Justus von Liebig

How to use this course

Justus von Liebig changed chemistry in three ways. He made the analysis of organic compounds fast and reliable, so that formulas could be measured rather than guessed. He invented, at Giessen, the research laboratory in which students learn by doing original work, and his pupils carried it across Europe and to America. And he argued, loudly and often wrongly in detail, that agriculture, nutrition and medicine should rest on chemistry. His journal, the Annalen, was for decades the leading chemical periodical in the world.

Each technical lesson opens with a paragraph for the general reader, then gives precise statements, worked chemistry and exercises with folded solutions. Formulas are written in modern notation; where Liebig’s own formulas differ, the lesson says so. His lifelong collaborator Friedrich Wöhler has a companion module, and the two courses share several episodes from different sides.

Timeline

Darmstadt, 1803–1819

Justus Liebig was born in Darmstadt on 12 May 1803, the son of Johann Georg Liebig and Maria Caroline Möser. His father sold and made drysalters’ goods: dyes, varnishes, pigments, and the acids and salts used to prepare them. The shop had a small workshop where the boy watched and copied the preparations. He borrowed chemistry books from the grand-ducal court library and worked through them by trying the experiments, a habit of learning by hand that shaped his later teaching.

School went badly. He attended the Ludwig-Georgs-Gymnasium, where classics dominated, and left at about fourteen without a leaving certificate. In 1816, when he was thirteen, the eruption of Tambora the year before brought the “year without a summer” to central Europe. Harvests failed and grain prices soared in Hesse. Liebig’s later conviction that chemistry should raise the yield of the land is often traced to that hunger.

His father placed him with the apothecary Gottfried Pirsch in Heppenheim. The apprenticeship lasted less than a year, and he returned to Darmstadt to help in the family business. The young Liebig had already met fulminates, the shock-sensitive salts used in toy “bangers” sold at fairs. They would be the subject of his first research.

Bonn, Erlangen and Paris, 1819–1824

In the autumn of 1819 Liebig enrolled at the University of Bonn to study chemistry under Karl Wilhelm Gottlob Kastner. Kastner taught an older, speculative chemistry and had little practical laboratory instruction to offer. When Kastner moved to Erlangen in 1821, Liebig followed him. In Erlangen he joined a student corps and was drawn into the political agitation of the student movement. After a confrontation with the authorities in March 1822 he left the town and went home.

Kastner then obtained for him a grant from the Hessian government to study in Paris, at that time the centre of the chemical world. Liebig arrived in the autumn of 1822 and heard the lectures of Louis Jacques Thénard, Louis Nicolas Vauquelin and Joseph Louis Gay-Lussac. Meanwhile Erlangen awarded him a doctorate in absentia in June 1823 on the strength of his written work, at Kastner’s request.

The turning point was his research on fulminates. He presented results on silver fulminate to the Académie des sciences, and Alexander von Humboldt, who heard him, took an interest in the young German. Humboldt introduced him to Gay-Lussac, who admitted him to his private laboratory. There Liebig learned exact analytical method, and in 1824 the two men published a joint analysis of silver fulminate. He later said that his whole career had been decided by that introduction. He returned to Darmstadt in April 1824.

A professor at twenty-one, 1824–1831

On Humboldt’s recommendation, and over the objections of some of the faculty, the Grand Duke of Hesse appointed Liebig extraordinary professor at the University of Giessen on 26 May 1824. He was 21 and had no habilitation. On 7 December 1825 he became full professor of chemistry and pharmacy. In May 1826 he married Henriette (“Jettchen”) Moldenhauer; they had five children, and their daughter Johanna later married the surgeon Karl Thiersch.

Giessen was a small university with almost no laboratory. Liebig was given part of an old barracks, and he paid for much of his equipment himself. To support the work, he and colleagues ran a private institute for pharmacists and technical students from the late 1820s until 1833, when it was taken into the university. The arrangement suited his method. Students learned qualitative and quantitative analysis by practice, then undertook a piece of original research under his direction. Within a decade this became the model of the research laboratory (Lesson 11).

