pKa

What is pKa?

The pKa value is the negative base -10 logarithm of the acid dissociation constant (Ka) of a solution.

The quantitative behavior of acids and bases in solution can be understood only if their pKa values are known. In particular, the pH of a solution can be predicted when the analytical concentration and pKa values of all acids and bases are known; conversely, it is possible to calculate the equilibrium concentration of the acids and bases in solution when the pH is known. These calculations find application in many different areas of chemistry, biology, medicine, and geology. For example, many compounds used for medication are weak acids or bases, and a knowledge of the pKa values.

Table of Contents:

Definition of pKa

pKa is a number that describes the acidity of a particular molecule. It measures the strength of an acid by how tightly a proton is held by a Bronsted acid. The lower the value of pKa, the stronger the acid and the greater its ability to donate its protons. describe the acidity of a particular molecule

Ka denotes the acid dissociation constant. It measures how completely an acid dissociates in an aqueous solution. The larger the value of Ka, the stronger the acid as acid largely dissociates into its ions and has lower pka value. The relationship between pKa and Ka is described by the following equation:

pKa = -log[Ka]

Acid dissociation constants, or pKa values, are essential for understanding many fundamental reactions in chemistry. These values reveal the deprotonation state of a molecule in a particular solvent. There is great interest in using theoretical methods to calculate the pKa values for many different types of molecules.

Calulation of pKa

  • Let us consider a weak acid HA which ionises in the aqueous solution as:
    • HA(acid) + H2O ⇋ H3O+ (aq) + A (aq)(conjugate base)
  • The dissociation constant of acid is defined by

Ka = [H3O+] [A] / [HA]

pKa = – log Ka = – log { [H3O+] [A] / [HA]}

  • In the case of polyprotic acid which contains more than 1 proton dissociates stepwise and produces more than one dissociation constant and more than one pKa value.

Examples with Phosphoric acid which contain 3 protons that produced dissociation of the first proton may be denoted as Ka1 and the constants for the dissociation of successive protons as Ka2, and Ka3.

H3PO4 ⇋ H2PO4+ H+

Ka1 = [H2PO4] [H+] / [H3PO4]

pKa1 = – log Ka1 = – log {[H2PO4] [H+] / [H3PO4]}

H2PO4 ⇋ HPO42- + H+

Ka2 = [HPO42- ] [H+] / [H2PO4]

pKa2 = – log Ka2 = – log {[HPO42- ] [H+] / [H2PO4]}

HPO42- ⇋ PO43- + H+

Ka3 = [PO43- ] [H+] / [HPO42- ]

pKa3 = – log Ka3 = – log {[PO43- ] [H+] / [HPO42- ]}

pKa and pH of buffer solution

From the Henderson equation of acidic buffer, the pH of the solution is defined as the

pH = pKa + log { [salt] / [Acid]}

When [salt] / [Acid] = 10 then,

pH = pKa + 1

When [salt] / [Acid] = 1/10 then,

pH = pKa – 1

Note: So weak acid may be used for preparing buffer solutions having pH values lying within the ranges pKa + 1 and pKa – 1.

The acetic acid has a pKa of about 4.8. It may therefore be used for making buffer solutions with pH values lying roughly between the range 3.8 to 5.8.

Relation between pKa and pKb

Let us consider a weak acid HA which ionises in the aqueous solution as:

HA + H2O ⇋ H3O+ (aq) + A (aq)

(acid) (conjugate base)

The dissociation constant of acid is defined by

Ka = [H3O+] [A] / [HA] ……….. (1)

The conjugate base A- behaves as a weak base in water

A + H2O ⇋ HA + OH

For base Kb = [HA] [ OH] / [A ] …………. (2)

Multiply equation (i) and (ii)

Ka x Kb = {[H3O+] [A] / [HA]} x {[HA] [OH] / [A ]} = [H3O+] [OH] = Kw

Ka x Kb = Kw

On taking negative logarithm on both side

– log Ka – log Kb = – log Kw

pKa + pKb = pKw

List of pKa value of acids

Acid ionisation constants (Ka) and pKa values at 25℃

Compound Formula Ka value pKa value
Acetic acid CH3COOH 1.7 x 10 -5 4.75
Benzoic acid C6H5COOH 6.3 x 10-5 4.20
Boric acid H3BO3 5.9 x 10-10 9.15
Carbonic acid H2CO3

HCO3

4.3 x 10-7

4.8 x 10-11

6.35

10.33

Cyanic acid HOCN 3.5 x 10-4 3.46
Formic acid HCOOH 1.7 x 10-4 3.75
Hydrocyanic acid HCN 4.9 x 10-10 9.3
Hydrofluoric acid HF 6.8 x 10-4 3.20
Sulphuric acid H2SO4

HSO4

strong

1.1 x 10-2

-3

1.99

Hydrogen sulphide H2S

HS

8.9 x 10-8

1.2 x 10-19

7.05

19

Nitrous acid HNO2 4.5 x 10-4 3.25
Oxalic acid H2C2O4

HC2O4

5.6 x 10-2

5.1 x 10-5

1.2

4.2

Water H2O 10-14 14
Alkyne CH☰CH 10-25 25
Amine NH3 10-35 35
Alkane CH4 10-50 50
Phenol C6H5OH 10-10 10
Phosphoric acid H3PO4

H2PO4

HPO42-

6.9 x 10-3

6.2 x 10-8

4.8 x 10-13

2.16

7.21

12.32

Sulfurous acid H2SO3

HSO3

1.3 x 10-2

6.3 x 10-8

1.85

7.2

Frequently Asked Questions on pKa

Q1

How do I calculate pKa?

pKa value is the negative base -10 logarithm of the acid dissociation constant (Ka) of a solution. pKa = -log[Ka]

Q2

How does pKa change with temperature?

An endothermic reaction required heat during the reaction. Increase the dissociation constant (Ka) with an increase in temperature. Higher the value of Ka lowers the value of pKa. Hence pKa value decreases with increasing temperature.

Q3

Is high pKa more acidic?

No higher pka means lower acidity. So the higher the pKa the smaller Ka, and this means a weaker acid. Higher pKa indicates weaker acid.

Q4

What is the pKa of pure water at 25 ℃?

The pKa value of pure water at 25 ℃ is 14.

Q5

Why is pKa value important?

The quantitative behaviour of acids and bases in solution can be understood only if their pKa values are known. In particular, the pH of a solution can be predicted when the analytical concentration and pKa values of all acids and bases are known; conversely, it is possible to calculate the equilibrium concentration of the acids and bases in solution when the pH is known.

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