What happens if you heat potassium nitrate



Potassium nitrate, better known in common usage as Saltpetre, specifically as Potash nitrate, is the potassium salt of nitric acid.

properties

Potassium nitrate forms colorless crystals, which dissolve in water under strong cooling. It is therefore much more soluble in warm water than in cold water. Up to 130g of potassium nitrate can be dissolved in 1 liter of water at 0 ° C, and up to 2455g of potassium nitrate in 1 liter of water at 100 ° C. At such high concentrations, the density of the solution is much greater than that of pure water.

Potassium nitrate decomposes to potassium nitrite and oxygen when heated:

It is an excellent oxidizing agent at elevated temperatures. Charred residue in glass utensils dissolves quickly in molten potassium nitrate.

Potassium nitrate is significantly less hygroscopic than many other nitrates, e.g. B. Sodium Nitrate.

Extraction

  • Natural occurrence
    Potassium nitrate (min. "Nitrocalite") occurs as efflorescence on soils. The deposits in China and Southeast Asia were of economic importance, where in the first half of the 19th century more than 10,000 tons of saltpetre were extracted annually by leaching such soils.
  • Bacterial nitrification of nitrogen-rich organic waste
    From the end of the 14th to the 19th century, saltpetre was produced in Europe with the help of bacteria and atmospheric oxygen in order to be independent of the import of this raw material, which is indispensable for war purposes. (see "History"). Nitrogen-rich organic waste (manure and urine) is mixed with lime and wood ash (potash) and left to rot in loose, air-permeable piles of earth. The nitrogen compounds are converted into nitrates by bacteria. After two years, the mass is leached with water. Potash is added to the raw liquor, which converts calcium and magnesium nitrate into potassium nitrate and poorly soluble alkaline earth carbonate. The potassium nitrate is obtained by evaporating the filtered alkali and is purified by recrystallization.
  • Conversion nitrate
    From the middle of the 19th century to around 1920, the conversion of Chile's nitrate with potassium chloride was the most important process for the production of potassium nitrate: NaNO3 + KCl -> KNO3 + NaCl
    The slight increase in the solubility of sodium chloride with temperature is exploited: the KNO mother liquor3-Crystallization in the previous cycle is heated and raw sodium nitrate and potassium chloride are added in a stoichiometric ratio. The mixture is concentrated with the addition of a little soda at 100 ° C., sodium chloride and impurities (alkaline earth metal carbonates) precipitating and being filtered off. The filtrate is diluted again with the condensate from evaporation in order to avoid the precipitation of sodium salts during cooling, filtered until clear and then cooled to 5 ° C. for crystallization of the potassium nitrate and centrifuged. The deposited potassium nitrate is recrystallized for technical purposes.

Representation (manufacture)

There are many ways to represent potassium nitrate:





use

  • Potassium nitrate is used to preserve food (curing salt E 252)
  • For the production of black powder and other pyrotechnic mixtures
  • A mixture of 24% boron + 71% KNO3 + 5% binder (PMMA) serves as a reliable ignition mixture that burns even at very low temperatures (-196 ° C).
  • A mixture of 60% NaNO3+ 40% KNO3 melts at 222 ° C and is used as a heat transfer medium in solar thermal power plants. This molten salt is chemically stable up to 590 ° C, has a high specific heat capacity of 1.55 kJ / (kg K), a density of 1.79 g / cm³ and is as thin as water (viscosity: 2.1 mPa s). It wets metal surfaces very easily, which can lead to sealing problems if the construction and material selection are unsuitable. Stainless steels are largely resistant to molten saltpeter (removal rate: 6 - 15 µm / year at 570 ° C). The heat transfer coefficient on the pipe with a turbulent flow is around 6000 W / K m². Due to its high heat capacity (2.8 MJ / (K m³)), molten nitrate is also suitable as a heat storage medium. The melting temperature can be lowered further by adding sodium nitrite. A salt mixture called HiTec made of 53% KNO3 + 40% NaNO2 + 7% NaNO3 melts at 140 ° C and has particularly favorable properties as a heat transfer medium when the toxicity of sodium nitrite is irrelevant.
  • Saltpetre baths are used for the heat treatment of wrought aluminum alloys with a magnesium content of up to 10%. The maximum permissible temperature of the molten salt depends on the magnesium content; it drops from 550 ° C with 0.5% Mg to 380 ° C with 10% Mg.
  • in smoke grenades
  • in fertilizer

history

Already in the 11th century, saltpeter is mentioned in the book of Marcus Graecus, which also mentions the black powder mixture for the first time, as a new substance that is scraped off the earth and stones. The late 13th century book about mounted combat and the use of war machines by Hassan ar-Rammah (Al-Furusiyya wa al-Manasib al-Harbiyya) already contains several regulations for cleaning saltpetre with wood ash and for making incendiary devices and fuel for rockets.

Saltpetre was initially imported from India; Venice made high profits from the middlemen. With increasing demand and for reasons of independence, from the end of the 14th century governments promoted their own mining of saltpeter and secured all rights of manufacture, import and use by means of draconian laws through a "saltpeter shelf". Due to the rapid release of oxygen, saltpetre was the basis for the sudden combustion of sulfur and charcoal in gunpowder and therefore, as a chronically scarce substance, the strategic raw material for six centuries.

There were nine saltpeter works in Thuringia in the 16th century. The banks of the Vltava near Prague were covered with "sanitary benches", and the city of Halle granted a concession to extract saltpetre from the garbage dumps. The increasing demand for saltpetre was partly covered by further imports, especially from India, and by our own plants.

From the end of the 14th century, a systematic cultivation of saltpeter gardens took place. Animal waste (dung, excrement, urine and blood) was filled with calcareous earth, earth from cemeteries, slaughterhouses or bogs and with lime, rubble and ashes in pits or piled in piles and occasionally doused with liquid manure or urine. As a result of the decomposition, so much saltpeter was formed after one or two years that it could be washed out of the earth. The yield was about 6: 1, i.e. 1 kg of saltpeter was obtained from 6 kg of nitric earth.

Saltpeter as a special and very unpopular profession was allowed to enter properties at any time and look for saltpeter there. This was true even for churches in the 17th and 18th centuries, with the exception of the times of church services. In Sweden the farmers even had to pay some of their taxes in saltpeter.

In the 19th century there was even the saltpeter war that Chile waged against its neighboring countries in order to gain sole ownership of the huge desert deposits of sodium nitrate ("caliche"), which could now be converted into potassium nitrate immediately with potassium salts. This conversion process was finally replaced in 1916 by the Haber-Bosch process of generating ammonia from air and water with subsequent conversion to nitric acid.

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  1. abcdefG Entry to Potassium nitrate in the GESTIS substance database of the BGIA, accessed on September 3, 2007 (JavaScript required)

literature

  • Perry, R.H., Chemical Engineers' Handbook, 4th ed., McGraw-Hill Book Company, New York, 1963, pp. 9-77
  • Janz, G.H., Et al., Physical Properties Data Compilations Relevant to Energy Storage II. Molten Salts, NSRDS, April, 1979
  • Gartz, J, Cultural History of Explosives, E.S.Mittler & Sohn, Hamburg, 2006

Category: Oxidising substance