Dorthea Juliana Wallich was born in 1657 in Weimar, Germany. Around 1685, Wallich became interested in chymistry, which was the name for the chemical practices of the time. In Thurnigia and Saxony, she soon became known as a relatively renowned expert in the field. However, very little is known about her work prior to 1705.
Wallich’s contributions to chemistry mostly revolve around her research on cobalt. Cobalt is a chemical element. Its symbol is Co, and its atomic number is 27, meaning there are 27 protons in the nucleus of every atom of cobalt. Currently, it can be used to make magnets and lithium-ion batteries, in addition to other alloys.
Cobalt has a long history. Its name comes from the word kobold, which was a goblin or evil spirit in German mythology. Saxon miners gave cobalt this name because cobalt ores were dangerous and also did not, despite resembling it, contain any valuable silver. Another interesting and most distinguishable characteristic of cobalt is its color. While the element itself is more of a silvery-bluish color, its compounds were used to color glass and ceramics blue starting from 2000 B.C. Indeed, cobalt’s color-changing properties were at the heart of Wallich’s research.
Wallich worked with a material that she called either “minera,” “marcasite,” or “bismuth.” It contained bismuth metal and cobalt arsenide compounds. After she treated the material with nitric acid, she got a rose-colored solution. She then added sodium chloride solution, producing impure cobalt (II) chloride solutions and impure solid cobalt chloride salt (CoCl2). Her next steps involved performing tests on those two substances to discover their thermochromic effects.
Upon heating, the substances changed from rose to violet-blue to light blue to bright green. It returned to rose upon cooling. While the two substances experienced the same color changes, the mechanisms of doing so are different.
In standard conditions (room temperature, normal rates of humidity), solid cobalt (II) chloride exists as a rose-colored hexahydrate, which is a chemical compound with six molecules of water. Its equation is CoCl2・6H2O. When it’s heated, the water is removed, creating water-free cobalt (II) chloride with the equation CoCl2. And this substance is blue. Then, when the compound cools down, the hexahydrate is again formed. Here is the equation for the dehydration/hydration reaction: CoCl2・6H2O (pink) ⇌ CoCl2 (blue) + 6H2O.
The cobalt (II) chloride solution has to include a concentration and temperature-dependent equilibrium. The equation for this reaction is Co(H2O)62+ (pink) +4Cl-+heat ⇌ CoCl42- (blue)+6H2O. When at constant concentrations, heating shifts the equilibrium to CoCl42+, which is blue. Cooling favors pink Co(H2O)62+.
Wallich’s experiments had very useful repercussions. She was the first person to describe the thermochromic effects of solid cobalt chloride and of cobalt chloride-containing aqueous solutions. And in addition to her temperature-based experiments, Wallich continued her research, reporting on the reaction of her “minera” with nitric acid, citric acid, ammonium chloride, potassium nitrate, sodium chloride, sodium sulfate, antimony sulfide, mercury (II) chloride, and mercury. She also included reactions of her cobalt chloride product with ethanol-water mixtures and vinegar. This all means that, in addition to cobalt chloride, Wallich also produced cobalt nitrate and cobalt oxide and went on to describe some of the properties of these two compounds.
Indeed, in 1737, her thermochromic research served as the foundation for French chemist Jean Hellot’s (1685-1766) development of invisible ink in 1739 that Hellot ultimately presented to the French Académie des Sciences. In 1739, Johann Heinrich Pott (1692-1777), a Berlin chemist published a treatise about bismuth that often referenced Wallich’s work. And currently, chemistry students all over the world watch the thermochromic reactions of cobalt chloride to learn about chemical equilibrium.