Difference between revisions of "Potassium nitrate"

From Sciencemadness Wiki
Jump to: navigation, search
(Created page with "KNO<sub>3</sub> recrystallized from aqueous solution. '''Potassium nitrate''', also known as '''saltpeter''' or '''sa...")
 
 
(24 intermediate revisions by 3 users not shown)
Line 1: Line 1:
−
[[File:Recrystallized_potassium_nitrate.jpg|thumb|300px|KNO<sub>3</sub> recrystallized from aqueous solution.]]
+
{{Chembox
−
'''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 KNO<sub>3</sub> a widely used oxidiser with a variety of uses inside and outside pyrotechnics.  
+
| Name = Potassium nitrate
 +
| Reference =
 +
| IUPACName = Potassium nitrate
 +
| PIN =
 +
| SystematicName =
 +
| OtherNames = Chinese salt<br>Chinese snow<br>Indian saltpeter<br>Niter<br>Nitrate of potash<br>NPK 13-0-46<br>Saltpeter<br>Saltpetre
 +
<!-- Images -->
 +
| ImageFile = Recrystallized_potassium_nitrate.jpg
 +
| ImageSize = 300
 +
| ImageAlt =
 +
| ImageName =
 +
| ImageCaption = KNO<sub>3</sub> recrystallized from aqueous solution.
 +
| ImageFile1 =
 +
| ImageSize1 =
 +
| ImageAlt1 =
 +
| ImageName1 =
 +
| ImageFile2 =
 +
| ImageSize2 =
 +
| ImageAlt2 =
 +
| ImageName2 =
 +
| ImageFile3 =
 +
| ImageSize3 =
 +
| ImageAlt3 =
 +
| ImageName3 =
 +
| ImageFileL1 =
 +
| ImageSizeL1 =
 +
| ImageAltL1 =
 +
| ImageNameL1 =
 +
| ImageFileR1 =
 +
| ImageSizeR1 =
 +
| ImageAltR1 =
 +
| ImageNameR1 =
 +
| ImageFileL2 =
 +
| ImageSizeL2 =
 +
| ImageAltL2 =
 +
| ImageNameL2 =
 +
| ImageFileR2 =
 +
| ImageSizeR2 =
 +
| ImageAltR2 =
 +
| ImageNameR2 =
 +
<!-- Sections -->
 +
| Section1 = {{Chembox Identifiers
 +
| 3DMet =
 +
| Abbreviations =
 +
| SMILES =
 +
  }}
 +
| Section2 = {{Chembox Properties
 +
| AtmosphericOHRateConstant =
 +
| Appearance = White crystalline solid
 +
| BoilingPt =
 +
| BoilingPtC = 400
 +
| BoilingPt_ref =
 +
| BoilingPt_notes = (decomposes)
 +
| Density = 2.1 g/cm<sup>3</sup> (20 °C)
 +
| Formula = KNO<sub>3</sub>
 +
| HenryConstant =
 +
| LogP =
 +
| MolarMass = 101.1032 g/mol
 +
| MeltingPt =
 +
| MeltingPtC = 334
 +
| MeltingPt_ref =
 +
| MeltingPt_notes =
 +
| Odor = Odorless
 +
| pKa =
 +
| pKb =
 +
| Solubility = 13.3 g/100 ml (0 °C)<br>31.6 g/100 ml (20 °C)<br>246 g/100 ml (100 °C)
 +
| SolubleOther = Soluble in [[ammonia]], [[glycerol]]<br>Slightly soluble in [[ethanol]], [[ethylenediamine]], [[methanol]]<br>Insoluble in [[diethyl ether]], [[isopropanol]], [[toluene]]
 +
| Solubility1 = 0.6 g/100 ml (20 °C)
 +
| Solvent1 = ethanol
 +
| Solubility2 = 14 g/100 ml (20 °C)
 +
| Solvent2 = hydrazine
 +
| Solubility3 = 3 g/100 ml (20 °C)
 +
| Solvent3 = methanol
 +
| VaporPressure = ~0 mmHg
 +
  }}
 +
| Section3 = {{Chembox Structure
 +
| Coordination =
 +
| CrystalStruct = Orthorhombic, Aragonite
 +
| MolShape =
 +
  }}
 +
| Section4 = {{Chembox Thermochemistry
 +
| DeltaGf =
 +
| DeltaHc =
 +
| DeltaHf = -494.0 kJ/mol
 +
| Entropy =
 +
| HeatCapacity = 95.06 J·mol<sup>-1</sup>·K<sup>-1</sup>
 +
  }}
 +
| Section5 = {{Chembox Explosive
 +
| ShockSens =
 +
| FrictionSens =
 +
| DetonationV =
 +
| REFactor =
 +
  }}
 +
| Section6 = {{Chembox Hazards
 +
| AutoignitionPt = Non-flammable
 +
| ExploLimits =
 +
| ExternalMSDS = [https://www.docdroid.net/o9f4Gwg/potassium-nitrate-sa.pdf.html Sigma-Aldrich]
 +
| FlashPt = Non-flammable
 +
| LD50 = 1,901 mg/kg (rabbit, oral)<br>3,750 mg/kg (rat, oral)
 +
| LC50 =
 +
| MainHazards = Oxidizer
 +
| NFPA-F =
 +
| NFPA-H =
 +
| NFPA-R =
 +
| NFPA-S =
 +
  }}
 +
| Section7 = {{Chembox Related
 +
| OtherAnions =
 +
| OtherCations =
 +
| OtherFunction =
 +
| OtherFunction_label =
 +
| OtherCompounds = [[Lithium nitrate]]<br>[[Sodium nitrate]]<br>[[Rubidium nitrate]]<br>[[Caesium nitrate]]
 +
  }}
 +
}}
 +
'''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 '''KNO<sub>3</sub>''' a widely used oxidizer with a variety of uses inside and outside pyrotechnics.  
  
