The father of physical chemistry

Wilhelm Ostwald, best known for his work on catalysis and chemical affinity, was born 150 years ago this year. Michael Sutton describes the life of this energetic man.

In the course of a strenuous career, Wilhelm Ostwald published 45 books, over 500 articles, and about 5000 reviews. He also edited six scholarly journals, and organised the reprinting of a lengthy series of historic chemical papers. By his 50th birthday he had supervised 147 graduate students, 34 of whom became professors. He was honoured by over 60 universities and learned societies, and in 1909 won the Nobel prize for chemistry. Ostwald took an active interest in philosophical, social and political issues, and devoted much time and effort to the cause of world peace and international understanding. Always interested in the arts, he played the viola in a string quartet when young, and in retirement became an enthusiastic landscape painter. For 52 years he was happily married to Helene von Reyher, with whom he raised two daughters and three sons. It seems entirely appropriate that Ostwald devoted much of his life to the study of energy.

Ostwald’s family belonged to the German community in Riga - now the capital of Latvia, but then part of the Russian Empire. Gottfried Wilhelm Ostwald was a master cooper (barrel maker), and his wife Elisabeth (n?e Leuckel), the daughter of a master baker. Their second child, born on 2 September 1853, was christened Friedrich Wilhelm, and educated at the Riga Gymnasium (high school). His father hoped the boy would become an engineer, but in 1872 Wilhelm entered the University of Dorpat to study chemistry. (Dorpat was also under Russian rule then - renamed Tartu, it is now in Estonia.) At first, Ostwald neglected the official curriculum in favour of art, music, philosophy and the convivial student life. However, some last-minute cramming got him through the theoretical chemistry course, and in 1875 he was admitted to Karl Schmidt’s laboratory. Ostwald soon progressed from routine exercises to original investigations, gaining his doctorate in 1878. Part-time work as a laboratory assistant allowed him to continue research at Dorpat until he became professor of chemistry at Riga Polytechnic in 1881. 

Ostwald remained at Riga until called to a chair at Leipzig in 1887. In that year he founded the Zeitschrift f?r physikalische Chemie, which soon established him as an influential figure in the scientific world. But by then, much of his important research had already been done, far away from the major centres of European science, using inexpensive home-made apparatus. Ostwald believed it was this isolation from the academic mainstream that inclined him towards the unfashionable field of physical chemistry. While most of his contemporaries were busy discovering new substances and reactions, he sought to understand the forces that drove reactions and bound molecules together. For years, he measured the changes in physical parameters that accompanied chemical processes. One early project was an attempt to estimate the relative affinities of various substances, using pairs of compounds with one shared component. 

Suppose, for example, that A and A* are two acids, and B is a base with which they both combine. Then, if a solution of AB is mixed with a solution of A*, some quantity of A is displaced by A*, yielding a mixture of AB and A*B (with proportional quantities of both free acids). Ostwald used delicate volumetric measurements to investigate reactions of this type. Suppose a litre of aqueous solution containing a gram-molecule (mole) of A is added to a litre of a solution of B, also containing one gram-molecule. The final volume will differ slightly from 2l, by an amount we can call v. Again, if 1l of a similar solution of A* is added to a litre of a similar solution of B, the volume of this mixture will differ from 2l by a different amount, v*. And if 1l of an AB solution is mixed with 1l of an equivalent solution of A*, then the volume - call it v# - by which the mixture differs from 2l will indicate how much AB has been converted to A*B. From these volume changes, Ostwald could compare the strengths of the attractive forces between A and B, and between A* and B. 

In the late 1870s Ostwald had considerable success in comparing affinities by measuring the changes in volume (and other physical parameters like refractive index) that accompanied chemical reactions. His results generally agreed with the law of mass action, which relates the quantities of the products of a chemical reaction to the concentrations of the reactants. Two Norwegians, Peter Guldberg and Cato Waage, had stated the law and explored its implications in a series of papers beginning in 1864. However, their work remained virtually unnoticed until Ostwald publicised and extended it in the early 1880s. 

Meanwhile, the Dutch physical chemist Jacobus van’t Hoff (1852-1911) showed how the experimentally derived mass action law could be deduced directly from thermodynamic principles. He found that the molecules of dissolved substances behaved, in some respects, like molecules in the gaseous state. In particular, the universal gas equation, PV = RT often applied accurately to solutions (P representing osmotic pressure, and V the volume containing one gram-molecule of solute). Ostwald was one of the first to recognise the importance of this breakthrough. Unfortunately, there was a difficulty. The gas constant, R, sometimes had to be multiplied by an arbitrary figure to make the equation fit the observations. The significance of this multiplier - ’van’t Hoff’s i factor’ - became clear when the Swedish physical chemist Svante Arrhenius developed his theory of ionic dissociation. 

