Usanovich’s Concept of Acids and Bases

Michail Illyich Usanovich (1894—1981)

 Michail Illyich Usanovich (1894—1981), a brilliant Russian academic and chemist put forward his own theory of Acids and bases combining all the pre existing theories. He joined as a professor in Tomsk State University in 1930, Central Asian in 1935, and Kazakh in 1944. He developed several theories on law of dilution, quantitative solution theory, equation of non colligative properties in solution etc., but he was mostly known for his acid and base theory.

This concept is actually an extended and more generalized version of Lewis acid-base theory. This concept does not let itself be confined in just electron donation and acceptance. The theory is as follows

An acid is a substance capable of accepting anions, electrons or giving up cations whereas a base is a substance that can combine with cations or give up anions or electrons. Therefore this concept included the redox reactions as a category of acid-base reactions. Here theoretically Lewis and Usanovich concepts are almost identical but they differ on certain degree of process. In Lewis acid-base theory the electron is shared by both acid and base to make an adduct but in Usanovich’s concept the electrons are completely or partially transferred from base to acid. Examples of some Usanovich acid base redox reaction as well as Lewis acid-base reaction as follows

O + :SO₃^2- ⇋ SO₄^2-

C₆H₅N: + O ⇋ C₆H₅N⁺→O^-

The merits of this concept are:

(i) It is the most general of all the different definitions.

(ii) It includes all acids and bases defined by Lewis.

(iii) It states that donation or acceptances of electrons need to take place as shared pairs.

These theories make it clear how redox reactions are incorporated as acid- base reactions. Therefore this theory can be used further to explain certain reactions as acid-base reactions. Such as:

(i) SO₃ (Acid) + Na₂O (Base) → Na₂SO₄ (Salt)

This reaction takes place through the following stages:

Na₂O (Base) → 2Na⁺ + O^2-

Na₂O gives O^2- anion and hence acts as a base

SO₃ (Acid) + O^2- → SO₄^2-,

SO₃ accepts O^2- anion and hence behaves as an acid.

Na₂O (Base) + SO₃ (Acid) → 2Na⁺ + SO₄^2- or Na₂SO₄ (Salt)

(ii) SO₂ (Acid) + Na₂O (Base) → Na₂SO₃ (Salt)

The formation of Na₂SO₃ (salt) by the combination of SO₂ (acid) and Na₂O (base) can also be explained the formation of Na₂SO₄ by the combination of SO₃ and Na₂O.

(iii) Cl₂ (Acid) + 2Na (Base) → 2NaCl (Salt)

2Na (Base) → 2Na⁺ + 2e^-

Since Na furnishes electrons, it acts as a base.

2Cl + 2e → 2Cl^-

Cl accepts electron and is therefore an acid.

2Na (Base) +2Cl (Acid) → 2Na⁺ + 2Cl^- or 2NaCl (Salt)

(iv) Fe(CN)₂ (Acid) + 4KCN (Base) → K₄[Fe(CN)₆] (Salt)

4KCN (Base) → 4K⁺ + 4CN^-,

KCN acts as a base, since it gives CN^- anions.

Fe(CN)₂ (Acid) + 4CN^- → [Fe(CN)₆]^4-,

Fe(CN)₂ behaves as an acid because it adds CN^- anions.

4KCN (Base) + Fe(CN)₂ (Acid) → 4K⁺[Fe(CN)₆]^4- or K4[Fe(CN)₆] (Salt)

Usanovich also stressed on unsaturation involving some acid-base reactions:

OH^- + O=C=O → HOCO₂^-

Limitations of the Concept

a) The Usanovich definition of acids and bases has not been widely used, probably because of (i) the relative inaccessibility of the original to non-Russian reading chemists and (ii) the circularity of Usanovich’s original definitions.

b) The dividing line between electron pair donation and acceptance (Lewis definition) and oxidation-reduction (Usanovich definition) is not a sharp one.

c) It is an extremely generalized concept, so general that almost all chemical reactions are considered as acid-base reaction in this concept.

Reference

1) Concise inorganic chemistry by J. D. Lee.

2) Inorganic Chemistry by James E. Huheey, Ellen A Keither, Richard L. Keither, Okhil K. Medhi.