Acid-base concept – Arrhenius concept, Bronsted Lowry theory, Hard and soft acid base concept(1)

Acid-Base Concept

acid base concept

Acid base concept, An acid may be defined as any substances which has sour taste and its aqueous solution turns blue litmus red. A base may be defined as any substance which has it bitter taste and its aqueous solution turns red litmus blue.
But limitation of this concept could not explain behavior of all acids and bases.

Arrhenius concept

The concept of acid and base was the first introduced by Arrhenius. According to Arrhenius theory, an acid may be defined as any hydrogen containing substance which gives H⁺ ions in aqueous solution. A base may be defined as a substance containing hydrail groups or groups complete of providing hydroxide ions(OH⁻) in aqueous solution.
Example:- HCl → H⁺ + Cl⁻
NaOH → Na⁺ + OH⁻
acid and base undergo a reaction known as neutralization. The neutralization reaction result informal water and salt.
HCl + NaOH → NaCl + H₂O

✓ Limitations of Arrhenius concept

1. The definition of acid and base are only in terms of aqueous solution and not in terms of the substances.
2. The theory does not explain acidic and basic properties of substances in non aqueous solvents.
3. The neutralization of acid and base in absence of solvent is not explained.
4. The basic substances which don’t to contents OH⁻ is not explained by this theory.

Bronsted Lowry theory

In 1923 proposed a new definition of acid and base according to the Bronsted and Lowry.
Acid is a substance that can donate a proton. Base is a substance that can accept a proton.
Ex:- acid is a proton donor and base is a proton acceptor.
HCl → H⁺ + Cl⁻
OH⁻ + H⁺ → H₂O

✓ conjugate acid-base pairs

HCl + H₂O → H₃O + Cl⁻
In this reaction HCl donates a proton to H₂O and is in an acid. On the other hand water is accept a proton from HCl and is therefore a base. In the reverse reaction which at equilibrium proceeds at the rate at the forward to H₃O ion donates a proton to Cl⁻ ion and hence H₃O is an acid, while Cl⁻ ion accepts a proton from H₃O is a base. The acid base pair the membrane of which can be formed from each other mutually by gain or loss of proton are called conjugate acid base pair.

Limitation of Bronsted Lowry theory

1. This concept say expressive emphasis on Photon transfer. Although it is true that most common acids are proton in nature, yet these are many which are not.
2. A large number of acid base reaction are known in which no proton transfer can take place.

Hard and soft acid base concept

It is commonly known that contain substances or electrolytes are strong or weak acids or base. The nature of their being a strong or weak depends often their nature, degree of extent of its ionization and the solvent.
✓ hydrochloric acid in water is a strong acid while acetic acid in water is a weak acid. However, CH₃COOH of dissolved in ammonia shows strong acidic character.
✓ when an acid is dissolved in water, it will dissociate or ionize of the amount. The amount of ionization is depend on the strength of acid. A strong electrolyte is completely dissociated, while a weak electrolyte is partially dissociated. Thus, the degree of ionization of a great use to differentiate between strong acids and weak acids or strong bases and weak bases.

Importance of acid and base in pharmacy

Acid base of their reaction play vital roles in pharmacy practice some applications are:-
1. Acids and bases are used in analytical procedures which are involving acid base titration.
2. Acid-base fixed use as the therapeutic agents in the control of adjustment of adjustment the pH of the GIT tract.

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