Potassium nitrate

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Potassium nitrate
Recrystallized potassium nitrate.jpg
KNO3 recrystallized from aqueous solution.
Names
IUPAC name
Potassium nitrate
Other names
Chinese salt
Chinese snow
Indian saltpeter
Nitrate of potash
Saltpeter
Saltpetre
Properties
KNO3
Molar mass 101.1032 g/mol
Appearance White solid
Odor Odorless
Density 2.1 g/cm3 (20 °C)
Melting point 334 °C (633 °F; 607 K)
Boiling point 400 °C (752 °F; 673 K) (decomposes)
13.3 g/100 ml (0 °C)
31.6 g/100 ml (20 °C)
246 g/100 ml (100 °C)
Solubility Soluble in ammonia, glycerol
Slightly soluble in ethanol, ethylenediamine
Insoluble in diethyl ether, isopropanol, toluene
Solubility in ethanol 0.6 g/100 ml (20 °C)
Solubility in hydrazine 14 g/100 ml (20 °C)
Solubility in methanol 3 g/100 ml (20 °C)
Vapor pressure ~0 mmHg
Thermochemistry
-494.0 kJ/mol
Hazards
Safety data sheet Sigma-Aldrich
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
1,901 mg/kg (rabbit, oral)
3,750 mg/kg (rat, oral)
Related compounds
Related compounds
Sodium nitrate
Calcium nitrate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Potassium nitrate, also known as saltpeter or saltpetre or Indian saltpeter, is an important chemical compound in both industry and chemistry. Known for centuries for its role in early pyrotechnics, the nitrate ion makes KNO3 a widely used oxidiser with a variety of uses inside and outside pyrotechnics.

Properties

Chemical

Potassium nitrate is a good source of both potassium and nitrate ions.

When heated to to temperatures between 550 and 790 °C, under an oxygen atmosphere, it loses oxygen and converts into potassium nitrite:

2 KNO3 → 2 KNO2 + O2

Molten potassium nitrate will react with carbon dioxide and water vapors to form potassium carbonate and hydroxide.[1]

Potassium nitrate will react with hydrochloric acid to release nitric acid, that will give off nitrogen dioxide fumes:

2 KNO3 + 2 HCl + H2O → 2 KCl + HNO3 + NO2 + H2O

By adding an excess of HCl, this reaction can be used to generate "poor man's aqua regia".

A substantial amount of energetic materials are based on the nitrate ion, and it is arguably the most important ion when it comes to explosives, such as black powder.

10 KNO3 + 3 S + 8 C → 2 K2CO3 + 3 K2SO4 + 6 CO2 + 5 N2

A less known reaction is the synthesis of potassium cyanide, by reacting a mixture of potassium nitrate and charcoal in a cast iron bowl, in an inert atmosphere to prevent combustion:

KNO3 + 4C -> KCN + 3CO[2]

If you attempt to try this reaction, AVOID ADDING ACID TO THE RESULTED SLAG AS IT WILL GIVE OFF HYDROGEN CYANIDE GAS WHICH CAN BE DEADLY (see the Sciencemadness thread below).

A mixture of sucrose (most sugars work) and potassium nitrate known as rock candy can be ignited, producing pink flames and large amounts of white smoke, useful for improvised smoke bombs. The solid end product of this reaction is potassium carbonate, plus some of the leftover reactants.

Physical

Potassium nitrate is white solid with a melting point of 334°C. Its solubility curve makes recrystallization easy, being only somewhat soluble in freezing water (13.3 g/100 ml at 0 °C), but very soluble in boiling water (246 g/100 ml at 100 °C). It is not hygroscopic, absorbing about 0.03% water in 80% relative humidity over a period of 50 days. It is soluble in glycerol and ammonia, but only slightly soluble in ethanol.

Availability

In many American hardware stores, potassium nitrate is sold in a reasonably pure form as a stump remover, which can be purified by recrystallization. Care must be taken if it is being bought this way, as some stump removers mostly consist of other agents, such as sodium metabisulfite, which can be a rude awakening to one treating it as potassium nitrate.

It can also be bought as fertilizer, either pure or mixed with traces of other substances. If it's present as prills, recrystallization might be required as the granules are difficult to grind properly in a mortar. Some types of prills tend to be covered with an anti-caking agent, such as calcium stearate, and recrystallization is required for purification.

Preparation

Potassium nitrate can be prepared by neutralizing KOH with nitric acid or adding ammonium nitrate to it.

KOH + HNO3 → KNO3 + H2O
KOH + NH4NO3 → KNO3 + NH3 + H2O

It can also be made by a double displacement reaction between ammonium nitrate and potassium sulfate or potassium chloride, and then recrystallizing the KNO3 out at low temperatures. This reaction can also be done using potassium sulfate and sodium nitrate, but instead of crystallizing potassium nitrate, sodium sulfate is crystallized out along with some of the potassium nitrate at low temperatures, leaving most of the desired product in solution.

A reaction between a small amount of ice or water, potassium hydroxide, and ammonium nitrate easily proceeds to completion very exothermically, driving ammonia gas out of the mixture and producing potassium nitrate as an end product. This can also be done with potassium carbonate, but the carbonate route requires heating the two in solution.

The nitrate ion itself is very difficult to make from other OTC chemicals, and despite the increasing difficulty in obtaining this salt, buying it in pure or impure from is the only real viable option.

An old method of making saltpeter involves separating nitrates from manure, and adding potash. The resulting solution is recrystallized repeatedly until fairly pure saltpeter is obtained. This method requires large amounts of manure, but can be done fairly easy if one has a farm or a big garden.

Projects

Handling

Safety

Potassium nitrate is virtually non-toxic, and is even approved for use in food as a preservative (E252). Soldiers of the early modern era even used the saltpeter-based black powder as a table salt substitute to sprinkle on their rations.

While generally quite stable, it is an oxidizing agent, so mixtures with reducing agents like sucrose, red phosphorus, sulfur, magnesium or aluminium should not be ground or pressed together, and the risk of spontaneous ignition is ever present. Sulfur is especially dangerous, as acidic traces can greatly increase the sensitivity of energetic mixtures. As it is not hygroscopic, like its sodium cousin, it poses a greater fire hazard.[3]

Storage

Potassium nitrate should be stored in closed bottles, away from any flammable materials as well as organic and acidic vapors. It is not hygroscopic, so there's no need to seal it.

Disposal

Potassium nitrate is a good fertilizer, and unless it's contaminated with heavy metals or other toxicants, it can be disposed pretty much anywhere.

References

  1. Proceedings of the Symposia on Corrosion in Batteries and Fuel Cells and Corrosion in Solar Energy Systems, Chris J. Johnson, Steven L. Pohlman, 1984, p. 407-408
  2. http://www.google.com/patents/US579988
  3. Tdep - experience working with and setting fire to said chemical.

Relevant Sciencemadness threads