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	93.7 %
Solubility (g/l) at 20°C	3.108 mol/kg
Density (g/cm³)	2.103 g/cm3
Molar volume	48.04 cm3/mol
Molar weight	101.10 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
[Steiger.etal:2014]Title: Weathering and Deterioration Author: Steiger, Michael; Charola A. Elena; Sterflinger, Katja [Robie.etal:1978]Title: Thermodynamic properties of minerals and related substances at 298.15 K and 1 bar pressure and higher temperatures Author: Robie R.A., Hemingway B.S.; Fisher J.A.

Abstract

Niter is one of the most important salts responsible for damages of building materials and murals. It is mostly observed indoors, often found in cotton-like efflorescence. Pictures, microphotographs and examples of their habits illustrate and complement the description.

General

Potassium nitrate was an important ingredient in the production of gun powder and explosives in the 19th century, were obtained from natural sources potash deposits contain a maximum of 10% KNO₃ among various other potassium salts) and obtained through the reaction of sodium nitrate with 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

Natural accumulations of potassium nitrate 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. Large deposits of potassium and sodium nitrate, called "salitre" in Spanish, are found in the desert region of Atacama, nothern Chile, which have been exploited since the 19th century.

Information on the origin and formation of Niter on monuments

Contaminated water at the base of monuments is the major input source of both potassium and nitrate ions, the latter having a biogenic origin, such as nitrifying bacteria. These ions will be transported into the material via capillarity. Nitrate ions can also be deposited from atmospheric pollution. Once within the porous system of the material, creep of the formed salt will also contribute to its redistribution. 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.

Solution behavior

Figure 1: Solubility of potassium nitrate in water. The molality m [n(KNO₃)•kg(H₂O)^-1] is plotted versus the temperature.

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 figure 1.

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

Hygroscopicity

In the temperature range of 0°C up to 30°C the deliquescence humidity of potassium nitrate lies always above 90% RH and the temperature dependence is more or less linear. Under the influence of foreign ions the deliquescence humidity is shifted towards lower values.

Figure 2:Deliquescence behaviour of potassium nitrate. The water activity a_w is plotted versus the temperature.

Table 1: Deliquescence humidity of potassium nitrate and its temperature dependence [Steiger.etal:2014]Title: Weathering and Deterioration
Author: Steiger, Michael; Charola A. Elena; Sterflinger, Katja
.

0°C	5°C	10°C	20°C	30°C	40°C
97.0% RH	95.5% RH	93.7% RH	91.5% RH	88.9% RH	85.9% RH

Niter and damages caused by niter in the image

On objects

• 

  fluffy salt crystals of niter break-up the surface

Under the polarizing microscope

• KNO3 crystallized from aqueous solution on a slide
• 

  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.

concentration given as amount of substance (mol) of the dissolved substance per kilogram solvent; unit: mol•kg-1.

relative humidity

the relative humidity, above which a material, e.g. a salt, absorbes humidity from the air and dissolves.

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