Microscopic identification of salts

Author: Hans-Jürgen Schwarz
English Translation by Sandra Leithäuser

back to Polarized light microscopy or Analysis of Salts

Abstract

This section describes the identification of salts using polarized light microscopy.

Method

The examination of salts can be carried out using several different methods, such as microscopy, spot test analysis, X-ray diffraction. It is important to determine both the anions and cations in the salt(s) and, if possible, the specific phases of the actual salts. In general, laboratories carrying out routine chemical analysis do not usually determine the presence of carbonate ions, and therefore this salt is not identified, although they are often present and can be responsible for the observed efflorescence and eventual deterioration.

Two different approaches to microscopic analysis of salts are presented here:

The first approach considers mainly the analysis of the salts, that is, the salt crystals actually taken from the object.

The second approach analyses the recrystallized salts obtained from the aqueous extraction of either the salts themselves or of a sample of the salt-contaminated material. Both possibilities lead to salt crystallizations that vary, depending on the different kind of salt, as described below. See also Micro-chemical testing.

Determination of individual salts in aqueous extract

Before the actual determination of salts, the sample to be examined (pure salt or material/salt mixture) is mixed with a few drops of distilled water, producing an aqueous extract as discussed by Bläuer. A few drops of this extract are placed onto a microscope slide and observed under the polarizing microscope. It is important to observe the crystallization of the salts from the beginning, i.e., from the appearance of the first small crystals until the end of the crystallization process. Only then, the right conclusions can be drawn, because salts that crystallize later may cover up some salts completely, and therefore their crystal shape may become difficult to recognize and to identify. The continuous documentation of the process is therefore an important option.

Halite (NaCl)

Figures 1-3 show halite crystals. Halite and sylvite (KCl) are the most commonly found isotropic salt crystals, i.e., belonging to the cubic crystal system in building materials. This does not mean that all crystals that appear isotropic on the microscope slide are cubic, like halite and sylvite. On the slide, some salts crystallize at an orientation that make them appear optically isotropic. Therefore caution is vital. In comparison with typical cubic crystal shapes, however, the identification is unambiguous.

Salt	chemical formula	Birefringence	Refractive indices	Crystal system	Optical orientation
Halite	NaCl		nD=1.5443	cubic	isotropic

• Halite, crystallized from aqueous solution on a microscope slide
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  Figure 1: Halite crystals under polarized light

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  Figure 2: Halite crystals under polarized light with analyzer

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  Figure 3: Halite crystals under polarized light with analyzer and red I

Calcium chloride

Calcium chloride only crystallizes at relatively low relative humidity levels (pure salts at a RH <30, 8% and 20°C). Because the relative humidity on objects and in the laboratory usually lies above this value, calcium chloride will only rarely crystallize on an object. To have this salt crystallize so as to observe its crystals under the polarizing microscope, the slide with the solution has to be warmed up until they form. However, the crystals dissolve in the ambient air when the temperature cools down and the RH rises. Figures 4- 6 show calcium chloride crystals while warming.

Salt	chemical formula	Birefringence	Refractive indices	Crystal system	Optical orientation
Antarcticite	CaCl2•6H2O	Δ= 0.024	no =1.417-1.494 ne = 1.393-1.550	trigonal	negative

• Calcium chloride, crystallized from aqueous solution on a microscope slide
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  Figure 4: Calcium chloride crystals under polarized light

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  Figure 5: Calcium chloride crystals under polarized light with analyzer

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  Figure 6: Calcium chloride crystals under polarized light with analyzer and red I

Niter

Salt	Chemical formula	Birefringence	Refractive indices	Crystal system	Optical orientation
Niter	KNO3	Δ = 0.171	α = 1.335 β = 1.505 γ = 1.506	orthorhombic	biaxial negative

• Niter crystallized from aqueous solution on a microscope slide
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  Figure 7:Potassium nitrate crystals under polarized light

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  Figure 8: Potassium nitrate crystals under polarized light with analyzer

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  Figure 9: Potassium nitrate crystals under polarized light with analyzer and red I

Calcium nitrate

Salt	Chemical formula	Birefringence	Refractive indices	Crystal system	Optical orientation
Nitrocalcite

