Non-Aqueous Titrations: Theory

THEORY

The concepts of the Lowry-Bronsted theory may explain the various reactions that take place during many non-aqueous titrations. Thus, an acid is a proton donor and a base is a proton acceptor. Therefore, when an acid HA undergoes dissociation it gives rise to a proton and the conjugate base A of the acid :

In other words, the liberated base A shall unite with a proton to give the corresponding conjugate acid HA of the base A because every base has its conjugate acid and vice versa.

Hence, from the above definitions it may be implied that :

(a) an acid : could be either an electrically neutral molecule e.g., HNO₃ ; or a negatively charged anion e.g., HSO₄^– ; or a positively charged cation e.g., C₆H₅NH₂⁺ , H₃O ;

(b) a base : could be either an electrically neutral molecule e.g., C₆H₅NH₂ ; or an anion e.g., Cl^–, NO₃^–.

1. SOLVENTS

These are of three types and they will be discussed briefly here :

(a) Protophillic Solvents : They are essentially basic in nature and normally react with acids to form solvated protons :

Example :

Perchloric acid displays more strongly acidic characteristics than a weak acid, for instance : acetic acid when dissolved in a weakly basic solvent.

(b) Protogenic Solvents : They are acidic in nature and character e.g., sulphuric acid. They exert a ‘levelling effect’ on bases i.e., they become indistinguishable in strength when dissolved in strongly basic solvents due to their enhanced affinity of strong bases for protons.

(c) Amphiprotic Solvents : They possess both protophillic and protogenic characteristics.

Examples : Acetic acid, water and alcohols.

They undergo dissociation to a very less extent. Acetic acid is mostly employed as a solvent for the titration of basic substances and its dissociation can be depicted as shown below :

CH₃COOH   < == >  H⁺  +  CH₃COO^–

In the above instance acetic acid is behaving as an acid.

Perchloric Acid : It is a very strong acid and when it is made to dissolve in acetic acid, the latter can behave as a base and forms an ‘onium ion’ after combining with protons donated by the perchloric acid. Thus, we have :

As the CH₃COOH₂⁺ ion can instantly donate its proton to a base, therefore, a solution of perchloric acid in glacial acetic acid, behaves as a strongly acidic solution.

Pyridine, a weak base, when dissolved in acetic acid, the latter exerts its levelling effect and subsequently increases the basic characteristics of the pyridine. Therefore, it is practically feasible to titrate a solution of a weak base in acetic acid against a mixture of perchloric acid in acetic acid. Thus, a sharp end point is achieved which otherwise cannot be obtained when the titration is performed in an aqueous medium.

The various reactions with perchloric acid, acetic acid and pyridine are summarized below :

Acetonitrile, acetone and dimethylformamide—these non-aqueous solvents exert a greater differen-tial in the protophillic properties of many substances than in the corresponding aqueous solutions, due to the levelling effect of water in the latter solutions. Hence, the most acidic substance in aqueous solutions of a number of acids is the formation of the hydronium ion as shown below :

It is pertinent to observe here that the following inorganic acids almost exhibit equal strength in aqueous solutions, whereas in non-aqueous solvents, their ‘acidity’ retards in the following order :

In glacial acetic acid (an acidic solvent) and in dioxane (a neutral solvent), the perchloric acid (HClO₄) behaves as more acidic (i.e., less protophyllic) than HCl; and, therefore, many base-hydrochlorides (i.e., chlorides) may be titrated with standard HClO₄, just as carbonates may be titrated in aqueous solution with standard HCl.

In short, it is possible to titrate mixtures of two or three components selectively with a single titration by wisdom of the right choice of solvent for the non-aqueous titrations.