Acid-base titration

In acid-base titration, H⁺ comes from the acid, and OH^– comes from the base; as a result neutral salt is formed. There are two main types of acid-base titrations.

1. Acidimetry      2. Alkalimetry

Acidimetry

In acidimetry titrations, a standard solution of acid will be used to determine the exact concentration of base, or for the quantitative determination of base

Alkalimetry

In alkalimetry, the standard solution of base is used to determine the quantitative determination of acid.

Advantages of acid-base titration

1). The relationship between acid and base is fast and instantaneous

2). The reaction is single-way, i.e., no side reaction takes place

3). The reaction goes to 100 % completion

4). The reaction is always stoichiometric.

Five theories describe the concept of acid-base titrations.

Arrhenius Concept

This theory was first proposed by Swedish physical chemist Svante Arrhenius in 1884. According to this theory, any substance that ionizes partially or completely in H₂O to give a H⁺ ion is called an acid.

HCl + H₂O →    H₃O⁺ + Cl^–

H₂SO₄ + H₂O    →   H₃O⁺ + SO₄^-2

Similarly, bases are those substances that give OH^– ions in an aqueous medium.

e.g. NaOH → Na⁺ + OH^–

An acid will react with a base, and a base will react with an acid and form a H₂O molecule

According to the Arrhenius concept, H₂O acts as a neutral and also a solvent only.

Limitations

• In aqueous medium, the behavior of acids and bases is explained. This is also the main limitation of the Arrhenius concept: if a non-aqueous medium is present, the Arrhenius theory is unable to explain the acidic or basic behavior.
• It does not explain the conjugate acid-base pair
• It is unable to explain the acid or base that does not contain H or OH ions.

Lowery Bronsted concept

This theory was first introduced by J.N. Bronsted and T. M. Lowery independently in 1923. According to this concept, an acid is a species that tends to donate H⁺ ions in any method, any way, or in any solvent, e.g., HCl, HF, and HCN. Similarly, base is a species that tends to accept the proton in any method, any way, or in any solvent, e.g., NaOH, KOH, Cl^–, Br^–, and F-. According to this concept, HSO₄^– and HCO₃^– act as both an acid as well as base.

  e.g.       HCl + HCO₃^–   →   Cl^– + H₂CO₃

 In this reaction, HCl is an acid, and Cl^– is a conjugate base of an acid. So, it is a conjugate acid base pair. Similarly, H₂SO₄ is an acid, and HSO₄^– is a conjugate. So, it is a conjugate acid base pair. H₂O is considered an amphiprotic according to this concept. That is, H₂O acts as an acid as well as a base.

Advantages

• This concept can explain the acidic and basic behavior of a species in any solvent
• This theory is also able to explain the acidic and basic behavior of a specie which do not contain H⁺ ion e.g., Cl^–, Br^-, and I^–.
• This theory is also able to categorize the acids and bases according to their strength.

Lewis concept

Lewis theory is also known as electronic theory and first time introduced by G.N. Lewis in 1923. According to this theory, an acid is a specie which accepts an electron pair, and a base is a species which donate electron pair. So, according to this concept base is a nucleophile and the acid is an electrophile.

Octet rule: Atoms of the main group tend to accept electrons, so that their outermost shell becomes complete. Their outer shells are complete and become noble gases or inert gases.

Lewis concept: If the octet of that base is complete and that base still contains a lone pair of electron than that compound acts as a base. For example,

NH₃ + H⁺  → NH₄⁺,  In this case NH₃ acts as base.

Similarly, AlCl₃ + Cl^– → AlCl₄^– , It acts as a Lewis acid.

According to Lewis’ concept, H₂O acts as a predominantly Lewis base, and it also acts as a Lewis acid. So, by nature, water is amphoteric in nature.

Advantage: this concept is able to define acid and base without any type of solvent.

Usanovich concept

This was first proposed by Russian physical chemist Mikhail Usanovich in 1938. This concept is broader and more applicable so according to this concept, oxidizing agents are acids and reducing agents are bases.

Explanation:                                   Fe⁺³ (acid)  + e^– → Fe⁺² (base) 

 Similarly,      Ce⁺⁴ (acid)  + e^– → Ce⁺³ (base)         

According to this concept, H₂O is amphoteric in nature

Advantages: it is applicable in a broader way

Lux flood concept

This is more acceptable and contains fewer limitations compared to other concepts. First time this theory was proposed by German chemist Hermann Lux in 1939, and later expanded by Hakoon Flood in 1947. It is, in fact, the combination of previous concepts like Arrhenius, lowery-Bronsted and Lewis concepts.

According to this concept, acids are a species that can react with bases, donate a cation, and accept an anion or electron pairs. And bases are those species which accept a proton, donate an anion, and an electron pair. This concept is based on the oxide acceptor-donor concept. According to this concept, bases are these species which donate O^-2 (Oxide ions), e.g., PbO, MgO, CaO, etc.

CaO  →  Ca⁺² + O^-2

MgO   →  Mg⁺² + O^-2

Al₂O₃   →  2Al⁺³ +3O^-2

Similarly, acids are those species that accept theO^-2 (Oxide ions), e.g.

SO₃  +   O^-2      →      SO₄^-2

CO₂   + O^-2      → CO₃^-2

SiO₂  + O^-2    →  SiO₃^-2

Other examples of the Lux flood concept

CaO (base) + SO₃ (acid)      →    CaSO₄ (salt)

MgO (base) + SiO₂ (acid)  →     MgSiO₃ (salt)

MgO (base) + CO₂ (acid) →  MgCO₃ (salt)

ZnO (base) + SO₃  (acid) →     ZnSO₄ (salt)

ZnO (acid) + Na₂O (base)   →    Na₂ZnO₂   (ZnO is amphoteric in nature)

Lux flood concept limitation

According to this concept, for being an acid or base, they must contain oxide ions (O^-2)