 
==Properties==
 
==Properties==
Line 9: Line 123:
  
 
:2 KNO<sub>3</sub> → 2 KNO<sub>2</sub> + O<sub>2</sub>
 
:2 KNO<sub>3</sub> → 2 KNO<sub>2</sub> + O<sub>2</sub>
 +
 +
Molten potassium nitrate will react with [[carbon dioxide]] and water vapors to form [[potassium carbonate]] and hydroxide.<ref>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</ref>
  
 
Potassium nitrate will react with [[hydrochloric acid]] to release [[nitric acid]], that will give off [[nitrogen dioxide]] fumes:
 
Potassium nitrate will react with [[hydrochloric acid]] to release [[nitric acid]], that will give off [[nitrogen dioxide]] fumes:
  
−
:2KNO<sub>3</sub> + 2HCl + H<sub>2</sub>O → 2KCl + HNO<sub>3</sub> + NO<sub>2</sub> + H<sub>2</sub>O
+
:2 KNO<sub>3</sub> + 2 HCl + H<sub>2</sub>O → 2 KCl + HNO<sub>3</sub> + NO<sub>2</sub> + H<sub>2</sub>O
  
 
By adding an excess of HCl, this reaction can be used to generate "poor man's aqua regia".
 
By adding an excess of HCl, this reaction can be used to generate "poor man's aqua regia".
Line 20: Line 136:
 
:10 KNO<sub>3</sub> + 3 S + 8 C → 2 K<sub>2</sub>CO<sub>3</sub> + 3 K<sub>2</sub>SO<sub>4</sub> + 6 CO<sub>2</sub> + 5 N<sub>2</sub>
 
:10 KNO<sub>3</sub> + 3 S + 8 C → 2 K<sub>2</sub>CO<sub>3</sub> + 3 K<sub>2</sub>SO<sub>4</sub> + 6 CO<sub>2</sub> + 5 N<sub>2</sub>
  
−
A less known reaction is the synthesis of [[potassium cyanide]], by reacting a mixture of potassium nitrate and [[Carbon|charcoal]] in a cast iron bowl, in an inert atmosphere to prevent combustion:
+
A less known reaction is the synthesis of [[potassium cyanide]], by reacting a mixture of potassium nitrate and [[Carbon|charcoal]] in a cast iron bowl, in an inert atmosphere to prevent combustion or oxidation to potassium cyanate:
  
−
:KNO<sub>3</sub> + 4C -> KCN + 3CO<ref>http://www.archpatent.com/patents/0579988</ref>
+
:2 KNO<sub>3</sub> + 7 C → KCN + KOCN + 5 CO<ref>http://www.google.com/patents/US579988</ref>
  
 
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).
 
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).
 +
 +
Presence of iron may also lead to formation of [[Prussian blue]].
  
 
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.
 