The same years brought the dispute that founded his most important friendship. In 1824 Friedrich Wöhler, working in Stockholm, analysed silver cyanate and found the same composition as Liebig’s silver fulminate. Liebig at first suspected an error. Rechecking convinced both men that the analyses were right and that two different substances could share one composition. They met in person soon after, and from 1826 they corresponded until Liebig’s death. Lesson 04 tells the chemistry.

Fulminates, cyanates and isomerism

For the general reader. Silver fulminate explodes if you scratch it. Silver cyanate is a harmless white powder. Yet when Liebig and Wöhler analysed them in the 1820s, they found the same elements in the same proportions. Chemists had assumed that composition determines a substance. The fulminate–cyanate pair, with a handful of other cases, forced them to accept that the arrangement of atoms also matters. Berzelius called this isomerism.

The evidence

Mercury fulminate had been prepared by Edward Howard in 1800 from mercury, nitric acid and alcohol, and silver fulminate is made the same way from silver. Liebig and Gay-Lussac determined its composition in 1824 by decomposing it and measuring the silver and the gases released. Wöhler, studying the salts of cyanic acid under Gmelin and Berzelius, prepared silver cyanate from cyanogen compounds. Both salts contain one atom each of silver, carbon, nitrogen and oxygen, \(\mathrm{AgCNO}\). After an exchange of polemics in 1824–25, each confirmed the other’s numbers.

Berzelius’s other examples were tartaric and racemic acid, and the two forms of tin oxide. Wöhler’s synthesis of urea from ammonium cyanate in 1828, which Liebig welcomed, gave a further case.

The modern picture

Both anions are linear and contain sixteen valence electrons. They differ in the order of the atoms:

In cyanate the carbon sits in the middle and every atom has a satisfactory formal charge. In fulminate the nitrogen is central, carrying a formal positive charge, with a negative charge on the terminal carbon. The weak, polarized N–O bond and the high energy of the arrangement explain why fulminates detonate. Free fulminic acid, HCNO, was isolated pure only in 1966.

Combustion analysis and the Kaliapparat

For the general reader. To know what an organic compound is, a chemist first needs its formula, and that means knowing how much carbon and hydrogen it contains. Before 1830 the measurement took days and only a few masters could do it. Liebig designed a simple glass device, five bulbs of potash solution, that caught the carbon dioxide from a burned sample so it could be weighed. A student could now do a reliable analysis in an afternoon, and organic chemistry became a science of measured formulas.

Before Liebig

Gay-Lussac and Thénard (1810) burned organic substances with potassium chlorate and measured the gases by volume. Berzelius improved the method with copper oxide and weighed absorbers. Both procedures were slow and delicate. Gas volumes had to be corrected for temperature, pressure and moisture, and losses were hard to detect.

The method of 1831

The dried sample, mixed with an excess of copper(II) oxide, is placed in a glass tube and heated progressively along its length over charcoal. The copper oxide oxidizes it completely:

The gases pass through a weighed tube of calcium chloride, which absorbs water, and then through the Kaliapparat, a blown-glass device of five bulbs arranged in a triangle. The three lower bulbs hold concentrated potassium hydroxide solution, which absorbs carbon dioxide; the end bulbs prevent the liquid from being sucked back or blown out. At the end, air is drawn through to sweep the last gas into the absorbers. Both absorbers are reweighed.

Two features made it robust. Everything was weighed, so no gas-law corrections were needed. And the bubbles in the last bulb told the operator whether absorption was complete. The device was cheap and easy to copy. A stylized Kaliapparat has been part of the emblem of the American Chemical Society since 1909.

Radicals: benzoyl and ethyl

For the general reader. Inorganic chemistry had elements that pass unchanged through reactions. Liebig and Wöhler found that in organic chemistry whole groups of atoms behave this way. Starting from oil of bitter almonds, they made a family of compounds that all contained the same seven-carbon group, which they called benzoyl. Organic chemistry, Liebig hoped, would be the chemistry of such compound radicals.

