Niter

Authors: Hans-Jürgen Schwarz, Nils Mainusch
Translation of the German Version by Hans-Jürgen Schwarz
back to Nitrate

Niter
Mineralogical name	Nitrokalite, Niter, Kalisalpeter
Chemical name	Potassium nitrate
Trivial name	Saltpetre , Nitrate of potash, Vesta powder, Kali-Salpeter, Kehrsalpeter, Konversionssalpeter
Chemical formula	KNO3
Other forms	none
Crystal system	orthorhombic
Crystal structure	orthorhombic - dipyramidal; 2/m 2/m 2/m , see
Deliquescence humidity 20°C	94.6 % (20°C), 93.6% (25°C)
Solubility (g/l) at 20°C	315 g/l
Density (g/cm³)	2.109 g/cm3
Molar volume	48.04 cm3/mol
Molar weight	101.11 g/mol
Transparency	translucent to transparent
Cleavage	very good on {001}; good on {010}h
Crystal habit	include crusts and acicular crystals formed as efflorescence on cave and mine walls
Twinning
Phase transition
Chemical behavior	easily soluble in water
Comments
Crystal Optics
Refractive Indices	α = 1.335 β = 1.505 γ = 1.506
Birefringence	Δ = 0.171
Optical Orientation	biaxial negative
Pleochroism
Dispersion	weak, r < v
Used Literature

Abstract

Niter is one of the most important salts that is responsible for damages of buildings and murals. He is mostly observed indoors, often found in cotton-like efflorescences. Pictures, microphotographs and examples of habits illustrate and complement the description.

General

Potassium salts were an important ingredient for the production of gun and explosives in the 19th century, and were both originating from natural sources (potash deposits contain a maximum of 10% KNO₃ among various other potassium salts) and obtained through the transformation of sodium nitrate into potassium chloride. Currently various methods of large-scale production of this salt are based on the reaction between crude potassium salts and nitric acid.
Potassium nitrate is also used as a fertilizer in agriculture.

Occurrence of Niter

Potassium nitrate in natural accumulations can occur anywhere where nitrogen compounds are synthesized in presence of a sufficient quantity of potassium ions (for example during the decomposition of organic matter). Large quantities of nitrifying bacteria and nitrogen compounds are known to be present in manure and urine of living organisms.

Information on the origin and formation of Niter on monuments

Contaminated ground water is the major input source of potassium ions in monuments, and nitrates most commonly come from atmospheric pollution. In addition, building and restoration materials can also contain soluble potassium compounds. We can also mention potassium silicate, potassium hydroxide (used as a cleaning agent) and cements.
Nitrates can also originate from biogenic sources and be transported into the material structure via capillary transport of moisture. Microbial activity is an important potential source of nitrates through nitrifying bacteria.

Solution behavior

Figure 1: Influence of temperature on the water solubility of potassium nitrate in comparison with other damaging salts (after [Stark.etal:1996]Title: Bauschädliche Salze
Author: Stark, Jochen; Stürmer, Sylvia
)

Potassium nitrate has a high water solubility, belonging to the group of mobile salts. It can be associated with frequent changes in the accumulation zones in the porous material. The temperature effect on the water solubility is strong, which is reflected in the steep curve in Diagram 1.

A consequence of this is the danger of solution supersaturation during rapid temperature drop, and a subsequent crystallization of the salt.

Niter and damages caused by niter in the image

On objects

• 

  fluffy salt cystals (?)of Niter destroys the surface

Under the polarizing microscope

• KNO<sub
• 

  in simple polarized light

• 

  under crossed polarizers

• 

  under crossed polarizers and Red I

• 

  in simple polarized light

• 

  under crossed polarizers

• 

  under crossed polarizers and Red I

• 

  under crossed polarizers and Red I

• 

  under crossed polarizers and Red I

Literature

[Mainusch:2001]	Mainusch, Nils (2001): Erstellung einer Materialsammlung zur qualitativen Bestimmung bauschädlicher Salze für Fachleute der Restaurierung, Diplomarbeit, HAWK Hochschule für angewandte Wissenschaft und Kunst Hildesheim/Holzminden/Göttingen, file:Diplomarbeit Nils Mainusch.pdf
[Robie.etal:1978]	Robie R.A., Hemingway B.S.; Fisher J.A. (1978): Thermodynamic properties of minerals and related substances at 298.15 K and 1 bar pressure and higher temperatures. In: U.S. Geol. Surv. Bull, 1452 ()
[Steiger.etal:2014]	Steiger, Michael; Charola A. Elena; Sterflinger, Katja (2014): Weathering and Deterioration. In: Siegesmund S.; Snethlage R. (eds.): Stone in Architecture, Springer Verlag Berlin Heidelberg, 223-316, 10.1007/978-3-642-45155-3_4.
[Vogt.etal:1993]	Vogt, R.; Goretzki, Lothar (1993): Der Einfluss hygroskopischer Salze auf die Gleichgewichtsfeuchte und Trocknung anorganischer Baustoffe, unveröffentlichter Bericht.

crystallization of a dissolved salt

A phase is a physically distinctive form of matter and is characterized by having relatively uniform chemical and physical properties.

Hochschule für angewandte Wissenschaft und Kunst Hildesheim/Holzminden/Göttingen, University of Applied Sciences and Arts Hildesheim/Holzminden/Göttingen