition reaction of H2O2 is:
2 H2O2 → 2 H2O + O2
But for the in solution catalyzed (making up words now I know) decomposition reactions that take place in the body there are free H+ ions created. There can also be OH- (hydroxide ions) as well as OH+ (hydroxyl radical) produced in different decomposition reactions. The hydroxyl radical is extremely reactive and is capable of degrading most organic materials.
I think You are right in a since, and that Dr. Young is also correct. H2O2 is acidic, and it can produce more than just oxygen and water. I hope this was not inconveniently long (though I'm sure it was) and I hope it cleared up the issue. Take care, Mike