Introduction to Non-Aqueous Titration

Non- Aqueous Titration

Non Aqueous titration is a process of titration where all the procedure is free from water. By this titration, we can easily determine the substances which are

                                                -water insoluble,

                                                -water sensitive

                                                -very very weakly acidic or basic solution

                                     

-The substances which are either too weakly acidic or too weakly basic to give sharp end point in aqueous solution, can easily be titrated with accuracy in non-aqueous solvent.

-Organic acids and bases that are insoluble in water are soluble in non-aqueous solvent.

-A non-aqueous solvent may help two or more acids in mixture. The individual acid can give separate end point in different solvent.

-By the proper choice of the solvents or indicator, the biological ingredients of a substance whether acidic or basic can be selectively titrated.

-Non-aqueous titrations are simple and accurate.

Types of Non-Aqueous Solvent:

There are four types of non-aqueous solvent. They are-

1. Aprotic solvent
2. Protophilic solvent
3.  Protogenic solvent
4.  Amphiprotic solvent. 

Aprotic Solvent

Aprotic solvent include those substances which may be considered chemically neutral and virtually unreactive under the conditions employed. Aprotic solvent neither accept nor donate protons. They possess low dielectric constants, do not cause ionization in solutes and do not undergo reactions with acids and bases. Aprotic solvents are frequently used to dilute reaction mixture.

Example: Hydrocarbons, Carbon-tetra-chloride (Chloroform), Toluene, Benzene.

Protophilic Solvent:

Protophilic solvents are the substances that possess a high affinity for protons. The overall reaction can be represented as-

                            HB+S <=========> SH⁺ + B^-

The equilibrium in this reversible reaction will be generally influenced by the nature of the acid and the solvent. Weak acids are normally used in the presence of strongly Protophilic solvents as their acidic strengths are then enhanced and then become comparable to these of strong acids. This is known as the levelling effect.

Example: Acetone, Ethers, Amine, Ethylene diamine, Liquid ammonia etc.

Protogenic solvent:

Protogenic solvents are acidic in nature and readily donate protons. Anhydrous acids such as hydrogen fluoride and sulphuric acid fall in this category. Because of their strength and ability to donate protons, they enhance the strength of weak bases.

Amphiprotic solvent:

Amphiprotic Solvents act as both proton donor and proton acceptor. These solvents are both acidic and basic to a significant extents. Ethanoic acid displays acidic properties in dissociating to produce protons.

                                CH₃COOH    <=====> CH₃COO^- + H⁺

But in the presence of perchloric acid, a far stronger acid, it will accept a proton.

                         CH₃COOH + HClO₄   < ==========> CH₃COOH₂⁺  + ClO₄^-

The CH₃COOH₂⁺   ion can very readily give up its proton to react with a base, so basic properties of a base is enhanced. So, titration between weak base and perchloric acid can often be accurately carried out using Ethanoic acid as solvent.

Note that, the most valuable and widely used titrant is perchloric acid in glacial acetic acid or dioxane. Perchloric acid is a strong acid. It gives excellent result with all weak bases.