Qualitative Analysis of Metal Ions in Solution

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  

In the presence of H, this equilibrium shifts to the left. Therefrae, only the metal sulfides with the smallest values precipitate under acidic conditions. These are Bi2S3, CdS, CuS, and SnS (see Table 17.4). The solution is then filtered to remove the insoluble sulfides. 

and SOU ions. Which compound will precipitate first as silver nitrate is added to the solution  

2 Barium nitrate is added slowly to a solution that is 0.10 M in SOj ions and 0.10 M in F ions. Calculate the concentration of Ba ions (in mol/L) required to initiate the precipitation of BaS04 without precipitating Bap2. 

CHAPTER 17 Acid-Base Equilibria and Solubility Equilibria 

Most toothpastes contain fluoride, which helps to reduce tooth decay. The F ions in toothpaste replace some of the OH ions during the remineralization process  

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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. 

Separation of Ions Using Differences in Solubility Fractional Precipitation Qualitative Analysis of Metal Ions in Solution... 

The flame of an alcohol lamp looks almost colourless. When a length of platinum wire which is dipped into a metal salt solution is put into the flame, the flame is coloured in the upper part by the wire. The colour is peculiar to the kind of the metal strontium colours the flame red, sodium yellow, barium pale green and copper blue. This is applied to the qualitative analysis of metal ions as the colour flame test. The emission of the coloured light is caused by atomic metal gas or a gas consisting of molecules of metal compound, and the process may be set out as follows ... 

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. 

Precipitation reactions have many applications. One is to make compounds. The strategy is to choose starting solutions that form a precipitate of the desired insoluble compound when they are mixed. Then we can separate the insoluble compound from the reaction mixture by filtration. Another application is in chemical analysis. In qualitative analysis—the determination of the substances present in a sample—the formation of a precipitate is used to confirm the identity of certain ions. In quantitative analysis, the aim is to determine the amount of each substance or element present. In particular, in gravimetric analysis, the amount of substance present is determined by measurements of mass. In this application, an insoluble compound is precipitated, the precipitate is filtered off and weighed, and from its mass the amount of a substance in one of the original solutions is calculated (Fig. 1.6). Gravimetric analysis can be used in environmental monitoring to find out how much of a heavy metal ion, such as lead or mercury, is in a sample of water. 

The nitrite ion is an excellent coordinating agent, generally forming metal —N02 bonds (although there are a few cases of metal —ONO bonds reported). One of its most interesting reactions takes place with dipositive cobalt in the presence of dilute acid. In this reaction, part of the nitrite coordinates With the cobalt, and part of the nitrite oxidizes the cobalt to the +3 state (NO being formed). Thus one cobalt ion consumes seven nitrite ions to yield the complex Co(N02)e8, the ion often used in qualitative analysis to precipitate potassium from solution. 

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. 

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

Coordination compounds are used extensively in qualitative analysis as a means of separating certain metal ions and also as a means of positively identifying certain unknown ions. For example, you may have performed an experiment used to identify silver ion in solution. If silver ion is present, the addition of chloride ion gives an immediate white precipitate of silver chloride. 

He also concluded that at such concentrations the ions in the vicinity of the electrode are exhausted by reduction and that the current intensity depends mostly on the number of metal ions transported into this space by diffusion. He also pointed out that the position of the wave on the potential axis is characteristic of the reduced species and can be used for qualitative analysis. From occurrence of waves at -0.4 and -0.6 V in solutions prepared from metallic zinc, it was possible to conclude the presence of two impurities, attributed tentatively to indium and gallium. M. Shikata at the same meeting reported (11) the possibility of carrying out the reduction at the D.M.E. in a nonaqueous system - in sodium ethoxide. In the following year the Japanese chemist reported also the first reduction of an organic compound, nitrobenzene (12,13). 

Describe how you would separate the metal ions in a solution containing silver ion, copper(II) ion, and nickel(II) ion, using the sulfide scheme of qualitative analysis. 

Flame tests are not commonly used to identify transition metals. The presence of a certain transition-metal ion in a solution is sometimes obvious from the solution s color. Some transition-metal ions can be more accurately identified using a procedure called qualitative analysis. Qualitative analysis is the identification of ions by their characteristic reactions. The transition-metal ions most often identified through qualitative analysis include copper, nickel, zinc, chromium, iron cobalt, cadmium, manganese, and tin. Most tests to identify the presence of an ion in a mixture involve causing the ion to precipitate out of solution. Some of the more dramatic precipitation reactions for transition metals are shown. 

Acid-base (neutralization) reactions are only one type of many that are applicable to titrimetric analysis. There are reactions that involve the formation of a precipitate. There are reactions that involve the transfer of electrons. There are reactions, among still others, that involve the formation of a complex ion. This latter type typically involves transition metals and is often used for the qualitative and quantitative colorimetric analysis (Chapters 8 and 9) of transition metal ions, since the complex ion that forms can be analyzed according to the depth of a color that it imparts to a solution. In this section, however, we are concerned with a titrimetric analysis method in which a complex ion-forming reaction is used. 

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. 

Precipitation and dissolution of metal hydroxides The solubility product principle can also be applied to the formation of metal hydroxide precipitates these are also made use of in qualitative inorganic analysis. Precipitates will be formed only if the concentrations of the metal and hydroxyl ions are momentarily higher than those permitted by the solubility product. As the metal-ion concentration in actual samples does not vary much (10—1 —10 3 mol -1 is the usual range), it is the hydroxyl-ion concentration which has the decisive role in the formation of such precipitates. Because of the fact that in aqueous solutions the product of hydrogen- and hydroxyl-ion concentrations is strictly constant (A = 10 14 at 25°C, cf. Section 1.18), the formation of a metal-hydroxide precipitate depends mainly on the pH of the solution. Using the solubility product principle, it is possible to calculate the (minimum) pH required for the precipitation of a metal hydroxide. 

In most of the systems of qualitative analysis for the metal ions use is made of the procedure of sulfide precipitation. This involves the treatment of the solution with hydrogen sulfide, leading to the precipitation of about fifteen of the twenty-three or twenty-four metals that are commonly tested for. 

If the various metals are present in a solution which has been acidified with 6.3 N hydrochloric acid, some of the metal ions precipitate as sulfides and others do not. The metal ions that precipitate as sulfides under these conditions are Hg+-, Cu+, Cd++, Sn++, Sn+ + + +, As+ + +, AS+ + + ++, Sb+ + +, Sb+ + + + -, and Bi+ ++. The solubility products for the corresponding sulfides, HgS, CuS, CdS, PbS, SnS, SnSs, AsgSg, AsoS,v SboSjj, SboS and BigSg, have values corresponding to precipitation under these conditions. These metals are said to constitute the hydrogen sulfide group in the system of qualitative analysis. 

Detection of halogens. Very few organic compounds contain halogen atoms which form halide ions in aqueous solutions and so can be detected by the addition of silver ion. It is necessary, therefore, to convert the organic halogen to a metallic halide by fusion with metallic sodium or calcium oxide. By such decomposition nitrogen and sulfur, if present, form ions which can be detected by the usual methods of qualitative inorganic analysis. 

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