The benzoyl series (1832)

In the summer of 1832 Wöhler spent several weeks in Liebig’s laboratory, and the two men studied oil of bitter almonds. They showed that the oil, benzaldehyde, oxidizes in air to benzoic acid, and that chlorine converts it to a chloride from which a whole series follows. In modern notation, with \(\mathrm{Bz}=\mathrm{C_6H_5CO}\):

They also made the bromide and iodide, and the ethyl ester. The group \(\mathrm{C_7H_5O}\) survived every step. Berzelius greeted the paper as the dawn of a new day in organic chemistry, although the radical contained oxygen, which did not fit his dualism comfortably.

The ethyl theory (1834)

Liebig extended the idea to alcohol and ether. He treated ether as the oxide of a radical ethyl, \(\mathrm{(C_2H_5)_2O}\) in modern formulas, and alcohol as its hydrate. Ethyl chloride and ethyl esters were salts of the same radical. This competed with Dumas’s etherin theory, in which ether was a compound of ethylene and water. In 1837 Dumas and Liebig issued a joint note declaring that organic chemistry was the chemistry of compound radicals. The alliance broke apart within a year, when Dumas embraced substitution.

Substitution of hydrogen by chlorine (Lesson 07) eventually showed that radicals are not unalterable. The concept survived in a changed form: the substituent groups of structural chemistry, and the alkyl and acyl groups of every textbook. The free radicals of modern chemistry, with an unpaired electron, are a separate idea from the twentieth century.

Chloral, chloroform and aldehyde

For the general reader. In the early 1830s Liebig passed chlorine through alcohol and isolated a new oily liquid, chloral. Treating it with alkali gave another liquid, chloroform, which within two decades would put surgical patients to sleep. A few years later he oxidized alcohol to a compound he named aldehyde, and found that it deposits a mirror of silver from silver salts. Each of these reactions is still taught.

Chloral and chloroform (1831–1832)

Exhaustive chlorination of ethanol oxidizes and chlorinates it to trichloroacetaldehyde, which Liebig named chloral from the first syllables of chlorine and alcohol. With aqueous alkali chloral splits cleanly:

Chloroform was found independently in 1831 by Samuel Guthrie in the United States and Eugène Soubeiran in France, both from alcohol and bleaching powder. Liebig at first gave it a formula without hydrogen and called it a carbon chloride. Dumas established the correct composition in 1834 and named it, noting that alkali converts it to a formate. James Young Simpson introduced it as an anaesthetic in Edinburgh in November 1847.

Aldehyde and the silver mirror (1835)

Liebig oxidized alcohol and isolated acetaldehyde, naming it from alcohol dehydrogenatus, alcohol deprived of hydrogen. He noticed that aldehydes reduce ammoniacal silver solutions to metallic silver. In 1856 he developed the reaction into a method for silvering glass for the Munich optician Carl August von Steinheil. Silver deposited chemically on glass replaced the toxic mercury–tin amalgam of older mirrors and later coated the glass mirrors of reflecting telescopes.

Polybasic acids and the hydrogen theory

For the general reader. What makes an acid an acid? Lavoisier said oxygen; Berzelius treated acids as oxides. Liebig, in 1838, argued that the essential ingredient is hydrogen that a metal can replace, and that some acids carry two or three such hydrogens. This is the view every student now learns, and it began in a study of citric, tartaric and other plant acids.

Background

Humphry Davy had shown in 1810 that hydrochloric acid contains no oxygen. Pierre Louis Dulong suggested in 1815 that oxalic acid is a compound of hydrogen with a carbon–oxygen group, and that salts form when a metal replaces the hydrogen. In 1833 Thomas Graham showed that there are three phosphoric acids, ortho-, pyro- and meta-, combining with three, two and one equivalents of base. Liebig took Graham’s result as the model for organic acids.

Liebig’s evidence was analytical. He prepared salts of citric, tartaric, meconic and other acids, measured their metal and water content, and showed that, for example, citric acid takes up three equivalents of silver. The existence of acid salts and of double salts with two different metals on the same acid was the decisive criterion for polybasicity. Charles Gerhardt, a former student, later gave the hydrogen theory its general form within type theory.

