Selective precipitation qualitative analysis

Qualitative Analysis Selective Precipitation Qualitative Analysis... 

Section 16.2 ion product selective precipitation qualitative analysis... 

Complex formation, selective precipitation, and control of the pH of a solution all play important roles in the qualitative analysis of the ions present in aqueous solutions. There are many different schemes of analysis, but they follow the same general principles. Let s think through a simple procedure for the identification of five cations by following the steps that might be used in the laboratory. We shall see how each step makes use of solubility equilibria. 

Qualitative analysis involves the separation and identification of ions by selective precipitation, complex formation, and the control of pH. 

Seven chemical reactions were identified from the chemistry syllabus. These chemical reactions were selected because they were frequently encountered during the 2-year chemistiy course and based on their importance in understanding concepts associated with three topics, namely, acids, bases and salts, metal reactivity series and inorganic chemistry qualitative analysis. The seven types of chemical reactions were combustion of reactive metals in air, chemical reactions between dilute acids and reactive metals, neutralisation reactions between strong acids and strong alkalis, neutralisation reactions between dilute acids and metal oxides, chemical reactions between dilute acids and metal carbonates, ionic precipitation reactions and metal ion displacement reactions. Although two of the chemical reactions involved oxidation and reduction, it was decided not to include the concept of redox in this study as students had only recently been introduced to ion-electron... 

These methods may prove useful in the qualitative analysis of organic compounds, once the selectivities of the precipitants are understood. The metallic oxides suffer from the disadvantage of producing a precipitate which is difficult to filter, while calcite and zirconium phosphates produce relatively well-mannered precipitates. Even when the efficiencies of collection of various model compounds in seawater is known, the immense variety of organic compounds in seawater will keep this technique largely qualitative. 

Qualitative analysis is a procedure for identifying the ions present in an unknown solution. The ions are identified by specific chemical tests, but because one ion can interfere with the test for another, the ions must first be separated. In the traditional scheme of analysis for metal cations, some 20 cations are separated initially into five groups by selective precipitation (Figure 16.17). 

When solutions of soluble ionic compounds are mixed, an insoluble compound will precipitate if the ion product (IP) for the insoluble compound exceeds its fCsp. The IP is defined in the same way as /equilibrium concentrations. Certain metal cations can be separated by selective precipitation of metal sulfides. Selective precipitation is important in qualitative analysis, a procedure for identifying the ions present in an unknown solution. 

The qualitative analysis scheme for cations based on the selective precipitation procedure described above is summarized in Fig. 8.13. 

Note that the qualitative analysis of cations by selective precipitation involves all the types of reactions we have discussed and represents an excellent application of the principles of chemical equilibrium. 

In this chapter we will discuss the formation of solids from an aqueous solution and the resulting equilibria. We will also show how selective precipitation and the formation of complex ions can be used to do qualitative analysis. 

Two points regarding qualitative analysis must be mentioned. First, the separation of the cations into groups is made as selective as possible, that is, the anions that are added as reagents must be such that they will precipitate the fewest types of cations. For example, all the cations in group 1 also form insoluble sulfides. Thus, if HjS were... 

The principle of selective precipitation can be used to identify the types of ions present in a solution. This practice is called qualitative analysis. There are about 20 common cations that can be analyzed readily in aqueous solution. These cations can be divided into five groups according to the solubility products of their insoluble salts (Table 17.4). Because an unknown solution may contain fi om 1 to all 20 ions, any analysis must be carried out systematically from group 1 through group 5. The general procedure for separating these 20 ions is as follows ... 

Groups of cations can be identified through the use of selective precipitation. This is the basis of qualitative analysis. 

Selective precipitation as discussed in Section 16.6 can be used in a systematic way to determine which metal ions are present in an unknown solution. This method is known as qualitative analysis. The word qualitative means involving quality or kind. Qualitative analysis involves finding the kind of ions present in the solution. This stands in contrast to quantitative analysis, which is concerned with quantity, or the amounts of substances in a solution or mixture. 

In the past, qualitative analysis by selective precipitation was used extensively to determine the metals present in a sample. This process—dubbed wet chemistry because it involves the mixing of many aqueous solutions in the lab—has been replaced by more precise and less time-intensive instrumental techniques. Nonetheless, both for the sake of history and also because of the importance of the principles involved, we now examine a traditional qualitative analysis scheme. You may use such a scheme in your general chemistry laboratory as an exercise in qualitative analysis. 

Precipitation of sulphides Hydrogen sulphide gas is a frequently used reagent in qualitative inorganic analysis. When hydrogen sulphide gas is passed into a solution, metal sulphides are precipitated. For this precipitation the rule mentioned above can be applied precipitation may take place only if the product of concentrations of metal ions and sulphide ions (taken at proper powers) exceed the value of the solubility product. While the concentration of metal ions usually does fall into the range of 1-10 3 mol 1, the concentration of sulphide ion may vary considerably, and can easily be selected by the adjustment of the pH of the solution to a suitable value. 

Hydrogen sulfide used to have importance in analytical chemistry for well over a century, in the qualitative inorganic analysis of metal ions. In these analyses, heavy metal (and nonmetal) ions (e g., Pb(II), Cu(II), Hg(II), As(III)) are precipitated from solution upon exposure to H2S. The components of the resulting precipitate redissolve with some selectivity. 

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