So far, we have discussed a confluence of factors leading to the ability to understand and solve the practical problem of ammonia synthesis. Research in agricultural science led to an awareness of the need for a synthetic source of fixed nitrogen, while developments in organic chemistry helped lead to the infrastructure, capital, and expertise of the German chemical industry. Further conceptual steps in the natural sciences gave rise to the discipline of physical chemistry and a new theoretical framework for chemical reactions. The possibility of synthesizing ammonia from the elements was now plausible, meaning a solution was on the horizon. In 1898, William Crookes’ message to the world on the importance of ammonia synthesis for the production of foodstuffs not only framed the looming humanitarian crisis but also made its economic potential clear. All that was left to determine were the conditions under which ammonia could be synthesized.

In the century before Haber’s work, scientists had taken up the problem of ammonia synthesis and although several patents had been taken out, none were successful (Mittasch 1951; Tamaru 1991). One of the first chemists to undertake the challenge using a physicochemical approach was Wilhelm Ostwald in the 1890s. His experiments were conducted at atmospheric pressure with temperatures between 250 ∘C and 300 ∘C. He also used a catalyst to raise the reaction rate. Any ammonia formed was removed from the continuously circulating reaction gas mixture in a cycle that was also responsible for heat exchange. Ammonia formed at higher temperatures and was extracted at lower temperatures while the excess heat was used to warm incoming gas precursors. Today, we recognize that Ostwald’s methods were, in principle, correct. He did not, however, possess the necessary experimental sensitivity with which a measurement of ammonia equilibrium must be made, nor was he mindful enough of the impact of impure materials. What he thought was a successful synthesis of ammonia from the elements was actually the result of a contaminated experiment: the ammonia detected had been produced from nitrogen in the iron catalyst. It was a young Carl Bosch who, while attempting to reproduce Ostwald’s results at BASF, uncovered the error and dashed Ostwald’s dreams of a lucrative patent for ammonia production (Ostwald 1927, pp. 279–287), (Farbwerke Hoechst 1964), (Holdermann 1960, pp. 40–42).

While Ostwald moved on to different endeavors, work on ammonia synthesis from the elements continued. Reacting to a proposal by the Margulies Brothers from the Austrian Chemical Works in 1903, the physical chemist Fritz Haber began working on the ammonia system (Haber 1903c). Using what amounted to Ostwald’s laboratory method, Haber worked at even higher temperatures and tested different catalysts to further raise reaction rates. In 1905, he published his results (Haber and van Oordt 1905b). At the extreme temperature of 1023∘C he found only 0.012% ammonia in equilibrium with nitrogen and hydrogen. His experiments were successful, but his findings did not support the possibility of an economically viable synthesis as the engineering challenges appeared too great. Although working at higher pressures would have brought Haber more reliable results, he at first decided against the strategy due to technical difficulties. Haber’s publication led to a famous written response from Walther Nernst in 1906 and an exchange at the 14th Meeting of the Bunsen SocietyFootnote 1 in 1907 in Hamburg during which Nernst drew attention to “Haber’s very inaccurate numbers.”Footnote 2 They were, in Nernst’s view, not in accordance with his heat theorem (Nernst 1906, 1907).

Nernst proposed Haber repeat the measurements under higher pressures to increase ammonia yield and improve measurement accuracy. Haber knew this concept well, but only after the motivation of a senior scientist did he make use of it. For his part, Haber (correctly) cast doubt on Nernst’s methodology. The exchange is often characterized as a polemic between two self-absorbed scientists, but it was, in fact, an important event in the development of ammonia synthesis. The interaction between Haber and Nernst enabled a complementary exchange of knowledge and suggestions that guided Haber toward the solution of the ammonia mystery. The key was that they approached the synthesis of ammonia from different perspectives. Haber, experienced in the laboratory, was aware of the required measurement sensitivity as well as the importance of using pure substances. His experimental aptitude was also put to the test when determining equilibrium conditions in the laboratory “from both sides”Footnote 3 with the necessary accuracy. Nernst, ever the theorist, was known for his critical nature and hence focused on how the experiment could be improved on theoretical grounds. In this case, he drew on the physicochemical concept that higher pressures would increase the amount of ammonia present in equilibrium. He also wished to avoid fixing the integration constant for the free energy with an experimental result and so referred self-confidently to his theoretical work. He saw ammonia synthesis as a test for his conception of physical chemistry: theory and experiment must complement one another and ultimately arrive at the same result. We can see explicitly from these approaches how their interaction bridged the gap between experiment and theory. The roles of Haber and Nernst were not restricted to that of the experimentalist and the theorist, however—neither was purely one nor the other—and both knew it. Their experience and abilities assured their interaction would be an exchange of views rather than a lesson from one to the other. Haber, for example, had successfully used entropy in his theoretical thermodynamic derivations while Nernst had an aversion to the quantity. In his initial formulation of the third law, Nernst chose not to mention entropy although one of the quantities he used was equal to the entropy of the system. On the other hand, Nernst had experience in experimental methods reaching back two decades. He saw little difference between pure and applied research (Bartel 1989; Haberditzl 1960; Suhling 1972). Both men understood that the synthesis of ammonia was in principle a balancing act. High temperatures increased the reaction rate, but above a certain temperature, the catalyst became inactive and the decomposition of ammonia began to outpace production. Basic physicochemical principles were not in question.

The scientific breakthrough was only possible through a cooperative effort. As Rudwick puts it in the case of The Great Devonian Controversy, the outcome “was clearly not established by the straightforward victory of one interpretation or theory over its rival or competitor (Rudwick 1985, p. 405).” Haber began with Ostwald’s experimental plan and benefited from Nernst’s suggestions. Nernst’s criticism and insistence on theoretical stringency was the reason Haber was able to see more in his measurements than merely an approximate determination of the integration constant for the free energy. His comments drove Haber toward more fundamental questions. Haber himself can be credited with recognizing the need for increased measurement sensitivity and pure substances, and for his and his assistants’ ability to perform the experiments with high accuracy. They also recognized the effect of an inferior catalyst. In the end, the exchange at the meeting of the Bunsen Society was, along with Haber and Nernst’s other public and private exchanges, vital and fundamental to the scientific breakthrough. It was a dispute among scientists with beneficial results. A similar dynamic still plays out today.

The path to ammonia synthesis in a scientific laboratory setting had ended, but still unresolved was the technical engineering problem of bridging the divide between scientific understanding and an economically viable manufacturing process. It was unclear if it was possible at all.