Agricultural chemistry and the law of the minimum

For the general reader. In 1840 Liebig published a book that told farmers that plants do not eat the humus of the soil. They build themselves from carbon dioxide in the air, water, and a small set of mineral salts taken by the roots. Whatever the farm removes in crops must be returned, and the yield is held back by whichever nutrient is scarcest. The book made him famous and helped create the fertilizer industry, though several of its specific claims were wrong.

Against humus

Albrecht Thaer’s humus theory, dominant in German agriculture, held that plants draw their carbon from decayed organic matter in the soil. Liebig, building on the measurements of Nicolas-Théodore de Saussure, argued that the humus in a field could not supply a fraction of the carbon harvested year after year, while the carbon dioxide of the atmosphere could. The plant needs from the soil only water, nitrogen in some form, and minerals: potash, phosphate, lime, magnesia, silica and others. The book, Die organische Chemie in ihrer Anwendung auf Agricultur und Physiologie, was written in response to a request from the British Association for the Advancement of Science and appeared in 1840.

The principle had been stated earlier by Carl Sprengel, and is now often called the Sprengel–Liebig law. Liebig gave it wide currency and added non-chemical factors such as light and warmth. In a simple quantitative form, if nutrient \(i\) is available in amount \(x_i\) and supports at most yield \(a_i x_i\), then

This “linear-plateau” form is still used in fertilizer recommendation. A smoother alternative, introduced by Eilhard Alfred Mitscherlich in 1909, assumes diminishing returns: \(Y=A\big(1-e^{-c(x+b)}\big)\). Ecologists use the minimum of several saturating (Michaelis–Menten) terms to model phytoplankton limited by nitrogen, phosphorus or iron. The barrel with staves of unequal length, often called Liebig’s barrel, is a later illustration.

Errors and corrections

In the 1840s Liebig came to hold that plants obtain enough nitrogen as ammonia from rain and air, so that nitrogen fertilizer was unnecessary. The long-term field trials begun by John Bennet Lawes and Joseph Henry Gilbert at Rothamsted in England showed that crops such as wheat respond strongly to added nitrogen. A fertilizer marketed under Liebig’s name in the mid-1840s failed in the field: it lacked nitrogen, and he had fused the minerals into insoluble forms so that rain would not wash them away, which also kept them from the roots. In 1863 he acknowledged the importance of nitrogen fixed biologically by plants. Sulfuric-acid treatment of bone and rock phosphate, producing soluble superphosphate, became the basis of the industry instead. Late in life Liebig warned that cities which discharged their sewage into rivers were exporting the soil’s minerals for good, an early argument for recycling nutrients.

Animal chemistry, food and fermentation

For the general reader. After plants, Liebig turned to animals and people. He described the body as a chemical system that burns food to make heat and builds tissue from protein. He sold a meat extract that bore his name for a century, designed a formula for infants, and fought Pasteur over whether fermentation is the work of living yeast. Some of his ideas were sound and influential; others were disproved within his lifetime.

Animal Chemistry (1842)

In Die Thier-Chemie, published in English as Animal Chemistry, Liebig divided foods into nitrogenous “plastic” foods (proteins), which build tissue, and non-nitrogenous “respiratory” foods (fats and carbohydrates), which are burned to keep the body warm. Following Lavoisier, he treated animal heat as the product of slow combustion. He proposed that muscular work consumes the protein of the muscle itself, so that the nitrogen excreted in urine measures the work done.

Adolf Fick and Johannes Wislicenus climbed the Faulhorn in the Bernese Alps on a protein-free diet and collected their urine. The protein oxidized could not account for the work of lifting their bodies, let alone the heat produced. Fats and carbohydrates were the main fuel. Liebig’s own Annalen had in 1842 published Mayer’s first statement of the conservation of energy, and the experiment applied that principle against Liebig’s theory.

Meat extract

In 1847 Liebig published a method for concentrating the soluble parts of beef into a paste. He hoped to bring the nourishment of meat to the poor of Europe, but beef was too expensive there. In the 1860s the engineer Georg Christian Giebert proposed making it in Uruguay, where cattle were raised for hides and much meat was wasted. Liebig’s Extract of Meat Company was formed in 1865 with Liebig as scientific director, producing at Fray Bentos; the company later created the Oxo brand. The extract contains flavour and salts but little protein or energy, as critics pointed out, and Liebig came to present it as a stimulant and restorative.

