Metal ions qualitative analysis

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 halides of all the metals except silver, lead, mercurous mercury, and cuprous copper are soluble in water, but with the ions of these metals, the halide ions give characteristic precipitates. The precipitates are valuable as tests for identifying either the halogens or the metals in qualitative analysis. 

Precipitation is incomplete owing to the free mineral acid produced. If the hydrogen ions, arising from the complete ionization of the mineral acid, are removed by the addition of the salt of a weak acid, such as ammonium or sodium acetate, precipitation is almost complete. This is the basis of one of the methods for the removal of phosphates, which interfere with the precipitation of Group IIIA metals, in qualitative analysis. 

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. 

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... 

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... 

Metal salicylates are occasionally incorporated into mixtures of unknowns for qualitative inorganic analysis. During the conventional group separation, organic radicals are removed by evaporation with nitric acid. When salicylates are present, this can lead to formation of trinitrophenol through nitration and decarboxylation. This may react with any heavy metal ions present to form unstable or explosive picrates, if the evaporation is taken to dryness. The MAQA alternative scheme of analysis obviates this danger. 

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). 

Detailed procedures for separating and identifying all the ions can be found in general chemistry laboratory manuals. Although modern methods of metal-ion analysis employ sophisticated analytical instruments, qualitative analysis is still included in many general chemistry laboratory courses because it is an excellent vehicle for developing laboratory skills and for learning about acid-base, solubility, and complex-ion equilibria. 

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 activity in eChapter 16.15 shows the procedure for separating a mixture of metal ions into groups that may be further analyzed to determine the specific ions present. 

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. 

Another qualitative analysis technique is a flame test. A dissolved ionic compound is placed in a flame. Table 9.4 lists the flame colours associated with several ions. Notice that all the ions are metallic. The flame test is only useful for identifying metallic ions in aqueous 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. 

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. 

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. 

Therefore no longer qualitative statements are cast into a numerical framework but easily measurable values are linked to obtain information on complex stability or bond energies. Thus, by linear regression analysis, two new parameters are obtained which -once they are known for sufficiently many different metal ions, both essential and non-essential ones - in turn can be linked to this biochemical property of essentiality. Electrochemical ligand parameters for different complexes of the same metal ion are correlated... 

The sulfides of the alkali and alkaline-earth metals are colorless substances easily soluble in water. The sulfides of most other metals are insoluble or only very slightly soluble in water, and their precipitation under varying conditions is an important part of the usual scheme of qualitative analysis for the metallic ions. Many metallic sulfides occur in nature important sulfide ores include FeS, CugS, CuS, ZnS, AggS, HgS, and PbS. 

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. 

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