Analysis of Salts

Author: Hans-Jürgen Schwarz

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Abstract

To determine the presence of a salt or visible damage from it, a wide variety of analytical methods are used. Methods presented here may allow a qualitative determination of individual ions or salt phasesa and/or quantitative determination of the salt-forming ions. The fundamental principle of each method, its application and advantages are briefly outlined.

Polarized light microscopy

With the help of a polarizing microscope, the various salts present can be identified through their optical properties. This includes in particular the study of the birefringence and light refraction of the salts. The polarizing microscope differs from a regular microscope in that it has two polarizers - the polarizer and the analyzer. The analyzer is used to recognize the optical properties of a salt by the change of the linearly polarized light due to the salt.

Micro-chemical testing

Micro-chemical testing allows to identify salts in very small samples. The micro-chemical salt analysis is based on the detection of individual ions in more or less concentrated aqueous solutions. The salts are identified via the reaction products that can be observed with a microscope at magnifications up to 100 times. The method can be used on site.

Instrumental Analysis

X-Ray Diffraction Analysis (XRD)

XRD analysis allows both qualitative and quantitative salt analyses. When X-rays impinge on a crystalline material a series of "reflections" are produced that are characteristic for each phase, similar to a "fingerprint". It is a laboratory method and requires small samples or even very small ones if using special sample holders.

Atomic absorption spectroscopy(AAS)

Atomic absorption spectroscopy is a method that allows rapid and accurate quantitative determinations of many elements. Since the method relies on the specific energy absorption by free atoms, it can usually be used without prior separation or isolation steps of other elements present in the sample. The quantity of sample needed for analysis is low. Element concentrations of a millionth (ppm) or one billionth part (ppb) of the sample can be detected.

Inductive coupled plasma (ICP)

An inductively coupled plasma (ICP) is a type of plasma source in which the energy is supplied by electric currents produced by electromagnetic induction, that is, by time-varying magnetic fields. The inductively coupled plasma is used in a the high-frequency field (27 MHz) ionized gas (argon), which serves as an atomization and excitation medium for the sample. It is used in conjunction with atomic emission spectroscopy (ICP-AES) or with mass spectrometry (ICP-MS). It can detect very low concentrations of the analyzed elements in the sample, in the order of one millionth (ppm) or one billionth part(ppb).

Ion chromatography (IC)

Ion chromatography (or ion-exchange chromatography) is a process that allows the separation of ions and polar molecules based on their charge. It separates the substances as these are preferentially distributed between a stationary and a mobile phase. In ion chromatography, the ions bound to the stationary phase are later released by an eluent in the mobile phase. Quantification is achieved using a suitable detector. Specific columns are used to separate both anions and cations and these can be determined quantitatively even when in low concentrations.

Photometry

The basic principle of photometry is to produce a colored or light-absorbing solution. The ion or element to be determined is incorporated into a stable, colored and soluble compound, or it triggers a color reaction. With a photometer the absorption of monochromatic radiation through the solution is measured. Quantitative analysis of anions and cations in a sufficient accuracy is possible.

the mathematical difference between the largest and smallest refractive index for an anisotropic mineral

X-ray diffraction

In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. A very high heating over a few thousand K leads to ionization, thus turning it into a plasma, containing charged particles.

Inductive Coupled Plasma, method for analysing chemical elements.

carrier liquid or mobile phase of a gas

In physics, monochromatic refers to electromagnetic radiation of a single frequency.

Scanning Electron Microscope