The pH of Hydrogen Peroxide: Acidic or Basic?

Hydrogen peroxide, or H2O2, is a chemical molecule composed of two oxygen atoms and two hydrogen atoms.

In its purest state, it is a pale blue, slightly viscous liquid. It is the most basic peroxide with a single oxygen-oxygen link. Because it decomposes slowly in the presence of light, it is stored in dark bottles.

Is hydrogen peroxide an acid? Yes. The pH of hydrogen peroxide is 6.2, making it acidic. However, the pH value is dependent on the solution’s concentration and temperature. Depending on the concentration of hydrogen peroxide in the solution, the ph ranges between 4.2 and 7 at approximately 25°C.

Commercially produced hydrogen peroxide solutions contain acidic stabilisers, which further drop the pH of the solution to between 2.5 and 3.5.

Why is hydrogen peroxide a weak acid?

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Acids with a low acid dissociation constant are weak (Ka). The Ka value for hydrogen peroxide is approximately 1.55 x 10-12 at 298 Kelvin, showing that it is a weak acid.

In addition, the acid’s strength is determined by its susceptibility to lose protons, or H+ ions.

Pure hydrogen peroxide does not create hydrogen ions and has a pH of 7, which is neutral, while the pH of 0.1 M hydrogen peroxide aqueous solution is 6.4.

Hydrogen peroxide is therefore a weak acid.

The dissociation equation for hydrogen peroxide in aqueous solution is stated as:

H2O2 + H2O <=======> H3O+ + HO2- (Hydroperoxide ion)

HO2- + H2O <=======> H3O+ + (O2) 2- (Peroxide ion) (Peroxide ion)

The double arrow in reaction shows a reversible reaction, suggesting that the ions generated during hydrogen peroxide dissociation tend to rebuild the original molecules, demonstrating that H2O2 is a weak acid.

How can the pH of hydrogen peroxide be calculated?

pH is a quantitative indicator of a solution’s acidity or basicity. On the pH scale, values range from 1 to 14, with a neutral solution having a pH value of 7.

Solutions with a pH below 7 are acidic, whereas solutions with a pH over 7 are alkaline.

The Danish biologist S.P.L. Srensen was the first to introduce the term pH. He described it as the concentration of hydrogen ions in a solution, which runs from 1 to 10-14. It follows from the formula

pH = – log [H+]

In the case of H2O2, the pH of 0.1 M H2O2 solution is calculated as follows:

Calculation of the concentration of hydrogen ions:

[H+] = √Ka X M

As Ka = H2O2 has a value of 1.55 x 10-12

In addition, we are computing the concentration of hydrogen ions in a 0.1 M H2O2 solution.

Consequently, [H+] = 1.55 x 10-12 x 0.1

= 3.93 X 10^-7

The pH of a 0.1 M solution can be determined as follows:

pH = – log [H+]

= – log [3.93 X 10-7]

= – [log 3.93 – 7 log 10]

= 6.41

Since the pH value is below 7, it is evident that H2O2 is acidic in nature.

The constant of acid dissociation (Ka) and the logarithmic constant (pKa):

In the preceding section, the pH of the H2O2 solution was calculated using the Ka value.

You may wonder what this Ka is and how it relates to the acidity of a solution. Continue reading to comprehend this.

The acid dissociation constant (Ka) is utilised to assess the degree of ionisation of an acid. The equilibrium constant is utilised to determine the acidity of a solution.

The acid dissociation constant is expressed in units of mol/L. Ka has a greater value for strong acids compared to weak acids, which have a lower Ka value.

Occasionally, it is also stated as the logarithmic constant pKa. Strong acids have a pKa value below -2, whereas weak acids have a pKa value between -2 and 12.

Calculating Ka and pKa values for acids:

The values of Ka and pKa provide a measure of the acid’s strength.

As previously mentioned, strong acids almost entirely ionise in a solution, but weak acids do not completely dissociate; thus, their Ka values can be estimated using the concentration of their ions in the solution.

The following formula is used to compute it:

Ka = ([A-] [H+]) / [HA] Or Ka = ([A-] [H3O+]) / [HA]

The pH value of a solution can also be used to calculate a solution’s Ka value. It is stated as:

pH = -log [H+] or pH = -log [H30+]

This phrase can also be rewritten as

[H30+] = 10^-pH

If the molar concentration of the acid in a solution is known, it is possible to compute the pH of the solution, which may then be used to calculate the Ka value of the solution.

