Potassium nitrate, also known as saltpeter or niter, has played quite a notable role in history, mainly because of its use in the manufacture of gunpowder. While this use has greatly diminished over time, potassium nitrate still plays a significant role in the farming industry as fertilizer, as well as in the manufacture of nitric acid, fireworks and even in food preservation.
Chemistry
Each molecule of potassium nitrate, chemical formula KNO3, contains one atom of potassium, one atom of nitrogen and three atoms of oxygen. Elemental potassium is a soft, lightweight, silver white metal. Nitrogen and oxygen are colorless, odorless gases. Structurally, three oxygen atoms surround the nitrogen atom, forming a tight, flat, triangular NO3 group. This structure is very similar to carbonate or CO3 groups, placing potassium nitrate in the carbonate class of mineral compounds.
Physical Properties
Pure potassium nitrate is white or gray, but impurities can cause the mineral to be tinted slightly yellow or brown. Rarely, potassium nitrate forms large clusters of crystals in a hexagonal shape, most often forming small thin, gray crystal systems that appear orthorhombic, or containing three unequal axes intersecting at right angles. Only individual crystals are translucent or transparent. The specific gravity or relative density of potassium nitrate compared with water is 2.1, and it is easily soluble in water.
Natural Sources
Potassium nitrate is naturally found in caves or other sheltered areas, particularly on soils rich in organic matter. Ancient sea beds in Europe and brine lakes in California are principal natural sources of potassium nitrate. Other major natural sources include crystal deposits on cave walls or from decomposing organic, mostly urea material. Prior to modern techniques for manufacturing potassium nitrate, dung-heaps were a particularly common source for harvesting potassium nitrate. Chile contained one of the largest sources of potassium nitrate due to the uric acid waste or "guano" from sea birds that nested along its coasts. Ammonia from guano decomposition underwent bacterial oxidation, producing large quantities of potassium nitrate as a natural part of the nitrogen cycle.
Synthetic Manufacture
During World War I, Germans needed nitrogen to make explosives. However, the Allies blocked all trade routes, effectively cutting off the supply of potassium nitrate, their source of nitrogen. Using air, which contains 80 percent nitrogen, Fritz Haber and Charles Bosch, German chemists, developed and patented the Haber process, which takes molecular nitrogen from the air and combines it with molecular hydrogen under high pressure and heat to form ammonia gas. Iron oxide is often used as a catalyst to speed up the process for commercial purposes. Prior to the development of this process, potassium nitrate was prepared by reacting potassium chloride with sodium nitrate.
Common Uses
Gunpowder and fertilizer are the two major uses of potassium nitrate. Large quantities of potassium nitrate are used annually as fertilizer, as a source of both potassium and nitrogen for plants. It can also hasten the decay of tree stumps by acting as an oxidizer. Gunpowder was developed more than 2,200 years ago, in China, when potassium nitrate was mixed with carbon and sulfur, creating a black powder having explosive properties. This mixture, also known as "Chinese Snow" and "the Devil's Distillate" was manufactured into explosive grenades and bombs as early as 1000 A.D. Potassium nitrate also fuels fireworks, model rocket propellants and matches. Minor uses include its use in the manufacture of nitric acid and glass.
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