9 julio, 2024

Potassium nitrate (KNO3): structure, properties, uses, synthesis

What is potassium nitrate?

He potassium nitrate It is a ternary salt composed of potassium, the alkali metal, and the oxoanion nitrate. Its chemical formula is KNO3, which means that for every K+ ion, there is an NO3- ion interacting with it. Therefore, it is an ionic salt and constitutes one of the alkaline nitrates (LiNO3, NaNO3, RbNO3…).

KNO3 is a strong oxidizing agent due to the presence of the nitrate anion. That is, it works as a reserve for solid and anhydrous nitrate ions, unlike other salts that are highly soluble in water or highly hygroscopic. Many of the properties and uses of this compound are due to the nitrate anion, rather than the potassium cation.

The image above shows some needle-shaped KNO3 crystals. The natural source of KNO3 is saltpeter, known by the names saltpeter either saltpeter, in English. This element is also known as nitrate of potash or nitro mineral.

It is found in arid or desert areas, as well as efflorescences of the cavernous walls. Another important source of KNO3 is guano, the excrement of animals that inhabit dry environments.

Potassium Nitrate Chemical Structure

In the image above the crystalline structure of KNO3 is represented. The purple spheres correspond to the K+ ions, while the red and blue ones are the oxygen and nitrogen atoms, respectively. The crystalline structure is orthorhombic at room temperature.

The geometry of the NO3– anion is that of a trigonal plane, with the oxygen atoms at the vertices of the triangle, and the nitrogen atom at its center. It presents a positive formal charge on the nitrogen atom, and two negative formal charges on two oxygen atoms (1-2= (-1)).

These two negative charges of NO3– are delocalized between the three oxygen atoms, always maintaining the positive charge on nitrogen. As a consequence of the above, the K+ ions in the crystal avoid being located just above or below the nitrogen of the NO3– anions.

In fact, the image demonstrates how the K+ ions are surrounded by the oxygen atoms, the red spheres. In conclusion, these interactions are responsible for crystal arrangements.

Other crystalline phases

Variables such as pressure and temperature can modify these arrangements and give rise to different structural phases for KNO3 (phases I, II and III). For example, phase II is the one in the image, while phase I (with a trigonal crystalline structure) is formed when the crystals are heated up to 129 ºC.

Phase III is a transitional solid that is obtained from the cooling of Phase I, and some studies have shown that it exhibits some important physical properties, such as ferroelectricity. In this phase the crystal forms layers of potassium and nitrates, possibly sensitive to electrostatic repulsions between the ions.

In the phase III layers, the NO3– anions lose a bit of their planarity (the triangle curves slightly) to allow this arrangement, which, in the face of any mechanical disturbance, becomes the phase II structure.

Potassium Nitrate Uses/Applications

Salt is of great importance, since it is used in numerous human activities, which are manifested in industry, agriculture, food, etc. These uses include the following:

The preservation of food, especially meat. Despite the suspicion that it is involved in the formation of nitrosamine (a carcinogenic agent), it is still used in delicatessen.
Fertilizer, because potassium nitrate provides two of the three macronutrients of plants: nitrogen and potassium. Along with phosphorus, this element is necessary for the development of plants. That is, it is an important and manageable reserve of these nutrients.
It accelerates combustion, being able to produce explosions if the combustible material is extensive or if it is finely divided (greater surface area, greater reactivity). In addition, it is one of the main components of gunpowder.
Facilitates the removal of stumps from felled trees. The nitrate supplies the necessary nitrogen for the fungi to destroy the wood in the stumps.
It intervenes in the reduction of dental sensitivity by incorporating it into toothpastes, which increases protection against painful sensations in the tooth caused by cold, heat, acid, sweets or contact.
It is involved as a hypotensive in the regulation of blood pressure in humans. This effect would be given or interrelated with a change in sodium excretion. The recommended treatment dose is 40-80 mEq/day of potassium. In this regard, it is pointed out that potassium nitrate would have a diuretic action.

Synthesis

Most of the nitrate is produced in the mines in the deserts of Chile. It can be synthesized by several reactions:

NH4NO3 (aq) + KOH(aq) => NH3 (aq) + KNO3 (aq) + H2O(l)

Potassium nitrate is also produced by neutralizing nitric acid with potassium hydroxide in a highly exothermic reaction.

KOH (aq) + HNO3(conc) => KNO3 (aq) + H2O (l)

On an industrial scale, potassium nitrate is produced by a double displacement reaction.

NaNO3 (aq) + KCl (aq) => NaCl (aq) + KNO3 (aq)

The main source of KCl is from the mineral silvina, and not from other minerals such as carnallite or cainite, which are also composed of ionic magnesium.

Physical and chemical properties

Solid state potassium nitrate occurs as a white powder or in the form of crystals with an orthorhombic structure at room temperature, and a trigonal structure at 129 ºC. It has a molecular weight of 101.1032 g/mol, is odorless, and has a pungent, salty taste.

It is a highly soluble compound in water (316-320 g/litre of water, at 20 ºC), due to its ionic nature and the facility that water molecules have to solvate the K+ ion.

Its density is 2.1 g/cm3 at 25 ºC. This means that it is about twice as dense as water.

Its melting (334 ºC) and boiling (400 ºC) points are indicative of the ionic bonds between K+ and NO3–. However, they are low compared to those of other salts, due to the fact that the crystal lattice energy is lower for monovalent ions (that is, with charges ±1), and they also have not very similar sizes.

It decomposes at a temperature close to the boiling point (400ºC) to produce potassium nitrite and molecular oxygen:

KNO3(s) => KNO2(s) + O2(g)

References

Pubchem. (2018). Potassium Nitrate. Retrieved from pubchem.ncbi.nlm.nik.gov
Cryst Act. (2009). Growth and single-crystal refinement of phase-III potassium nitrate, KNO3. B65, 659–663.
Marni Wolfe. (October 3, 2017). Potassium Nitrate Risks. Recovered from livestrong.com
Amethyst Galleries, Inc. (1995-2014). The mineral niter. Recovered from galleries.com

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