Ionisation of Polybasic Acids

Introduction to Polybasic Acids

Polybasic acids are acids that generate more than one hydronium per molecule. Depending on the number of hydrogen atoms they can replace in their structure, they can be dibasic, tribasic, and so on. 

Sulphuric acid (Dibasic) and phosphorous acid (Tribasic) are two common examples of polybasic acids.

Definition of Polybasic Acids

Monobasic acids lose one Hydronium ion (H3O+) on combination with bases. Some inorganic or organic acids have the ability to lose more than one Hydronium ion and are known as polybasic acids.

Another name for polybasic acids is polyprotic acids. The ionisation process occurs in a stepwise way. Within each step, one proton is released. A characteristic of this ionisation process is the removal of one proton within each step. The negative charge on the species increases with loss of Hyrdonium ions and hence its capability to lose more ions keeps decreasing.

Polybasic Acids Formula

In a single reaction, the basic acid donates more than two H+ ions (protons). That is why the epithet polybasic. A good example is H3PO4 (phosphoric acid)

Types of Polybasic acids

  1. With the first dissociation being complete (Ka1 very large)
  2. With the first dissociation being partial (K relatively small)

Let us understand this concept with the help of a derivation.

H₂SO4 (aq) + H₂O(aq) H30+ (aq) + HSO4 (aq); Ka1= 1.4 x 10-2

HSO4 (aq) + H₂O(aq) ≤ H3O+ (aq) + SO2(aq); Ka2= 6.5 x 10-8

For the first dissociation is complete since Ka1 is is very large;

[H30]+ = 0.1M; pH = -log[H3O+].

Now consider 0.1 M of hydrogen sulphide to discuss case 2

For H2S, the first dissociation is in a similar format as case 1

pH = -log[H3O+] = -log√Ka1 C = -log√1.1 x 10-7 × 0.1 = 3.98

Here Ka1 is the first ionisation constant and C is the concentration.

It shows that hydrogen sulphide is a weak acid. Likewise, large Ka1 corresponds to stronger acid, and large Ka2 corresponds to weaker acid as Ka1>>Ka2 always.

Polybasic Acid Ionisation

Polybasic acid or polyprotic acid has many interchangeable hydrogen atoms. It means that the acid can create more than one ionisable ion per molecule, indicating high ionisability. As we are aware, an acid is a proton giver.

The following is the general representation for a dibasic acid:

  • H2A is the representation for dibasic acid.
  • HA– is a representation for intermediate form.
  • A-2 is completely basic or completely deprotonated form

Consider the ionisation reaction of a common polybasic acid, 

H₂X(aq) → H+ (aq) + HX¯(aq)

 HX (aq) → H+ (aq) + X2- (aq)

This case demonstrates how a dibasic acid dissociates into its constituent ions.

The equilibrium constant for the reaction above can be calculated as follows:

Ka1 = {[H+][HX¯]}/[H₂X]

Ka2  =  {[H+][X²-]}/[HX-]

In this equation, Ka1 and Ka2 are the first and second ionisation constants of the acid H2X, respectively. We have three ionisation constants for a tribasic acid, Ka1, Ka2, and Ka3, and so on for other acids.

Polyacidic Bases

We can observe that some bases, such as calcium hydroxide and aluminium hydroxide, contain more than one ionisable ion per base molecule if we look at the data for a handful of these compounds. Polyacidic bases are acids that have many acidic bases.

Let us assume the following ionisation reaction of a typical poly-acidic base.

M(OH)2 (aq) → MOH+(aq) + OH–(aq)

MOH+(aq) → M+2 (aq) + (OH)2- (aq)

The equilibrium constant for the reaction will be:

Kb1  = {[M+2OH¯] [OH-]}/[M(OH2) ]

Kb2 =  {[M+2][(OH)²¯]}/[M+2OH–]

The first and second ionisation constants of base M(OH)2 are denoted by Kb1 and Kb2, respectively.

Conclusion

Polybasic acids ionise and produce more than one proton when dissolved in water. As a result, they are also called polyprotic acids. 

The ionisation reaction proceeds in a stepwise fashion. Each step results in the release of one ionisable proton. As a result, each stepwise reaction has a specific ionisation constant (K). 

Generally, the first proton is released immediately upon addition to water. So, the ionisation constant (K) for the first proton is larger in magnitude, but successive protons are more difficult to release, and thus the ionisation constants become smaller in magnitude.

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