Indium (In), the soft, silvery-white metal with atomic number 49, belongs to the group of post-transition metals and possesses a low melting point. The history of its identification in the mid-19th century is a direct testament to the power of newly developed scientific techniques. Its discovery began with an investigation into mining byproducts.
The Context and Initial Search
The discovery took place in 1863 in Freiberg, Saxony, Germany. Professors Ferdinand Reich and Hieronymus Theodor Richter of the Freiberg School of Mines investigated the residues left over from the local industrial processing of zinc ores.
The primary objective was to confirm whether the zinc blend, known as sphalerite, contained the element thallium. Reich and Richter worked with the residue from the sulfide ore, the material remaining after the zinc had been extracted.
This residue was expected to concentrate trace elements, making it the ideal sample for detailed chemical examination. They performed chemical separations to isolate the suspected thallium compound before attempting its final identification.
The Critical Technique and Observation
To confirm the presence of thallium, the researchers employed spectrum analysis, or spectroscopy. This method involves heating a substance until it emits light, which is then separated by a prism into distinct colored lines that act as a unique signature for each element. The expectation was to observe the specific green spectral line that characterizes thallium.
Ferdinand Reich was colorblind, so he relied on his assistant, Hieronymus Richter, to observe the results. When the residue was heated and analyzed, they did not see the expected green line. Instead, the spectrum revealed a previously unknown, brilliant, deep indigo-blue line.
This single, distinct feature was unlike the signature of any known element. The line displayed a remarkable “glow, sharpness and consistency,” leading the researchers to conclude that a new metallic element must be present in the zinc ore residue. This intensely colored line was measured to have a specific wavelength of approximately 451 nanometers.
Isolation, Naming, and Early Characteristics
The observation of the unique indigo line immediately shifted the focus of the investigation to chemically isolating the new element. Reich and Richter began the painstaking process of purification. They succeeded in isolating the new metal from the complex ore matrix.
The element was named “Indium” directly from the color of its spectral signature. The name is derived from the Latin term indicum, which refers to the color indigo, reflecting the deep blue line that announced its existence.
Richter continued the work and managed to isolate the first pure sample of the metal, presenting a 0.5 kilogram ingot at the World Fair in Paris in 1867. The earliest characterizations noted its low density and the peculiar high-pitched “cry” or squeak it emits when a bar of the pure metal is bent.
Despite the collaborative nature of the initial discovery, a dispute arose when Richter later claimed sole credit. Ultimately, both men are recognized for the work that led to the identification of element 49.