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.
 +
 +
: KNO<sub>3</sub> + C<sub>12</sub>H<sub>22</sub>O<sub>11</sub> → K<sub>2</sub>CO<sub>3</sub> + CO<sub>2</sub> + N<sub>2</sub>
 +
 +
The ignition also releases copious amounts of caramelized sugar particles, giving the smoke a sweet fragrance.
  
 
===Physical===
 
===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 (13.3g/100ml at 0°C), but very soluble in boiling water (246g/100ml 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.
+
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.
 +
 
 +
Potassium nitrate is soluble in anhydrous basic solvents, like [[ammonia]] and [[hydrazine]], as well as [[glycerol]], but only slightly soluble in [[ethanol]] and [[methanol]]. Pure KNO<sub>3</sub> is odorless and has a slight bitter taste.
  
 
==Availability==
 
==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.
+
In many American hardware stores, potassium nitrate is sold in a reasonably pure form as a stump remover, which can be purified by recrystallization from hot water. 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.
 +
 
 +
Potassium nitrate is much more commonly encountered 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 filtration before recrystallization is required for purification. It's easy to determine if the saltpeter source has anticaking agent, as when the solution gets more concentrated solution, it will darken, turning yellow. Potassium nitrate fertilizers can be identified after the NPK number 13-0-46.
 +
 
 +
Potassium nitrate occurs naturally as the mineral ''niter'' (or ''nitre'').
  
 
==Preparation==
 
==Preparation==
−
Potassium nitrate can be prepared by neutralizing [[Potassium hydroxide|KOH]] with [[nitric acid]] or ammonium nitrate.
+
Potassium nitrate can be prepared by neutralizing [[Potassium hydroxide|KOH]] with [[nitric acid]] or adding [[ammonium nitrate]] to it.
  
−
:KOH + HNO<sub>3</sub> → KNO<sub>3</sub> + H<sub>2</sub>O
+
: KOH + HNO<sub>3</sub> → KNO<sub>3</sub> + H<sub>2</sub>O
−
:KOH + NH<sub>4</sub>NO<sub>3</sub> → KNO<sub>3</sub> + NH<sub>3</sub> + H<sub>2</sub>O
+
: KOH + NH<sub>4</sub>NO<sub>3</sub> → KNO<sub>3</sub> + NH<sub>3</sub> + H<sub>2</sub>O
  
 
It can also be made by a double displacement reaction between [[ammonium nitrate]] and [[potassium sulfate]] or [[potassium chloride]], and then recrystallizing the KNO<sub>3</sub> 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.
 
It can also be made by a double displacement reaction between [[ammonium nitrate]] and [[potassium sulfate]] or [[potassium chloride]], and then recrystallizing the KNO<sub>3</sub> 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.
Line 44: Line 172:
 
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.
 
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.  
+
Another method is a double displacement from [[barium nitrate]] with potassium carbonate or potassium sulfate. The resulting [[barium carbonate]] or sulfate can be filtered off, leaving potassium nitrate 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==
 
==Projects==
−
*Making nitric acid
+
*Making [[nitric acid]]
 
*[[Potassium nitrite]] synthesis
 
*[[Potassium nitrite]] synthesis
 
*[[Nitrocellulose]] synthesis
 
*[[Nitrocellulose]] synthesis
Line 55: Line 187:
 
*Dissolving gold when mixed with concentrated [[Hydrochloric acid|HCl]] (poor man's [[aqua regia]])
 
*Dissolving gold when mixed with concentrated [[Hydrochloric acid|HCl]] (poor man's [[aqua regia]])
 
*Electrolyte in a [[salt bridge]]
 
*Electrolyte in a [[salt bridge]]
−
*[[Potassium cyanide]] sythesis
+
*[[Potassium cyanide]] synthesis
 +
*Grow large crystals
  
 
==Handling==
 
==Handling==
 
===Safety===
 
===Safety===
−
Potassium nitrate is virtually non-toxic, and is even approved for use in food as a preservative ([http://en.wikipedia.org/wiki/E_number E252]).
+
Potassium nitrate is virtually non-toxic, and is even approved for use in food as a preservative ([http://en.wikipedia.org/wiki/E_number 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 nitrate|sodium cousin]], it poses a greater fire hazard.
+
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 nitrate|sodium cousin]], it poses a greater fire hazard.<ref>Tdep - experience working with and setting fire to said chemical.</ref>
  
 
===Storage===
 
===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.
+
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 the container.
  