Fermentation

In 1837 Charles Cagniard-Latour, Theodor Schwann and Friedrich Kützing independently concluded that yeast is a living organism that multiplies during alcoholic fermentation. Liebig rejected this. In his theory, fermentation is a chemical decomposition: an unstable substance in the process of decay communicates its molecular motion to the sugar, which falls apart into alcohol and carbon dioxide. In 1839 his journal printed an anonymous satire, generally attributed to Wöhler with Liebig’s help, that depicted yeast as tiny animals eating sugar and excreting alcohol. From 1857 Pasteur showed that fermentation accompanies the growth of living yeast and that different microbes produce different fermentations. Liebig replied in 1869–70. The question was resolved in 1897, when Eduard Buchner obtained fermentation from a cell-free yeast extract. The agent is a set of enzymes made by living cells: Pasteur was right about the cause, Liebig about the chemical nature of the process.

The Giessen school and the Annalen

Liebig did not invent laboratory teaching. Thomas Thomson in Glasgow, Friedrich Stromeyer in Göttingen and others had let students work at the bench. What Liebig built at Giessen was a system. Beginners worked through a graded course of qualitative and quantitative analysis, much of it organic analysis with the Kaliapparat. Once competent, each student was given a research problem, usually a piece of Liebig’s own programme, and the results appeared in the Annalen. The method produced a steady stream of new compounds and trained researchers in large numbers. By 1852 more than 700 students of chemistry and pharmacy had passed through his hands. The laboratory was rebuilt and enlarged as numbers grew; the building survives as the Liebig Museum in Giessen.

His students came from all over Europe and from America. They included August Wilhelm von Hofmann, who studied and assisted at Giessen from 1836 to 1845 and then became the first director of the Royal College of Chemistry in London; August Kekulé; Hermann Kopp, the historian of chemistry; Hermann von Fehling; Lyon Playfair; Henri Victor Regnault; Nikolay Zinin; Emil Erlenmeyer; Adolph Strecker; Max von Pettenkofer; Carl von Voit; and Charles Gerhardt. Agricultural chemists trained in his tradition founded experiment stations in Germany, Britain and the United States.

From 1832 Liebig edited the Annalen der Pharmacie, renamed Annalen der Chemie und Pharmacie in 1840. He soon pushed out co-editors who disagreed with him and made it his own journal, with Wöhler as co-editor from the late 1830s. It published the work of his school, of his allies, and, in sharp editorial notes, his attacks on opponents. After his death it was renamed Justus Liebigs Annalen der Chemie; it was absorbed into the European Journal of Organic Chemistry in 1998.

Liebig also wrote for the public. His Chemische Briefe, letters on chemistry first printed in the Augsburger Allgemeine Zeitung from 1841, appeared in English as Familiar Letters on Chemistry and went through many editions. They argued that chemistry was the key to agriculture, industry and health.

Munich, controversy, legacy, 1852–1873

On 29 December 1845 Grand Duke Ludwig II of Hesse made Liebig a Freiherr, a hereditary baron. Several universities tried to recruit him. In 1852 he accepted the call of King Maximilian II of Bavaria to Munich, on terms that relieved him of routine laboratory teaching. He was given a new institute, a residence and a lecture theatre seating about 300, where he lectured to students and gave public lectures attended by the court and the city. He became president of the Bavarian Academy of Sciences in 1858 and acted as scientific adviser to the king. After Maximilian’s death in 1864, he and other Protestant scientists brought in from the north faced growing hostility in Catholic Bavaria.

His research in Munich turned to applications: food, the silvering of mirrors, an infant food introduced in the mid-1860s, and the meat extract. He continued to fight. He quarrelled at various times with Dumas, Laurent, Gerhardt, Mulder, the Rothamsted agriculturists, and Pasteur, often in terms his friends regretted. Wöhler, whose temperament was the opposite, repeatedly counselled moderation. Their correspondence, edited by Hofmann and published in 1888, is one of the great documents of nineteenth-century science.