Ostwald saw the significance of this insight when it emerged - somewhat obscurely - in Arrhenius’ 1884 doctoral thesis on the conductivity of electrolytes. The two became friends and collaborators, and a more accessible version of Arrhenius’ theory appeared in Ostwald’s Zeitschrift f?r physikalische Chemie in 1887. They showed that van’t Hoff’s i factor was not arbitrary - it represented the proportion of dissolved molecules that separated into oppositely charged ions. The osmotic pressure exerted by a solution was recognised as being proportional to the number of dissolved particles, rather than to the number of dissolved molecules. Thus far, Ostwald’s principal role had been to clarify, coordinate and extend the ideas of van’t Hoff and Arrhenius. However, in 1887 he made an original contribution - Ostwald’s dilution law, usually expressed as follows: 

a²/(1-a)v = k

(where a represents the degree of ionic dissociation, v the volume of solution in litres containing one gram-molecule, and k is a constant.) Ostwald soon realised that k was also the equilibrium constant for many dissociation reactions of the form: 

AB 

 A⁺+ B^-

Ostwald and his collaborators showed that hundreds of water-soluble acids and bases obeyed the dilution law, while their behaviour in solutions conformed to the general ionic theory. They hailed this as the start of a new era in chemistry, but others were sceptical. Critics pointed out that only weak electrolytes obeyed the rules discovered by Ostwald, Arrhenius and van’t Hoff - indeed, it was not until the 1920s that Pieter Debye, Erich H?ckel and Lars Onsager developed a satisfactory theory of strong electrolytes, which explained their anomalous behaviour in terms of inter-ionic attraction. 

Besides studying chemical reactions that had already reached equilibrium, Ostwald, together with his research students, also measured the rates at which reactions proceeded towards the equilibrium state. These kinetic investigations demonstrated that while catalysts could alter the rates of reactions, they did not change the proportions of the final products. This discovery had profound implications for industry and for chemical theory. It was cited as the principal reason for Ostwald’s Nobel award - the presenter declaring: ’Catalysis, which formerly appeared to be a hidden secret, has thus become ... accessible to exact scientific study’. 

Ostwald was also interested in measuring the energy absorbed or released in chemical reactions. He quickly recognised that the thermodynamic studies of the American physicist Willard Gibbs were useful in this context. Gibbs’ heavily mathematical papers were hard reading for European chemists, but Ostwald strove to make them more accessible, while always giving the originator full credit. 

By the 1890s Ostwald was deeply absorbed in the theoretical and philosophical problems surrounding the concept of energy. He became convinced that matter was merely ’a mirage which the mind creates to comprehend the workings of energy’. Though accepting that atoms and molecules were convenient symbols for statistical regularities in our observations, he insisted that the basic truths of science should be expressed in terms of energetics, without reference to these hypothetical particles. In 1909, after much controversy, Ostwald was finally persuaded by new physical evidence (including Jean Perrin’s studies of Brownian motion) to accept the reality of atoms. Nevertheless, his stubborn scepticism had prompted a reassessment of the experimental grounds for belief in entities that were not directly observable. 

After retiring from academic life in 1906, Ostwald undertook a wide variety of philanthropic activities. Some produced valuable results, others failed utterly, but all were grounded in a commitment to scientific progress and humanitarian values. His attempt to promote Ido (an improved version of Esperanto) as an international language attracted few converts. His involvement with the utopian biologist-philosopher Ernst Haeckel and his ’Monist League’ accomplished little, while causing Ostwald considerable financial loss. His efforts for Der Br?cke (The Bridge) - an organisation seeking to stimulate and coordinate intellectual and cultural activity across national boundaries - were similarly unfruitful. In the more limited field of scientific collaboration, he played an important part in founding an international Association of Chemical Societies. However, the outbreak of war in 1914 marked the collapse of Ostwald’s hopes for wider international cooperation. 

As a patriot, but not a militarist, Ostwald hoped to see an honourable peace negotiated as quickly as possible. Consequently, he was criticised by fellow Germans for his lack of zeal, and by friends abroad for his unwillingness to condemn the war outright. In this depressing atmosphere, Ostwald turned away from public affairs, and developed one last research project. Always fascinated by the borderlands between science and art, he made a systematic analysis of colour phenomena and our perception of them. His aim was to supplement our subjective and qualitative classification of colours with a quantitative and objective one, and to establish principles of colour harmony, analogous to the harmonic rules of music. 

He achieved considerable success in this endeavour, and his colour classification system was widely adopted. In his final years, Ostwald moved on to more general speculations on the philosophy of aesthetics, and the scientific principles underlying our awareness of harmony and proportion. He died on 4 April 1932. Shortly afterwards, Wilder Bancroft, a former pupil, summed up the impact of his life in these words: ’He was loved and followed by more people than any other chemist of our time’. 

Source: Chemistry in Britain

Acknowledgements

Michael Sutton

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