 
===Disposal===
 
===Disposal===
−
Potassium nitrate is a good fertilizer, and unless it's contaminated with toxins or heavy metals, it can be disposed pretty much anywhere.
+
Potassium nitrate is a good fertilizer, and unless it's contaminated with heavy metals or other toxicants, it can be disposed pretty much anywhere.
 +
 
 +
==Gallery==
 +
<gallery widths="200" position="center" columns="4" orientation="none">
 +
Potassium_nitrate.jpg|Crystals of potassium nitrate, dried
 +
Potassium_nitrate2.jpg|Potassium nitrate, fresh out of solution
 +
</gallery>
  
 
==References==
 
==References==
 
<references />
 
<references />
−
Tdep - experience working with and setting fire to said chemical.
 
−
 
 
===Relevant Sciencemadness threads===
 
===Relevant Sciencemadness threads===
−
*[http://www.sciencemadness.org/talk/viewthread.php?tid=52 Preparation of ionic nitrites]
+
*[http://www.sciencemadness.org/talk/viewthread.php?tid=74 Obtaining Potassium Nitrate]
−
*[http://www.sciencemadness.org/talk/viewthread.php?tid=23 Preparation of cyanides]
+
*[http://www.sciencemadness.org/talk/viewthread.php?tid=171 KNO3]
 +
*[http://www.sciencemadness.org/talk/viewthread.php?tid=126396 Convert NaNO3 to KNO3 and purify it]
 +
*[http://www.sciencemadness.org/talk/viewthread.php?tid=73415 Cheap nitrates to potassium nitrate]
 +
*[https://www.sciencemadness.org/whisper/viewthread.php?tid=71448 The Electrochemical Oxidation of Ammonia to Potassium Nitrate]
 +
*[https://www.sciencemadness.org/whisper/viewthread.php?tid=156708 shock sensitive mixtures of KNO3]
 +
*[http://www.sciencemadness.org/talk/viewthread.php?tid=29163 Potassium nitrate and potassium nitrite...]
 +
 
 +
[[Category:Chemical compounds]]
 +
[[Category:Inorganic compounds]]
 +
[[Category:Potassium compounds]]
 +
[[Category:Neutral salts]]
 +
[[Category:Nitrates]]
 +
[[Category:Oxidizing agents]]
 +
[[Category:Readily available chemicals]]
 +
[[Category:Essential reagents]]
 +
[[Category:Minerals]]

Latest revision as of 17:12, 27 May 2023

Potassium nitrate
Recrystallized potassium nitrate.jpg
KNO3 recrystallized from aqueous solution.
Names
IUPAC name
Potassium nitrate
Other names
Chinese salt
Chinese snow
Indian saltpeter
Niter
Nitrate of potash
NPK 13-0-46
Saltpeter
Saltpetre
Properties
KNO3
Molar mass 101.1032 g/mol
Appearance White crystalline 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, methanol
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
 Lithium nitrate
Sodium nitrate
Rubidium nitrate
Caesium 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 oxidizer 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 or oxidation to potassium cyanate:

2 KNO3 + 7 C → KCN + KOCN + 5 CO[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).

Presence of iron may also lead to formation of Prussian blue.

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.

KNO3 + C12H22O11 → K2CO3 + CO2 + N2

The ignition also releases copious amounts of caramelized sugar particles, giving the smoke a sweet fragrance.

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.

Potassium nitrate is soluble in anhydrous basic solvents, like ammonia and hydrazine, as well as glycerol, but only slightly soluble in ethanol and methanol. Pure KNO3 is odorless and has a slight bitter taste.

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 from hot water. 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.

Potassium nitrate is much more commonly encountered 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 filtration before recrystallization is required for purification. It's easy to determine if the saltpeter source has anticaking agent, as when the solution gets more concentrated solution, it will darken, turning yellow. Potassium nitrate fertilizers can be identified after the NPK number 13-0-46.

Potassium nitrate occurs naturally as the mineral niter (or nitre).

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.

Another method is a double displacement from barium nitrate with potassium carbonate or potassium sulfate. The resulting barium carbonate or sulfate can be filtered off, leaving potassium nitrate 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 the container.

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.

Gallery

  • Crystals of potassium nitrate, dried

  • Potassium nitrate, fresh out of solution

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

Retrieved from "https://www.sciencemadness.org/smwiki/index.php?title=Potassium_nitrate&oldid=14987"