The Alchemy of Mineral Acids

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The Alchemy of Mineral Acids

Overview

Geber was the pseudonym of a fourteenth-century alchemist whose books were highly influential during the Middle Ages. He is credited with the discovery of sulfuric acid, whose preparation he described along with that of other strong acids. These acids were capable of dissolving many metals—a property that helped to spur interest in alchemy throughout Europe. Today, these compounds are among the most important industrial chemicals.

Background

Little is known of the alchemist Geber, although he is believed to have been Spanish and to have published his work during the early part of the 1300s. Geber was not his real name (he is often known as the False Geber or Psuedo-Geber), but one he took from Jabir ibn Hayyan (c. 721-815), an influential Arabic alchemist. (Geber is the Latin form of the Arabic name Jabir.) Before the invention of movable type in the fifteenth century, books were produced largely by hand and in only small numbers. As a result, medieval alchemists such as Geber sometimes attributed their work to earlier, well-known scholars in order to increase the likelihood of their books being published.

Geber is known to have produced at least four books: The Sum of Perfection, The Book of Furnaces, The Investigation of Perfection, and The Invention of Verity. (The word perfection in the titles refers to the perfection of metals—the attempt by alchemists to convert base metals such as lead into gold.) Geber's work is unusual in that, unlike the authors of other works on alchemy of the time, he does not merely present the ideas of previous scholars. Instead, he apparently had considerable firsthand experience in working with chemicals and performing his own experiments. In his books, he describes basic chemical procedures such as filtration and distillation (a process in which a liquid is converted to a gas and then condensed back to a liquid). Geber gives instructions for preparing certain compounds, and his writing attempts to teach these methods to others. Compared to the writing of earlier medieval alchemists, his work is fairly clear and less cloaked in mystery. At least partly as a result, his books were widely read among the alchemists of the next two centuries.

Geber's most important contribution to science, however, was his work with acids. Acids are an important class of compounds. According to one definition, an acid is any substance that increases the concentration of hydrogen ions when it is dissolved in water. Certain acids are capable of dissolving a variety of metals. Common organic (carbon-containing) acids, such as citric acid and acetic acid, had long been known to scientists by the Middle Ages. Citric acid was obtained from citrus fruits such as lemons; acetic acid was obtained from vinegar. Organic acids, however, are weak acids. They are very limited in their ability to dissolve metals.

Mineral acids (those that do not contain carbon) are much stronger. They include sulfuric, nitric, and hydrochloric acids, all of which were described by Geber. In fact, he is credited with the discovery of sulfuric acid. To produce this chemical, he described heating what were called vitriols (certain sulfur-containing compounds) to a high temperature. This heating resulted in a chemical reaction that produced sulfur and an oxygen-containing gas as its byproducts. When the gas was cooled, it would condense with water vapor, forming a liquid that could dissolve metals. This liquid, which Geber called oil of vitriol, consisted of sulfuric acid (H₂SO₄) dissolved in water.

Geber described a similar process, using a chemical called niter (today known as potassium nitrate) along with vitriols, to produce nitric acid, which was found to dissolve silver. Geber also explained how mixing sal ammoniac (ammonium chloride) with nitric acid produced an acidic liquid aqua regia. Aqua regia consisted of a mixture of nitric and hydrochloric acids that could even dissolve gold.

Impact

The discovery of mineral acids, along with the translation of earlier Arabic and Greek alchemical texts into Latin, marked the beginning of European alchemy during the Middle Ages. Some consider the discovery of sulfuric acid to be the greatest achievement in chemistry during the medieval period.

One of the central goals of alchemy that was promoted by Geber was the transformation of base metals into gold. (A base metal was considered to be any metal that was not gold.) In order to do so, it was believed that base metals would first need to be "unmade", or dissolved, as one step in their transformation. Therefore, part of the alchemists' quest was to find a so-called universal solvent, a substance that was capable of dissolving any other substance. The strong mineral acids provided them with a solution to this problem. Because aqua regia could dissolve gold along with all other known metals, they considered it to be a close approximation of the universal solvent. Since alchemists could now "unmake" gold, it seemed within the realm of possibility of that they would soon be able to reverse this process and create gold from other substances.

However, alchemists incorrectly assumed that when a metal was dissolved in acid, it was broken down into its elements. Geber, for example, held to the belief that all metals are composed of the elements sulfur and mercury, an idea that had been proposed by his role model Jabir. Because only the proportions of the two substances were thought to differ from one metal to another, it was believed that one metal could be transformed into another simply by changing these proportions. What the alchemists didn't realize was that most of the metals with which they worked—gold, silver, iron, tin, and lead—were actually elements themselves. As a result, they cannot be broken down into simpler substances. (Dissolving metals in acid causes them to lose electrons, changing neutral metal atoms into ions—atoms bearing an electrical charge. However, this process does not change the identity of the element itself. Metallic lead, for instance, dissolves as lead ions, not as the ions of some other element.)

The discovery of mineral acids helped to bring respect to the study of chemicals and their properties. It also allowed alchemists to perform experiments that were previously impossible. However, the experiments involving these acids continued to focus primarily on potential gold-making techniques. As a result, mineral acids were produced only in small quantities by individual alchemists usually working in isolation. (One practical application was soon found for nitric acid. When a silver-coated gold coin was placed in a nitric acid solution, the silver would appear to be "transformed" to gold as the silver dissolved. This trick was used by swindlers to convince wealthy patrons to invest their money in gold-making schemes. Once the funds were handed over, however, the swindlers would never be heard from again.)

It would be several centuries before this class of chemicals had more than a minor impact on society at large. Sometime in the mid-1700s, it was discovered that sulfuric acid was useful in dyeing wool with indigo. Normally, wool does not absorb the rich blue color of indigo very easily. However, when the wool was first treated with sulfuric acid, the dye would bind to the cloth quite well. Because indigo blue (the same color still used to dye many blue jeans today) proved to be such a popular color, great quantities of sulfuric acid were soon in demand. Cheaper and quicker methods of manufacturing the acid were soon developed. As the price of sulfuric acid fell, chemists found more and more uses for it.

Meanwhile, in England, hat-makers had discovered that a solution of mercury and nitric acid could be used in the manufacture of popular felt hats. And in the Netherlands, it was found that when tin was dissolved in aqua regia and then added to a purple dye, a brilliant scarlet hue was produced that could be used to color silk.

It wasn't until the late 1700s that chemists first began to investigate the chemical nature of acids. They began to ask how and why acids are capable of behaving as they do instead of simply investigating what they can do. At that time, Antoine Lavoisier proposed that acidity was caused by the presence of oxygen in a compound. His proposal turned out to be incorrect, but it turned attention toward further study of this class of compounds. It was not until the twentieth century that a full understanding of the chemical nature of acids was finally achieved. Today, Geber's oil of vitriol is the most important industrial chemical worldwide. Its chief use is in the manufacture of fertilizers, but it is also used in the production of drugs, explosives, detergents, and automobile batteries. Nitric acid is used to make many of the same products.

STACEY R. MURRAY