Qualitative analysis for metallic elements

In this final section, we look at how solubility equilibria and complex-ion formation can be used to detect the presence of particular metal ions in solution. Before the development of modem analytical instmmentation, it was necessary to analyze mixtures of metals in a sample by what were called wet chemical methods. For example, an ore sample that might contain several metallic elements was dissolved in a concentrated acid solution that was then tested in a systematic way for the presence of various metal ions. 

Qualitative analysis determines only the presence or absence of a particular metal ion, whereas quantitative analysis determines how much of a given substance is present. Even though wet methods of qualitative analysis have become less important in the chemical industry, they are frequently used in general chemistry laboratory programs to 

A scheme in general use divides the common cations into five groups (T FIGURE 17.23). The order in which reagents are added is important in this scheme. The most selective separations—those that involve the smallest number of ions—are carried out first. The reactions used must proceed so far toward completion that any concentration of cations remaining in the solution is too small to interfere with subsequent tests. 

Let s look at each of these five groups of cations, briefly examining the logic used in this qualitative analysis scheme. 

Group I. Insoluble chlorides Of the common metal ions, only Ag, Hg2, and Pb form insoluble chlorides. When HCl is added to a mixture of cations, therefore, only AgCl, Hg2Cl2, and PbCl2 precipitate, leaving the other cations in solution. The absence of a precipitate indicates that the starting solution contains no Ag Hg2 +,orPb  

Group 1. Insoluble chlorides Of the common metal ions, only Ag and 

Group 2. Acid-insoluble sulfides After any insoluble chlorides have been removed, the remaining solution, now acidic from HCl treatment, is treated with H2S. Since H2S is a weak acid compared to HCl, its role here is to act as a source for small amounts of sulfide. Only the most insoluble metal sulfides— CuS, Bi2S3, CdS, PbS, HgS, AS2S3, Sb2S3, and SnS2—precipitate. (Note the very small values of for some of these sulfides in Appendix D.) Those metal ions whose sulfides are somewhat more soluble—for example, ZnS or NiS—remain in solution. 

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QUALITATIVE ANALYSIS FOR METALLIC ELEMENTS We explain how the principles of solubility and complexation eqiilibria can be used to identify ions in solution. 

Symbol of the element whose presence can interfere with the qualitative analysis for Pb2+, Hg22+, and Ag+. When chloride ions are added to an aqueous solution of this metal ion, a white precipitate forms with formula MOCl. 

Qualitative analysis AES is an almost comprehensive methods for qualitative elemental analysis for metals, metalloids, and nonmetals with the exception of some of the permanent gases. Its sensitivity range is great, varying from parts per biUion to percent levels. Many elements can be detected simultaneously. Spectral overlap is the major limitation. 

SECTION 17.7 Metallic elements vary a great deal in the solubilities of their salts, in their acid-base behavior, and in their tendencies to form complex ions. These differences can be used to separate and detect the presence of metal ions in mixtures. Qualitative analysis determines the presence or absence of species in a sample, whereas quantitative analysis determines how much of each species is present. The qualitative analysis of metal ions in solution can be carried out by separating the ions into groups on the basis of precipitation reactions and then analyzing each group for individual metal ions. 

Analytical electron microscopy permits structural and chemical analyses of catalyst areas nearly 1000 times smaller than those studied by conventional bulk analysis techniques. Quantitative x-ray analyses of bismuth molybdates are shown from lOnm diameter regions to better than 5% relative accuracy for the elements 61 and Mo. Digital x-ray images show qualitative 2-dimensional distributions of elements with a lateral spatial resolution of lOnm in supported Pd catalysts and ZSM-5 zeolites. Fine structure in CuLj 2 edges from electron energy loss spectroscopy indicate d>ether the copper is in the form of Cu metal or Cu oxide. These techniques should prove to be of great utility for the analysis of active phases, promoters, and poisons. 

The analytical measurement of elemental concentrations is important for the analysis of the major and minor constituents of pharmaceutical products. The use of atomic spectroscopy in this regard has been the subject of several reviews (2,3,35,36). Metals are major constituents of several pharmaceuticals such as dialysis solutions, lithium carbonate tablets, antacids, and multivitamin-mineral tablets. For these substances, spectroscopic analysis is an important tool. It is indispensable for the determination of trace-metal impurities in pharmaceutical products and the qualitative and quantitative analysis of metals, essential and toxic, in biological fluids and tissues (37). Beyond this, several drugs which do... 

The experiments to illustrate the properties of metals and their compounds are followed by an outline of a general method for testing a simple salt for the metal and acid radical present. The short time available in the first year for qualitative analysis makes it impossible for the student to master the methods used in the separation of the metallic elements. He can, however, by the simple procedure outlined learn to identify pure salts. The method described has been found to be an excellent introduction to qualitative analysis. 

This chapter deals with optical atomic, emission spectrometry (AES). Generally, the atomizers listed in Table 8-1 not only convert the component of samples to atoms or elementary ions but, in the process, excite a fraction of these species to higher electronic stales.. 4, the excited species rapidly relax back to lower states, ultraviolet and visible line spectra arise that are useful for qualitative ant quantitative elemental analysis. Plasma sources have become, the most important and most widely used sources for AES. These devices, including the popular inductively coupled plasma source, are discussedfirst in this chapter. Then, emission spectroscopy based on electric arc and electric spark atomization and excitation is described. Historically, arc and spark sources were quite important in emission spectrometry, and they still have important applications for the determination of some metallic elements. Finally several miscellaneous atomic emission source.s, including jlanies, glow discharges, and lasers are presented. 

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