Introduction

This contribution discusses the little-known universal acid–base concept of Mikhail I. Usanovich. It is quite interesting and useful to see how one can get from the proton-based and quantitative Brønsted theory and the electron-based Lewis approach to a comprehensive and unified acid–base concept that includes exchange of all kinds of ions and even electrons. When the consideration of electron exchange touches even redox reactions, however, the Usanovich concept appears to most chemists as too far of a stretch. This paper will show that the full concept of Usanovich is even inconsistent or wrong.

Publishing the original idea in a Soviet journal in Russian language did not help either in promoting it (Usanovich 1939). Later articles were published in German (Gehlen 1954; Ussanowitsch 1964), and even some English chemistry textbooks (Huheey 1972) considered the contribution of Usanovich. These chapters, however, would disappear in later editions, and so, what remains today is the fascinating history of acid–base concepts that can teach us on how competing ideas contribute to the progress in chemistry; in particular, Reiners et al. (2022) see Usanovich as a pioneer of a general donor–acceptor-principle in chemistry.

Reiners et al. (2022) who gained access to the Russian original (1939) article of Usanovich explained his approach:

“Based on extensive experimental research on the conductivity and viscosity of solutions of selected metal halides and various mineral acids in oxygen- and nitrogen-containing solvents, Usanovich would consider that the formation of electrolytes, i.e. salts, from those acid–base reactions, that he had examined, might be the key to a new understanding of the terms “acid” and “base”. Exemplary, with the reaction of a strong acid (HCl) with a strong Base (NaOH) he discusses the difference between electric and acid–base neutrality and concludes that an aqueous NaCl solution could only considered acidic-base-neutral when assigning the Na+ cation an acidic function, since otherwise the electrical effect of the chloride anion would be compensated, not however, its basic effect. This led to the widening of the donor function of the acids and the acceptor function of bases: The acid function can express itself therein such that protons or any cations are donated. The base function is reflected accordingly in the ability to accept protons or cations.

The limitation to protons, being fundamental for the Brønsted concept was thus repealed.” (translated from the German original text, Reiners et al. 2022).

Another part of Usanovich’ concept was to assign both acids and bases not only acceptor but also donor functionalities, in order to overcome “asymmetry” of the classic acid–base theories. (Reiners et al. 2022).

Brief history—the way to Usanovich

The idea of acidic properties of substances is very old and has its origins in liquids with sour taste like vinegar and lemon juice. People early on realized that the sour taste and acidic behavior could be removed or “neutralized” using a different kind of substance. Both together would form salts, so that different kind of substance was called “base”, i.e. “base of the salt” (Jensen 2006). Since the Middle Ages alchemists discovered stronger acids with inorganic origin, so called “mineral acids” that showed the same acidic properties and behavior towards bases like organic acids.

It was Lavoisier who assigned the newly discovered chemical element oxygen as an essential part of all acids. In the German language, this chemical element is known till today as “Sauerstoff” i.e. “sour substance”. Known bases at the end of the eighteenth century included especially carbonates of sodium and potassium, their hydroxides (obtained from the carbonates by means of calcium hydroxide) as well as ammonia solution.

Sir Humphrey Davy noticed in 1810 that acids like muriatic acid did not contain oxygen (Hall 1940). Justus von Liebig assigned the element hydrogen as the essential constituent of acids instead of oxygen (Kurzweil and Scheipers 2011).

In the dissociation theory of Arrhenius which greatly explains the differences in conductivity of the solutions of acids, bases and salts, acids are the compounds that release hydrogen ions, whereas bases would release hydroxide ions. The neutralization is the reunification of hydrogen ions with hydroxide ions forming water, whereas the remaining acid anion and base cation form a salt. Quantification is possible by consideration of the dissociation degree.

Some problems remained such as basic behavior of compounds like sodium carbonate that do not contain hydroxide ions; however, this was addressed by considering hydrolysis of the carbonate ion in water.

The acid–base concept of Johannes Nicolaus Brønsted (1923) allows to use other solvents beside water. In a great generalization, acids are now compounds that donate hydrogen ions (now “protons”), whereas bases would be ions, molecules or even atoms that accept protons. Also the molecules of the solvent take part in these processes, and they can even protonate and deprotonate each other which leads to autoprotolyses. The exchange of protons can be described quantitatively using the concepts of Guldberg and Waage (Lund 1965), which results in equilibrium constants that describe the strength of acids and bases (acids and base constants). This concept is used till today and proved very useful in most areas of chemistry and connected fields.

The story could end here since the classic properties and very essential behavior of acids and bases can be described wonderfully using Brønsteds theory. However, it does not tell much about the binding and release of the protons. This part would be left to Gilbert N. Lewis (1923).

Lewis (1923) paid great attention to those particles that keep the molecules together. His valence bond theory is used till today to explain properties of molecules and reaction mechanisms. A major role plays his way of how to consider acids and bases. Bases possess lone electron pairs, whereas acids possess a gap into which an electron pair can step in forming a bond. The other way to form a binding electron pair is that each binding partner contributes one electron.

Brønsted acids and bases greatly fit in this scheme of Lewis, they share the same idea of a base, with one specialty in that Brønsted acids would donate the Lewis acid proton. This makes the Brønsted concept a special case of the broader Lewis acid–base theory.

Beside these two main theories some more specialized concepts have been published. Hermann Lux (1939) as well as Flood and Förland (1947) considered molten salts as a medium where acids and bases can react without water, hydroxide or hydrogen ions. Here, oxide ions are the particles that are exchanged; with bases like carbonate ions that donate oxide ions and acids like silicon dioxide that accept oxide ions. Also this acid–base concept is in accordance with Lewis.