He received the Copley Medal of the Royal Society in 1840 and the Albert Medal of the Royal Society of Arts in 1869, and was a member of most European academies. He died of pneumonia in Munich on 18 April 1873, aged 69, and was buried in the Alter Südfriedhof.

Legacy

• Method: routine organic combustion analysis, which turned formulas into measured quantities and underlies elemental analysis today.
• Institutions: the teaching-and-research laboratory and the research school, copied across Germany, Britain and America; the Annalen.
• Theory: isomerism (with Wöhler), compound radicals, the hydrogen theory of polybasic acids.
• Applied science: the mineral theory of plant nutrition and the law of the minimum; the idea that agriculture, nutrition and physiology are chemical subjects.
• Eponyms: the Liebig condenser, a counter-current water condenser he popularized (he credited earlier designers); Liebig’s law; Justus-Liebig-Universität Giessen.

Connections

People linked to Justus von Liebig across this collection. A name without a link has no page here yet. See Justus von Liebig in the network of all 337 lives →

Teachers and advisors

• Joseph Louis Gay-Lussac 1822-1824
• Karl Wilhelm Gottlob Kastner 1819-1823

Students

• August Kekulé
• Adolph Strecker
• August Wilhelm von Hofmann 1836-1845
• Carl von Voit
• Charles Gerhardt
• Emil Erlenmeyer
• Henri Victor Regnault
• Hermann Kopp
• Hermann von Fehling
• Lyon Playfair
• Max von Pettenkofer
• Moritz Traube
• Nikolay Zinin

Influenced by

• Carl Sprengel

Colleagues and collaborators

• Alexander von Humboldt
• Friedrich Wöhler 1826-1873
• Julius Robert Mayer
• Jöns Jacob Berzelius
• Robert Bunsen 1832

Rivals

• Louis Pasteur
• Jean-Baptiste Dumas

Friends

• Alexander von Humboldt

Family

• Karl Thiersch

Also mentioned in this life

• Stanislao Cannizzaro
• John Dalton
• Humphry Davy
• Joseph Henry
• Antoine Lavoisier

Fields

• organic chemistry 4 lives
• analytical chemistry 5 lives

Sources and further reading

Biography

1. Justus von Liebig, Wikipedia; and the German article Justus von Liebig.
2. W. H. Brock, Justus von Liebig: The Chemical Gatekeeper, Cambridge University Press, 1997.
3. W. H. Brock, Justus, baron von Liebig, Encyclopaedia Britannica.
4. Science History Institute, Justus von Liebig and Friedrich Wöhler.
5. A. W. Hofmann (ed.), Aus Justus Liebig’s und Friedrich Wöhler’s Briefwechsel in den Jahren 1829–1873, Vieweg, 1888.
6. J. B. Morrell, The chemist breeders: the research schools of Liebig and Thomas Thomson, Ambix 19 (1972).

Chemistry

1. J. Liebig and J. L. Gay-Lussac, Analyse du fulminate d’argent, Annales de chimie et de physique 25 (1824).
2. J. Liebig, Ueber einen neuen Apparat zur Analyse organischer Körper, Annalen der Physik und Chemie 21 (1831). See also Kaliapparat, Wikipedia.
3. F. Wöhler and J. Liebig, Untersuchungen über das Radikal der Benzoesäure, Annalen der Pharmacie 3 (1832).
4. J. Liebig, Ueber die Constitution der organischen Säuren, Annalen der Pharmacie 26 (1838).
5. J. Liebig, Die organische Chemie in ihrer Anwendung auf Agricultur und Physiologie, Vieweg, 1840.
6. J. Liebig, Die Thier-Chemie oder die organische Chemie in ihrer Anwendung auf Physiologie und Pathologie, Vieweg, 1842.
7. Liebig’s law of the minimum, Wikipedia.
8. Liebig–Pasteur dispute, Wikipedia.
9. Chloroform and Annalen der Chemie, Wikipedia, history sections.
10. A. J. Rocke, Chemical Atomism in the Nineteenth Century: From Dalton to Cannizzaro, Ohio State University Press, 1984.