Additional percentage dissociation may be estimated as follows:

percent Dissociation = ([A- (aq)] / [HA (aq)]) X 100

Type of acidKa valuepKa value
Very strong>0.1< 1
Strong10-3 – 0.11 – 3
Weak10-5 – 10-33 – 5
Very weak10-15 – 10-55 – 15
Extremely weak< 10-15 >15

Classes of Acids

In an aqueous solution, the compounds that dissociate are the acids. The acid’s strength is determined by the ease with which the acid molecules release H+ ions.

In addition, a material is classified as an acid if it possesses the following characteristics:

It causes blue litmus to become red.

• Its pH is less than seven.

• Together with its complimentary base, it creates a salt.

There are several theories in chemistry for determining whether a material is acidic or basic. Three hypotheses are presented below:

According to the Bronsted-Lowry theory, a molecule is acid if it readily gives up a proton in a solution.

According to Lewis theory, a molecule that accepts an unshared pair of electrons from another molecule.

A molecule that does not form hydroxide ion (OH-) in a solution, according to the Arrhenius theory.

H2O2 does form hydrogen ions in a solution, and it readily accepts unshared electron pairs, but it does not make hydroxide ions.

In addition, the pH value of H2O2 is below 7. Consequently, H2O2 is an acid.

After determining if a material is an acid or a basic, it is further classified as a strong or weak acid or a strong or weak base.

The acids that dissociate easily in an aqueous solution and release hydrogen ions are termed strong acids, for example, H2SO4, HCl, etc. while the acids that do not dissociate easily or quickly react with their conjugate base to reform the original molecule are termed as weak acids, for example, HCN, CH3COOH, H2O2, etc.

View the video demonstrating a litmus test for hydrogen peroxide.

Lewis Acid or Lewis Base?

In accordance with the Lewis theory, an acid is a chemical that receives electron pairs, whereas bases donate electron pairs.

By forming a coordinated covalent link between them, their combination results in the development of a product known as a Lewis adduct.

The following equation represents the reaction:

In fact, Lewis acids possess vacant orbitals, allowing them to receive electrons.

The ions hydrogen (H+) and hydronium (H3O+) are regarded to be Lewis acids.

For hydrogen peroxide, the dissociation equation is given as follows:

H2O2 + H2O ——-> H3O+ + HO2- (Hydroperoxide ion)

HO2- + H2O ——-> H3O+ + (O2) 2- (Peroxide ion)

These equations illustrate the emission of hydronium ions in the equation for dissociation. Hydrogen peroxide is therefore an acid.

Hydrogen peroxide is represented by the formula H2O2. Listed below are some of its key characteristics:

• H2O2 has a molar mass of 34.0147 g/mol

• It can dissolve in water

• Its density at 20 °C is 1.11 g/cm3

• H2O2 has melting and boiling temperatures of 0.43 and 150.2 degrees Celsius, respectively.

• Vapour Pressure is 5 mmHg

• This structure is non-planar and possesses C2 symmetry. However, the binding angle and bond length vary between the solid and gas phase.

• The value of pKa is 11.75

• The refractive index has a value of 1.4061.

• Viscosity measures 1.2445 cP

• Dipole moment is 2.26 D.

H2O2 Uses

• Hydrogen peroxide has numerous applications:

• It is used as a bleaching agent, particularly for bleaching pulp and paper.

• It is also utilised in the production of sodium percarbonate and sodium perborate, which are employed as bleaching agents in laundry detergents.

• It is employed alongside dibenzoyl peroxide in the synthesis of organic peroxides.

• It is utilised as an antibacterial and acne therapy.

• It is also utilised in the polymerization procedure.

• A 3-6 percent by weight solution is intended for consumer applications such as cleaning and disinfection.

• The explosive decomposition of highly concentrated hydrogen peroxide, also known as high-test peroxide, is utilised in rocket propellants.

• It is utilised to remove bloodstains.

• It is utilised in horticulture, particularly hydroponics, as the decomposition of hydrogen peroxide in watering solutions releases oxygen.

• It is also used to decrease mortality owing to microbial development in fish farming.

• It creates chemiluminescence when combined with certain di-esters, such as phenyl oxalate ester, and is consequently employed in the production of glow sticks.

Conclusion

Hydrogen peroxide is acidic because its pH is below 7.

The Ka value of hydrogen peroxide at 298 degrees Kelvin is around 1.55 x 10-12, showing that it is a weak acid.

The pH value of a 0.1 M hydrogen peroxide solution is 6.41.

H2O2 has a Ka value of 1.55 x 10-12.

Read more: Structure, Molecular Geometry, and Hybridization of PH3 Lewis

Misha Khatri
Misha Khatri is an emeritus professor in the University of Notre Dame's Department of Chemistry and Biochemistry. He graduated from Northern Illinois University with a BSc in Chemistry and Mathematics and a PhD in Physical Analytical Chemistry from the University of Utah.

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