pH of Hydrogen Peroxide – Acidic or Alkaline?

H2O2 Lewis Structure

Hydrogen peroxide or H2O2 is a chemical compound made up of two atoms of oxygen and two atoms of hydrogen.

It occurs as a pale blue, slightly viscous liquid in its pure form. It is the simplest peroxide having an oxygen-oxygen single bond. It slowly decomposes in the presence of light due to which it is stored in dark bottles.

Is hydrogen peroxide an acid? Yes. Hydrogen peroxide is acid as it has a pH of 6.2. However, the pH value depends upon the concentration as well as the temperature of the solution. At the temperature of about 25°C, depending upon the concentration of hydrogen peroxide in the solution, the ph value ranges from 4.2 to 7.

The hydrogen peroxide solutions made for commercial purposes contain stabilizers, which are also acidic, due to which the pH of the solution further reduces to 2.5-3.5.

 

Why is H2O2 a Weak Acid?

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Weak acids have a low value of acid dissociation constant (Ka). In the case of hydrogen peroxide, the Ka value is about 1.55 X 10-12 at 298 K indicating that it is a weak acid.

Further, the strength of an acid is estimated by its tendency to lose proton i.e. H+ ions.

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

Therefore, hydrogen peroxide is a weak acid.

In the aqueous solution the dissociation equation for hydrogen peroxide is given as:

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

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

The double arrow in reaction indicates a reversible reaction indicating that the ions formed after dissociation of hydrogen peroxide tend to reform the original molecules, hence, confirming that H2O2 is a weak acid.

 

How to calculate the pH of hydrogen peroxide?

pH is the quantitative measure to gauge the acidity or basicity of a solution. The value on the pH scale ranges from 1 to 14, amongst which 7 is the value for the neutral solution.

The solutions having a pH value below 7 are acidic while those having a pH value above 7 are alkaline.

pH was first used by Danish biochemist S.P.L. Sørensen who defined it as the concentration of hydrogen ions in a solution, which ranges from 1 to 10-14. It is given by the formula

pH = – log [H+]

In the case of H2O2, calculating the pH for 0.1 M H2O2 solution:

The concentration of hydrogen ions is calculated as:

[H+] = √Ka X M

As the value of Ka = H2O2 is 1.55 X 10-12

Also, we are calculating the hydrogen ion concentration for 0.1 M H2O2 solution.

Therefore, [H+] = √1.55 X 10^-12 X 0.1

= 3.93 X 10^-7

The pH value for 0.1 M solution can now be calculated as:

pH = – log [H+]

= – log [ 3.93 X 10-7 ]

= – [ log 3.93 – 7 log 10]

= 6.41

As the pH value is less than 7, therefore, it is clear that H2O2 is acidic in nature.

The acid dissociation constant (Ka) and Logarithmic Constant (pKa):

In the above section, we have used the Ka value for calculating the pH of the H2O2 solution.

Here, you may ask what is this Ka and how is it associated with the acidity of a solution? Keep reading to understand this.

The value of the acid dissociation constant (Ka) is used to estimate the extent of ionization of an acid. It is the equilibrium constant that helps measure the strength of an acidic solution.

The value of the acid dissociation constant is quoted in mol/L. For strong acids, the value of Ka is higher while weak acids have a lower Ka value.

It is also sometimes expressed as a logarithmic constant, pKa. The pKa value for strong acids is below -2 while for weak acids pKa value lies between -2 and 12.

pH scale

 

Calculating values of Ka and pKa for acids:

The value of Ka and pKa provides us the measure of the strength of any acid.

As we have already discussed earlier the strong acids almost completely ionize in a solution while weak acids do not dissociate completely, therefore, their Ka values can be calculated using the concentration of their ions in the solution.

The formula given below is used to calculate it:

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

The pH value of a solution can also be utilized in calculating the Ka value of a solution. It is given as:

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

The above statement can be rewritten as,

[ H30+] = 10^-pH

The pH of a solution can be calculated if the molar concentration of the acid in the solution is known and in turn, the pH value can be used for calculating the Ka value of that solution.

