Qualitative techniques for inorganic analysis

Qualitative techniques are used to identify cations and anions in aqueous solution by simple reactions, mostly involving the production of a precipitate, evolution of gas or a visual colour change. It is important to make observations accurately and to interpret them in a step-wise manner. 

The following basic equipment is required to carry out qualitative analysis  

Wash bottle - always keep a wash bottle of distilled water handy. 

At the start of your experimental work always check that the appropriate reagents are readily available (a list of commonly used reagents is given in Table 19.1). Note that it is essential to use distilled water in all qualitative analysis. Tap water contains ions such as Ca2+. Mg2+. Fe2, Fe3+, SOf and Cl and its use could lead to false-positive results. 

Spatula - never place the spatula in the test solution, it may lead to false positive tests for iron and chromium. 

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Qualitative techniques are used to identify cations and anions in aqueous 

Specific literature containing tests for the determination of anions and cations can be found in the resources section (p. 160). In general, however, the following tips are useful when carrying out qualitative analysis  

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Many methods used for qualitative analysis are destmctive, that is, the sample is consumed during the analysis or must be chemically altered in order to be analyzed. The most sensitive and comprehensive elemental analysis methods for inorganic analysis are inductively coupled plasma atomic emission spectrometry (ICP-OES or ICP-AES), discussed in Chapter 7, and ICP-MS, discussed in Chapters 9 and 10. These techniques can identify almost all the elements in the periodic table, even when only trace amounts are present, but often require that the sample be in the form of a solution. If the sample is a rock or a piece of glass or a piece of biological tissue, the sample usually must be dissolved in some way to provide a solution for analysis. We will see how this is done later in the chapter. The analyst can determine accurately what elements are present, but information about the molecules in the sample is often lost in the sample preparation... 

Environmental laboratories use a great variety of analytical methods for different types of organic and inorganic pollutant determinations. In this chapter, we will review the main instrumental techniques, their applications, and limitations in the analyses of environmental matrices, while focusing on qualitative aspects of environmental analysis. 

ICPMS can be used for both qualitative and quantitative trace and ultra trace elemental analysis of inorganics and in isotope ratio determinations. Both cations and anions can be determined. Normally, the sample is introduced in the form of a solution into the plasma, but direct analysis of gaseous or solid sample is also possible. Hence, ICP-MS has grown into a referral technique for the ultra trace analysis of REE in electronic materials, and metallurgical samples. The individual REE concentrations in natural and sea waters are so low and require preconcentration techniques prior to determination by ICP-MS. Separation in addition to preconcentration is also needed as high salt matrix of sea waters results in irreproducible results in ICP-MS analysis of individual REE. 

Inductively coupled plasma-atomic emission spectrometry allows the determination of anionic surfactants (LAS and AS) and inorganic compounds (phosphate, silicate, zeolite, sulfate). Other techniques, such as X-ray fluorescence spectroscopy and X-ray powder diffraction, have been used for the qualitative analysis of inorganic detergents. For surface analysis, optical light microscopy, scanning electron microscopy, and transmission electron microscopy characterize particles, deposition of surfactant, or other detergent ingredients on fabric. 

XRD is a technique that is useful for the analysis of solid crystalline or semicrystalline materials. Most organic and inorganic compounds, minerals, metals, and alloys, and many types of polymers form crystals and can be analyzed by XRD. XRD can provide the exact crystal structure of a pure single-crystal material. In addition, XRD can provide the qualitative and quantitative identification of the molecules present in pure crystalline powders or mixtures of crystalline powders. 

Raman spectroscopy is an accepted vibrational spectroscopic technique for determining and monitoring both qualitative and quantitative molecular information of organic and inorganic molecular species from almost any type of sample (i.e. as a solid, liquid or gas) [16, 17]. Raman spectroscopy is attractive for on-line analysis due to the relatively short analysis times (1-30 s on average) and ease of optical sampling. 

Thin-layer chromatography is used primarily as a qualitative analytical technique for the identification of organic and inorganic solutes by comparisons of samples with standards chromatographed simultaneously. Quantitative analysis is possible but precision is relatively poor. 

Raman spectroscopy is a good technique for qualitative analysis and discrimination of organic and/or inorganic compounds in mixed materials. A Raman spectrum can be obtained from samples that are as small as 1 xm. The intensities of bands in a Raman spectrum depend on the sensitivity of the specific vibrations to the Raman effect and are proportional to concentration. Thus, Raman spectra can be used for semiquantitative and quantitative analyses. The technique is used for identification of organic molecules, polymers, biomolecules, and inorganic compounds both in bulk and as individual particles. Raman spectroscopy is particularly useful in determining the structure of different types of carbon (diamond, graphitic, diamond-like-carbon, etc.) and their relative concentrations. 

This technique is useful in the analysis of compounded plastic samples. It is possible to obtain accurate quantifications of the principal compound constituents such as plasticiser, polymer and inorganic species. This data can be obtained on small (e.g., 10 mg) samples in a relatively short time (around 45 minutes). By the use of the temperature maxima at which weight events occur it is also possible to use the technique to obtain some qualitative assignments for the plasticiser and polymer. 

Faradaic rectification polarographic studies have been carried out for a mixture containing several metal ions together and also for individual inorganic depolarizers so as to explore the applicability and limitations of the method and to determine kinetic parameters for some of them. For comparison, some of the dc and ac polarograms have also been recorded simultaneously. In the following, the details of the experimental technique used will be described and the potentiality of the technique in qualitative and quantitative analysis will be examined. The applicability of the method in the... 

The SEM is also used to do X-ray/elemental analysis. This technique is qualitative. X-ray analysis and mapping of the particular elements present is useful for the identification of inorganic fillers and their dispersion in compounds as well as inorganic impurities in gels or on surfaces and curatives, e.g., aluminum, silicon, or sulfur in rubber compounds and Cl and Br in halobutyl blends. (Figure 9)... 

The qualitative analysis of inorganic substances in the solid state or in aqueous solution was the object of numerous Raman investigations, especially within the aim of the assignment of main lines to vibrational modes. Much less attention was devoted to the identification and quantitative determination of the content of each species in a mixture. The demand of new techniques and instruments for the real-time and on line monitoring of substances in water requires more reliable quantitative processes. Simultaneously the progress in the development of more compact transportable or portable Raman instruments has made possible the availability of appropriate Raman sensors. ... 

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