Potassium alum

Potassium alum is a vital chemical compound known by several names. Scientists call it potassium aluminium sulfate, and it is often referred to as potash alum. Its chemical formula is KAl(SO4)2. In many cases, it appears as a dodecahydrate, written as KAl(SO4)2·12H2O. This means twelve water molecules are attached to the compound.

It is the most important member of a group of substances called alums. This compound is highly versatile and serves many different industries.

The way alum forms crystals depends on its environment. In a neutral solution, it forms an octahedral structure, which is a shape with eight faces. However, if the solution is an alkali, it forms a cubic structure instead. The crystals have a specific lattice parameter of 12.18 Å. When the compound is heated to nearly a red heat, it turns into a porous, crumbly mass called burnt alum. It can also fuse at 147 °C in its own water of crystallization. These physical properties make it useful for many different chemical processes.

Potassium alum has many practical applications in modern life. In water purification, it is used to reduce turbidity, which is the cloudiness of water. In the textile industry, it acts as a mordant. A mordant is a substance that fixes dyes onto fabric to keep colors bright and fast. It is also used in leather tanning and making fireproof textiles. You might even find it in your kitchen as a component of baking powder, labeled as E522. For personal care, it serves as a deodorant, an aftershave, or a styptic to stop minor bleeding from shaving.

Humans have used alum since ancient times. As early as 1500 BCE, Ancient Egyptians used it to clear cloudy water. In Mesopotamia, people used it for glass-making, tanning, and dyeing cloth or wood. Archaeological evidence shows that people on the Greek island of Lesbos produced alum from a mineral called alunite. The Romans knew it as alumen or salsugoterrae. In India, ancient medical texts like the Charak Samhita mention it by names such as phitkari. In China, it is known as mingfan.

During the Middle Ages, alum became a major economic force. In the 13th and 14th centuries, Genoese and Venetian traders imported alum from Phocaea. This trade was so important that it caused wars between Genoa and Venice. Later, after the fall of Constantinople, a source of alunite was found at Tolfa in the Papal States in 1461. In England, alum was essential for the wool industry. Because imports from the Papal States were unreliable, England tried to make its own. In the 17th century, an industry in Yorkshire processed shale to produce alum. This industry helped contribute to the Industrial Revolution. However, the process was harsh. Workers burned shale for months, which used massive amounts of firewood and polluted the land with sulfuric acid and ash.

Scientists worked for many years to understand the true composition of alum. In the early 1700s, Georg Ernst Stahl mistakenly claimed it was made from sulfuric acid and limestone. This error was corrected by Johann Pott and Andreas Marggraf. They proved that the substance involved was alumina, which is found in common clay. Marggraf showed that perfect crystals could be made by dissolving alumina in sulfuric acid and adding potash or ammonia. In 1767, Torbern Bergman discovered that only potassium or ammonium sulfates could convert aluminium sulfate into alum.

The final piece of the puzzle came in the late 1700s. In 1797, Martin Klaproth discovered that potassium was present in minerals like leucite and lepidolite. This was surprising because people previously thought potassium only came from plants. Shortly after, Louis Vauquelin proved that alum is a double salt. He showed it is composed of sulfuric acid, alumina, and potash. Jean-Antoine Chaptal confirmed these findings by analyzing different types of alum from around the world. These discoveries connected the study of minerals to the broader field of chemical composition.