Simultaneously with Lewis, a Soviet chemist Mikhail Usanovich from Tashkent, developed a general theory that does not restrict acidity to hydrogen-containing compounds, but his approach, published in 1938, was even more general than Lewis theory. Usanovich's theory can be summarized as defining an acid as anything that accepts negative species or donates positive ones, and a base as the reverse. This pushed the concept of acid-base reactions to its logical limits, and even redefined the concept of redox (oxidation-reduction) as a special case of acid-base reactions, and so did not become wide spread, despite being easier to understand than Lewis theory, which required detailed familiarity with atomic structure. Some examples of Usanovich acid-base reactions include:
- Na2O (base) + SO3 (acid) → 2 Na+ + SO2−
4 (species exchanged: anion O2−)
- 3 (NH4)2S (base) + Sb2S3 (acid) → 6 NH+
4 + 2 SbS2−
4 (species exchanged: anion S2−)
- Na (base) + Cl (acid) → Na+ + Cl− (species exchanged: electron)
-
This acid-base theory was a revival of oxygen theory of acids and bases, proposed by German chemist Hermann Lux in 1939, further improved by Håkon Flood, circa 1947 and is still used in modern geochemistry and electrochemistry of molten salts. This definition describes an acid as an oxide ion (O2−) acceptor and a base as an oxide ion donor. For example,
MgO (base) + CO2 (acid) → MgCO3
CaO (base) + SiO2 (acid) → CaSiO3
NO−
3 (base) + S2O2−
7 (acid) → NO+
2 + 2 SO2−
4
In 1963, Ralph Pearson proposed an advanced qualitative concept known as Hard Soft Acid Base principle, later made quantitative with help of Robert Parr in 1984. 'Hard' applies to species that are small, have high charge states, and are weakly polarizable. 'Soft' applies to species that are large, have low charge states and are strongly polarizable. Acids and bases interact, and the most stable interactions are hard-hard and soft-soft. This theory has found use in organic and inorganic chemistry.
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