In the solvent theory, the idea of acid and base is extended starting with Brønsted, solvents that show self-ionization, and then considering acids as those particles that increase the concentration of the cation of that solvents self-ionization, whereas a base would increase the concentration of the regarding anion. This idea also corresponds well with Lewis’ concepts of electron pairs.

Independent from Lewis, and also from Lux, the Soviet–Russian chemist Mikhail I. Usanovich published his own very general concept of acids and bases (1939). The idea is basically as follows:

  • Acids are particles that donate cations or that accept anions or electrons

  • Bases are particles that donate anions or electrons or that accept cations

  • Acids and bases react with each other by forming a salt.

Accordingly, Usanovich identifies any particles that react with acids also as bases; and vice versa, any particles that react with bases are acids. Here, the close proximity between Lewis’ and Usanovich’ approaches becomes very clear as illustrated by Jensen (1980) in a Venn diagram.

An earlier translator and promotor of Usanovich’ 1939 article, Heinz Gehlen, thoroughly reported and discussed the new and very general acid–base theory in (1954). Gehlen considers Usanovich’ theory also in view of a contribution of Jannik Bjerrum, who obviously was not aware of the article of Usanovich and proposed to introduce “antibases” along with acids and bases (Bjerrum 1951). Briefly, the term “antibase” was proposed by Bjerrum in order to keep the Brønsted concept intact while applying the Lewis theory broadly. In particular, Bjerrum discovered a problem in case one would consider the electron as a Brønsted base that can form a hydrogen atom when combining with H+ (most people consider this merely a redox reaction). Furthermore, Brønsted acids could not be seen as Lewis acids, but rather as a product of the interaction of the Lewis acid “proton” with any Lewis base. To solve this issue, Bjerrum proposes “antibase” in lieu of “Lewis acid”. Now, the Lewis acid–base interaction takes place between antibase and base, and the term “acid” would be reserved for proton donators. Bjerrums idea is little known, and most chemists prevent that confusion by using the terms “Lewis acid”, “Lewis base” as well as “Brønsted acid” and “Brønsted base”.

Heinz Gehlen saw the similarities between Usanovich’ and Bjerrums ideas.

Inconsistencies

A major problem in Usanovich' theory seems to be that electrons are considered (being very similar to) anions. However, including electron transfer, and thus, redox reactions results in major inconsistencies. The reason for this is that electrons cannot be compared to chemical particles like atoms, ions and molecules. The mass of an electron is more than 2000 times smaller than that of a proton. Moreover, electrons are responsible for the chemical bonding be it covalent, ionic, or metallic bonds. Also van-der-Waals interactions find their causes in changes within and between electron shells of the regarding atoms and molecules. All this greatly distinguishes the electron from the proton let alone other chemical particles. Therefore, many analogies between proton exchange, anion exchange and electron exchange are questionable. The domains of acid–base chemistry and redox chemistry should be kept separated. In redox chemistry the change of oxidation numbers is essential, whereas in acid–base chemistry no matter what concept, it is not.

The foundation for all the classic acid–base concepts is Lewis’ valence bond theory and acid–base concept, where Lewis acids are searching for electron pairs and bases have electron pairs available, usually lone (free) electron pairs, but pi-electrons are possible either, sometimes even sigma bond electron pairs. In all these concepts, the oxidation state does not necessarily change. Oxidation and reduction would form different acids or destroy the acid (or base) altogether.

It is a break with the original acid–base idea to include electron transfer reactions that lead to change in oxidation state, i.e. redox reactions. Scheme 1 depicts an example of inconsistencies arising from this inclusion of redox chemistry: In the classic oxidative melt to detect manganese (or chromium), manganese dioxide would play both roles simultaneously: the acid because it accepts oxide ions, as well as the base because it donates electrons as the reducing agent.

Scheme 1.
scheme 1

Inconsistencies arising from inclusion of the electron alongside anions in the group of bases. Manganese dioxide appears as both Usanovich (Lux-Flood) acid and Usanovich base

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It has long been clear that redox chemistry and acid–base chemistry can meet in coupled equilibria in case that proton transfer plays a role. In this case, the hydrogen ions would appear as reactants together with the oxidizer in the Nernst equation.

$${\text{MnO}}_{{4}}^{ - } + {\text{ 8H}}^{ + } + {\text{ 5e}}^{ - } = {\text{ Mn}}^{{{2} + }} + {\text{ 4H}}_{{2}} {\text{O}}$$
$$E = E^{0} + \frac{0.059V}{5}\log \frac{{[{\text{MnO}}_{4}^{ - } ][{\text{H}}^{ + } ]^{8} }}{{[{\text{Mn}}^{2 + } ]}}$$

That has been accepted and is not to be confused with Usanovich’ idea of seeing electrons as bases.

Conclusion—a modified Usanovich concept

In view of these findings, the most popular acid–base concepts relate to each other and to redox chemistry as depicted in Scheme 2. Here, I took what I call the core definition of Usanovich removing the electron exchange i.e. redox reactions from consideration. Then we have

  • Acids, which either donate cations or accept anions, and

  • Bases, which donate anions or accept cations.

Scheme 2.
scheme 2

Relationships between the fundamental concepts of Guldberg and Waage, Lewis and Pearson, redox chemistry, as well as the classic protic and aprotic acid–base theories

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This way, Lewis’ acid–base concept is the foundation. Usanovich’ theory w/o redox chemistry may be seen as the roof of the building, covering the columns of the classic protic and aprotic acid–base theories, i.e. Arrhenius, Brønstedt, Lux-Flood, and the solvent theory. Solubility equilibrium and coordination chemistry share the same foundation, but are usually not seen as acid–base chemistries. Finally, on the same, common ground of the Guldberg-Waage quantitative chemical equilibrium theory, we find redox chemistry as a close neighbor; and indeed, all these equilibria are frequently interconnected.