Solubility qualitative cation analysis

Criteria for Precipitation and Its Completeness 18-6 Fractional Precipitation 18-7 Solubility and pH 18-8 Equilibria Involving Complex Ions 18-9 Qualitative Cation Analysis... 

Briefly describe each of the following ideas, methods, or phenomena (a) common-ion effect in solubility equilibrium (b) fractional precipitation (c) ion-pair formation (d) qualitative cation analysis. 

If phosphate has been detected previously (e.g. in the cation analysis), a qualitative test for arsenate may be made as follows. Pour 1 ml AgN03 solution, to which 2 drops of dilute acetic acid have been added, over the white precipitate. A brownish-red colouration of the precipitate confirms the presence of arsenate. The acetic acid is added to increase the solubility of the magnesium salt and thus facilitate the conversion of magnesium ammonium arsenate to the characteristic silver arsenate. 

When a precipitate has been formed during the qualitative analysis of the ions present in a solution, it may be necessary to dissolve the precipitate again to identify the cation or anion. One strategy is to remove one of the ions from the solubility equilibrium so that the precipitate will continue to dissolve in a fruitless chase for equilibrium. Suppose, for example, that a solid hydroxide such as iron(IIl) hydroxide is in equilibrium with its ions in solution ... 

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. 

In a laboratory environment, the method of qualitative analysis of cations is often required. The objective of qualitative analysis is to separate and identify the cations present in an unkonwn solution. Cations are first separated into five main groups depending on their solubilities ... 

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. 

Qualitative chemistry is an area of chemistry concerned with identifying substances. In Activity 9.1 you will perform a qualitative analysis to detect the presence of certain ions that, in turn, may reveal an art forgery. The ions could come from paints that were not available at the time of the artwork. In this qualitative analysis, metal ions (cations) and nonmetal ions (anions) are reacted with solvents and with each other. Then the cations and anions present are identified by the products produced. In addition, flame tests and pH determinations are used to identify ions. Qualitative analysis is an engaging opportunity for you to develop experience with chemical change and review solubility principles. Nowadays, however, most of the time a chemist analyzes a substance to detect ion content using quantitative analytical computerized instruments. 

The classic scheme for qualitative analysis of a mixture containing all the common cations (listed in Fig. 8.13) involves first separating the cations into five major groups based on solubilities. (These groups are not directly related to the groups of the periodic table.) Each group is then treated further to separate and identify the individual ions. We will be concerned here only with separation of the major groups. 

Complex ions are formed in solution by the combination of a metal cation with a Lewis base. The formation constant Kf measures the tendency toward the formation of a specific complex ion. Complex ion formation can increase the solubility of an insoluble substance. Qualitative analysis is the identification of cations and anions in solution. 

Here we will briefly discuss qualitative analysis, the determination of the types of ions present in a solution. We will focus on the cations. There are some 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 12.6). Because an unknown solution may contain from 1 to all 20 ions, any analysis must be carried out systematically from group 1 gqgedure... 

In the study of inorganic chemistry, especially qualitative analysis, it is useful to know the approximate solubility of common substances. The simple rules of solubility are given below. These rules apply to compounds of the common cations Na, K, Mg" " ", Ca, Ba" ", ... 

Qualitative analysis is the identification of cations and anions in solution. It is based largely on the principles of solubility equilibria. 

Fig. 9 shows the ellipsometric isotherm A — Ao(triangles) of the cationic surfactant C12-DMP bromide. A pronounced non-monotonic behaviour is shown with an extremum at an intermidiate concentration far below the cmc. Also shown is the number density of amphiphiles adsorbed to the interface (circles) as determined by Surface second harmonic generation (SHG). At these bulk concentrations the measured number density equals the surface excess F. SHG reveals a monotoneous increase in the surface excess in qualitative agreement to a thermodynamic analysis within the Gibb s framework. The data also clearly prove that the ellipsometric quantity need not be proportional to the adsorbed amount for a soluble ionic surfactant. What causes the nonmonotonous behaviour and how can it be understood ... 

Co-polymers usually dissolve in a greater number of solvents than homopolymers, eg. poly(vinyl chloride) is only very slightly soluble in acetone or methylene chloride, in which co-polymers with vinyl acetate or acrylates easily dissolve. Attempts have failed to separate plastics according to their solubility in the same way in which cations are separated in inorganic qualitative analysis. Table 6.4 summarizes the solubility of plastics. 

Qualitative analysis operates on the principle that a mixture of cations can be separated and analyzed based on the differences in the solubilities of their salts. 

Konig and Waldorf discuss analysis of hair and body shampoos (32). The shampoo is first evaporated to dryness and extracted with isopropanol or 95% ethanol to separate the surfactant, which is qualitatively identified by IR. The alcohol-insoluble residue is also examined by IR in case a less-soluble surfactant remains there. The alcohol extract is then separated into anionic, cationic, nonionic, and amphoteric surfactant fractions by ion exchange. Anionics can be separated into sulfonates and carboxylates by use of strongly basic anion exchange resins in the Cl" and OH" form, respectively. Anionics are further characterized by TLC. Nonionics are likewise characterized by TLC. Once the components are identified, quantitative analysis is by the usual methods, described elsewhere in this volume. 

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