Sodium hydroxide (\(\text{NaOH}\)), commonly known as caustic soda or lye, is an inorganic compound. This chemical is a white, odorless solid that functions as a strong base, making it one of the most important compounds in modern industrial chemistry. The question of when it was “discovered” is complex, as its caustic properties were utilized for millennia before chemists identified it as a distinct compound. Early civilizations harnessed its power from natural sources, but its true chemical identity was established only recently.
Early Use of Lye in Ancient Processes
Long before the chemical formula \(\text{NaOH}\) was known, people utilized a crude form of lye derived from plant ashes. Evidence from ancient Babylon, dating back to approximately 2800 BC, shows the use of a soap-like substance made by boiling fats with this alkali. The ancient Egyptians around 1550 BC also used alkaline salts, often called potash, mixed with oils for cleaning and medicinal purposes.
This early lye was produced by leaching, where water was filtered through wood ashes to dissolve the soluble alkaline materials. The resulting liquid was an impure mixture, predominantly containing potassium carbonate and potassium hydroxide (\(\text{KOH}\)). If made from ashes of seaside plants or mineral deposits, it sometimes contained sodium carbonate. Users understood the substance’s caustic nature and its ability to react with fats to create soap, but they could not chemically separate or identify the specific sodium compound. The practical application of this material in making soap and processing textiles was developed through empirical observation over thousands of years.
The Chemical Identification of Sodium Hydroxide
The chemical discovery of sodium hydroxide as a distinct compound occurred during the late 18th and early 19th centuries, following a revolution in chemical understanding. Chemists began to distinguish between “fixed” alkalis, like soda (sodium-based), and “volatile” alkalis, like potash (potassium-based). This conceptual groundwork led to the isolation of the elemental components.
The definitive breakthrough came in 1807 with the work of English chemist Humphry Davy. Davy used the voltaic pile, a powerful new tool, which allowed him to pass an electric current through molten compounds, a process known as electrolysis. He successfully isolated the element sodium by applying this current to caustic soda.
The isolation of metallic sodium proved that caustic soda was a compound of sodium, oxygen, and hydrogen, confirming the existence of sodium hydroxide (\(\text{NaOH}\)) as a unique chemical entity. Davy’s accomplishment, which also included the isolation of potassium from caustic potash in the same year, provided the first scientific method to definitively separate the two elements. This work established the compound’s chemical identity and shifted its nomenclature from the general term “lye” to Sodium Hydroxide.
Transition to Modern Industrial Production
The chemical identification of sodium hydroxide paved the way for its mass production as an industrial commodity. An early industrial method, the Leblanc process (invented in 1791), primarily yielded sodium carbonate (soda ash) from common salt. This soda ash could then be reacted with calcium hydroxide (slaked lime) in a process known as the causticizing reaction to produce sodium hydroxide, marking the first large-scale industrial route.
The next revolution in production began in the late 19th century with the development of the electrolytic Chloralkali process. This method involved passing electricity through an aqueous solution of sodium chloride (brine). The process simultaneously yielded chlorine gas, hydrogen gas, and high-purity sodium hydroxide.
The Castner-Kellner process, patented in the 1890s, utilized a mercury cell to prevent the chlorine and sodium hydroxide products from mixing and reacting. This electrochemical technology proved more economical and efficient than older chemical routes. The ability to cheaply and reliably manufacture high-purity sodium hydroxide facilitated its widespread adoption in industries such as paper pulp, aluminum production, petroleum refining, and the manufacturing of soaps and detergents.