Di Polybasic Acid Base

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Acids are any substance which changes the colour of blue litmus paper to red, have sour taste inside the water, reacts with metals to form hydrogen ions (for example iron), reacts with bases to form salts and acts as a catalyst with specific reactions. Examples include – sulphuric acid, nitric acid, hydrochloric acid (mineral acid) and carbonic acid, sulphonic acid (organic acid) etc.

Polybasic Acids

Polybasic acids are those acids which are capable of yielding more than one hydronium per molecule. These may be called dibasic, tribasic etc. according to the number of hydrogens they can replace. Some examples of polybasic acids are sulphuric acid and phosphorus acid.

The ionization of a typical polybasic acid is written below:

\[H_{2} X(aq) \leftrightarrow H^{+} (aq) + HX^{-} (aq)\]

\[HX^{-} (aq) \leftrightarrow H^{+} (aq) + X_{2}^{-} (aq)\]

What are Polybasic Acids?

Generally, inorganic acids, when combined with base loose one atom of acid to get united with one atom oxide, therefore termed to be monoxide. However, some acids are already present as pyrophosphoric,  who have the power to combine with a base, & then are termed as basic. Some examples of these types of acids are found among vegetables and the animal kingdoms.

Polybasic acids are also known as polyprotic acids. Ionization process takes place in a stepwise manner; within each step, one proton is released, the constant which is used to denote each step of the ionization process. The magnitude of the ionizing protons gets smaller and smaller with each step with the release of frequent protons subsequently.

What is Ionization?

Ionization is a process of losing or gaining molecules by an atom to form a charged ion. Ionization can be a result of the loss of electronic molecules after colliding with subatomic molecules and ions or interaction through electromagnetic radiation. Radiation energy knocks electrons from atoms, thereby agonizing them. This process is called ionizing radiation.

X + energy → X+ + e-

where x is any atom (to be ionized), x+ is any atom (with electron removed), and x- is any atom with (the removed electron).

The basicity of an acid depends on the number of hydrogen atoms it can replace in its molecule. If an acid can replace one atom of hydrogen ions, it is called a monobasic with the basicity of 1. Similarly, the acids which can replace two or three numbers of hydrogen ions are called as dibasic or tribasic with the basicity of 2 and 3 respectively. Some examples of acids with their basicities are given below.

Basicity of Acid







Basicity of Acetic Acid

Ethanoic acids contain four hydrogen atoms, with one of those atoms having direct contact with the oxygen atom, thus forming COOH- carboxyl group at the end of the carbon chains. The remaining of the hydrogen atoms would instinctively render the carbon chain unstable and is thus dramatically unfavourable.

CH3COOH (aq)+OH(aq) → CH3COO(aq) + H−O−H (l)

Acetic acid is an organic acid. It is a carboxylic acid which undergoes dissociation as follows:


The basicity of Acetic acid is always one because the covalent bond between Carbon and Hydrogen is very strong. For the dissociation to properly occur, the solvent cannot provide sufficient energy. This results in the production of only 1 Hydrogen ion, and this is the reason the basicity is 1.

FAQ (Frequently Asked Questions)

Q1. What are Polyprotic Acid and Bases?

Ans. Polyprotic acid is those types of acids which can release more than one atom within a molecule in an acid-base reaction. Polyprotic Bases are those types of bases that can accept at least one H+ ion, or proton, in acid-base reactions. In simpler terms, acid which can lose more than one H+ molecule during ionization. These contain multiple acidic protons which can disassociate themselves with the compound. They can do it with the help of a unique dissociation constant for each proton.

Q2. Why are Polyprotic Acids Weak?

Ans. Polyprotic acid loses protons gradually. However, an Arrhenius acid donates a proton. A polyprotic acid can release more than one hydrogen ion than a monoprotic acid. A monoprotic acid does not completely disassociate. But a polyprotic acid does, thus, making it weak.

Q3. How do you Identify if an Acid/Base is Polyprotic?

Ans. To identify a polyprotic acid or a base, you can look at the anions. In a polyprotic, the element must be able to donate more than 1 H+ protons. Thus, logically speaking, you can look at the anions to know whether it is polyprotic or not. For example, a Chloride anion has a negative one charge, and only one H+ proton can be attached to it. So, it cannot be polyprotic.

Q4. What are the Different Types of Bases?

Ans. The different types of bases are as follows:

  1. Strong Base: A strong base is a compound which contains the ability to kick out a proton from a weak acid. Examples are Potassium hydroxide and Sodium hydroxide.

  2. Weak Base: in this, the disassociation is incomplete when in water. The solvent contains both the weak base and the conjugate acid. Example, H2O, NH3.

  3. Super Base: these are much better at deprotonation than the strong base. These have very weak conjugate acids, and can’t sustain an aqueous solution as these are stronger bases than hydroxide ions. Example: (C6H4(C2)2)2-

  4. Neutral Base: It forms a bond with the neutral acid.