Karl Hoepfner (or Carl Hopfner) was a German electrochemist. He studied geology, mineralogy, physics and chemistry at Berlin, graduating as Doctor of Philosophy (D Phil) in 1881. His inaugural dissertation, Ueber das gesteine des Monte Tajumbina in Peru... was published in Halle that same year. In 1882 he was sent to southern Angola and western Hereroland (in present Namibia) in search of workable mineral deposits. Landing at Mossamedes about the middle of 1882 he first prospected in southern Angola before setting out overland for Hereroland. After a difficult journey via Humpata and the plains of Ovamboland he reached Okahandja towards the end of January 1883. Among others he visited the ore deposits at Otavi. He also collected some plants, which were sent to Berlin. Several species were named in his honour: Latuca hoepfneriana (Fam. Compositae); Sida hoepfneri (Fam. Malvaceae), the type specimen of which was collected at Mossamedes; and Selago hoepfneri (Fam. Selaginaceae), collected at Okahandja. He described his visit in a paper, "Ueber seine Reise an der Westkueste Sued-Afrikas", in the Verhandlungen der Gesellschaft fuer Erdkunde zu Berlin in 1883.
In August 1884 he returned to Namibia to lead an expedition into the interior for Adolf Luderitz. Accompanied by Luderitz's brother, August, he proceeded to present Okanhandja, where their negotiations with the Herero Chief, Kamaherero, were unsuccessful. They did, however, initiate a "protection treaty" and gain a mining concession from the Bastard Chief, Hermanus van Wyk, at Rehoboth. Hoepfner compiled a report on the copper ore deposits of the territory, "Die Kupfererzlagerstaetten von Sued West Afrika", which was published in Leipzig in the Bergmaennische Zeitung (1884).
In 1883 Hoepfner had registered a patent relating to innovations in the electrolysis of the halogen salts of light and heavy metals, aimed at the extraction of metals from their ores. Though his process could not be applied on an industrial scale it paved the way for the introduction of other electrolytic processes by several firms, especially the chemical firm Grieshelm. After his return from Africa, with support from W. Siemens, he continued his work on the electrolytic recovery of precious and heavy metals directly from their ores, registered further patents and became associated with several large manufacturers. One of the problems with which he achieved some success was the extraction of gold by means of a cyanide solution - a process that later came to be known as the MacArthur-Forrest process. From 1887 to 1889 he directed the chemical technology laboratory of Siemens and Halske. In the latter year he opened his own establishment in Giessen for the electrolytic extraction of copper from its ore, and also worked on the recovery of nickel and zinc. However, he did not publish any of his work in electrochemistry.
In 1899 he established the Hoepfner Refining Company in Hamilton, Ontario, Canada, for the recovery of copper, nickel and zinc. He was investigating the extraction of silver in Denver, Colorado, United States of America, when he died of typhoid.