• Calcium nitrate crystallized from aqueous solution on a microscope slide
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  Figure 10:Calcium nitrate crystals under polarized light

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  Figure 11:Calcium nitrate crystals under polarized light with analyzer

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  Figure 12:Calcium nitrate crystals under polarized light with analyzer and red I

Magnesium nitrate

Salt	Chemical formula	Birefringence	Refractive indices	Crystal system	Optical orientation
Nitromagnesite	Mg(NO3)2•6H2O	Δ = 0.166	nx = 1.34 ny = 1.506 nz = 1.506	monoclinic	negative

• Magnesium nitrate crystallized from aqueous solution on a microscope slide
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  Abbildung 13:Magnesium nitrate crystals under polarized light

• Mg-NO3-2 (1).jpg Figure 14:Magnesium nitrate kristalle unter polarisiertem Licht mit Analysator und Rot I

Gypsum

Salt	chemical formula	Birefringence	Refractive indices	Crystal system	Optical orientation
Gypsum	CaSO4•2H2O	Δ = 0.0092	α = 1.5207 β = 1.5230 γ = 1.5299	monoclinic	biaxial positive

• Gypsum crystallized from aqueous solution on a microscope slide
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  Figure 15:Calcium sulfate crystals under polarized light

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  Figure 16:Calcium crystals under polarized light with analyzer

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  Figure 17:Calcium sulfate crystals under polarized light with analyzer and red I

Magnesium sulfate

Salt	chemical formula	Birefringence	Refractive indices	Crystal system	Optical orientation
Epsomite	MgSO4•7H2O	Δ = 0.0284	nx = 1.432 ny = 1.453 nz = 1.4609	orthorhombic	biaxial negative

• Magnesium sulfate crystallized from aqueous solution on a microscope slide
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  Figure 18:Magnesium sulfate crystals under polarized light

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  Figure 19:Magnesium sulfate crystals under polarized light with analyzer

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  Figure 20:Magnesium sulfate crystals under polarized light with analyzer and red I

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  Figure 21:Magnesium sulfate crystals under polarized light

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  Figure 22:Magnesium sulfate crystals under polarized light with analyzer

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  Figure 23:Magnesium sulfate crystals under polarized light with analyzer and red I

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  Figure 24: Magnesium sulfate drop of crystallized solution under polarized light

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  Figure 25: Magnesium sulfate drop of crystallized solution under polarized light with analyzer

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  Figure 26: Magnesium sulfate drop of crystallized solution under polarized light with analyzer and red I

Sodium sulfate

Salt	chemical formula	Birefringence	Refractive indices	Crystal system	Optical orientation
Thenardite	Na2SO4	Δ = 0.015	nx = 1.468 ny = 1.473 nz = 1.483	orthorhombic	positive

• Sodium sulfate crystallized from aqueous solution on a microscope slide
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  Figure 27 : Sodium sulfate crystals under polarized light

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  Figure 28 : Sodium sulfate crystals under polarized light detail from previous photo

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  Figure 29 : Sodium sulfate crystals under polarized light with analyzer and red I

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  Figure 30: Sodium sulfate crystals under polarized light

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  Figure 31: Sodium sulfate under polarized light detail from previous photo

Sodium carbonate

Salt	chemical formula	Birefringence	Refractive indices	Crystal system	Optical orientation
Natrite	Na2CO3	Δ = 0.131	nx = 1.415 ny = 1.535 nz = 1.546	monoclinic	biaxial negative

• Sodium carbonate crystallized from aqueous solution on a microscope slide
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  Figure 32: Sodium carbonate crystals under polarized light

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  Figure 33: Sodium carbonate under polarized light detail from previous photo

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  Figure 34: Sodium carbonate crystals under polarized light with analyzer and red I

Sodium acetate

• Sodium acetate crystallized from aqueous solution
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  Figure 35: Sodium acetate crystals under polarized light

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  Figure 36: Sodium acetate crystals under polarized light, detail Figure 35

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  Figure 37: Sodium acetate crystals under polarized light with analyzer and red I, detail Figure 35

crystallization of a dissolved salt

relative humidity