Further percentage dissociation may also be calculated as:

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

Type of acid Ka value pKa value
Very strong >0.1 < 1
Strong 10-3 – 0.1 1 – 3
Weak 10-5 – 10-3 3 – 5
Very weak 10-15 – 10-5 5 – 15
Extremely weak < 10-15  >15

 

Types of Acids

The acids are the substances that dissociate in an aqueous solution. The ease with which the acid molecules release H+ ions determines the strength of the acid.

Further, a substance can be categorized as an acid if it displays the following characteristics:

It turns blue litmus to red.

• It has a pH of less than 7.

• It forms a salt with its complementary base.

A few theories have been given in chemistry to identify a substance as an acid or base. Three theories have been given below:

Bronsted-Lowry theory: According to this theory if a molecule easily gives away a proton in a solution, it is an acid.

Lewis theory: A molecule that takes up an unshared pair of electrons given away by another molecule.

Arrhenius theory: A molecule that does not produce hydroxide ion (OH-) in a solution.

H2O2 does produce hydrogen ions in a solution, also it easily takes unshared pair of electrons, and also, it does not produce hydroxide ions in a solution.

Further, the pH value for H2O2 is less than 7. Therefore, H2O2 is an acid.

Once the substance has been identified as an acid or base it is further categorized as strong or weak acid and 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.

Check out the video having a litmus test for hydrogen peroxide.

 

 

Is H2O2 a Lewis Acid or Base?

As per the Lewis theory, an acid is a substance that accepts electron pairs while bases are the substances that accept electron pairs.

When combined together they result in the formation of a product known as Lewis adduct by the formation of a coordinate covalent bond between them.

The reaction is represented by the following equation:

Lewis Theory

Actually, Lewis acids have empty orbital due to which they are able to accept electrons.

The hydrogen ions (H+) and hydronium ions (H3O+) ions are considered Lewis acids.

In the case of hydrogen peroxide the dissociation equation is written as:

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

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

These equations display the release of hydronium ions in the dissociation equation. Therefore, hydrogen peroxide is an acid.

 

H2O2 Properties

Hydrogen peroxide has the chemical formula H2O2. Some of its important properties are listed below:

• Molar mass of H2O2 is 34.0147 g/mol

• It is miscible in water

• It has a density of 1.11 g/cm3 20 °C

• The melting and boiling points of H2O2 are −0.43 °C and 150.2 °C, respectively.

• Vapour Pressure is 5 mmHg

• It has a non-planar structure with C2 symmetry. However, the bond angle and bond length differ for the gas and solid phase.

Dihedral angle of gaseous and crystalline HOOH - Chemistry Stack Exchange

• The pKa value is 11.75

• The value of the refractive index is 1.4061.

• Viscosity is 1.245 cP

• Dipole moment is 2.26 D.

 

H2O2 Uses

• Hydrogen peroxide is used for a variety of purposes:

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

• It is also used in the manufacturing of sodium percarbonate and sodium perborate which are used as bleaches in laundry detergents.

• Along with dibenzoyl peroxide, it is used for the production of organic peroxides.

• It is used for the treatment of acne and as an antiseptic.

• Also, it is used in the polymerization process.

• 3-6 % by weight solution is meant for consumer usage like cleaning and disinfection.

• Highly concentrated hydrogen peroxide, also known as high-test peroxide, is used in rocket propellants as it decomposes explosively.

• It is used for the removal of bloodstains.

• It is used in horticulture especially in hydroponics as hydrogen peroxide releases oxygen on decomposition in watering solutions.

• It is also used in fishkeeping for controlling mortality due to microbial growth.

• It produces chemiluminescence with certain di-esters like phenyl oxalate ester and is, therefore, used in manufacturing glow sticks.

 

Conclusion

Hydrogen peroxide is acid as it has a pH value below 7.

The Ka value of hydrogen peroxide is about 1.55 X 10-12 at 298 K indicating that it is a weak acid.

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

The Ka value for H2O2 is 1.55 X 10-12.

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