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WHAT TOPICS BELONG IN A MODERN COURSE IN QUALITATIVE ANALYSIS?' ESMARCH S. GILREATH Washington and Lee University, Lexington, Virginia
THE
title of this paper may appear somewhat presumptuous. Certainly, it seems proper to raise the questions as to what constitutes a modern course in qualitative analysis, and who is to he the judge as to whether a course is modern or not. If we assume that the majority of teachers in qualitative analysis keep abreast with the trends in their field, then the answer to what constitutes a modern course may be answered in part by the type of textbook which is most widely used. Two illustrative statements by authors are: . . w i t h the more advanced techniques of spectroscopy b e coming fully developed in their application to qualitative anslysin, the wet analysifi became ontmoded, and, considering that perhaps less than one per cent of the students would ever make prsetical application of the detailed laboratory knowledge of this course, we were faced with the rruestion of either twine t o eliminate this course altogether from-the curriculum or changing it so drasticallv t h a t the laboratorv work would be an adiunet but
The ronventional course in qualitative analysis has lost favor as a. suhjeet of stud" in the curriculums of many colleges and universities during the past several ycars. Oftentimes, where the course has disappeared, there has appeared a simplified version of the q~~alitrttiw scheme of analysis within the laboratory exercises of general chemistry. Perhaps there are many good reasons
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Presentedas part of theSymposium on Quslitat,ive AnalysisWhat, Why, Hoa-?-before the Division of Chemical Education a t the 130th Meeting of the American Chemical Society, Atlsntie City, Septemher, 1856. ' HOGNEFS,T. R.,A N D W. C. JOHNSON, "Qualitative Analysis and Chemical Equilibrium," 3rd ed., Henry Holt & Co., Inc., New York, 1947. GILREATH, E. S., "Qualitative Andysi8," McGmw-Hill Book Ca., Inc., 1954, p. v
VOLUME 34, NO. 8, AUGUST, 1957
underlying these changes, hut without any attempt to list or analyze t,hem, it is evident that qualitative analyais as taught in the past does not meet the requirements of a present-day course of instruction in chemistry. I n most schools where the subject of qualitative analysis is retained, there is s. demand for s. broader thearetied approach with less emphasis upon I ~ b o r a tory e~oreises.~
EMPHASIS ON THEORY
In view of the foregoing statements, we will assume for the purpose of this discussion that a modern course in qualitative analysis is one that lays heavy emphasis upon theory and less stress upon experimental operations. Such a course permits the development of theoretical aspects in an orderly manner with little regard to their immediate application to laboratory ~rocedures. This does not mean that correlation of theory with laboratory work is impossible, but it is certainly illogical to present unconnected theoretical items in an attempt to stress an outmoded scheme of analysis. Whereas general chemistry is a broad survey of all types of substances under varying conditions, qualitative analysis is a more restricted subject which should be primarily concerned with solutions of electrolytes. The theory of qualitative analysis is, therefore, a study of the behavior of ions in aqueous solution. Of necessity, this study is elementary and often incomplete, partly because students have not had the mathematical background for a more comprehensive treatment, and partly because much of the theory is still in the speculative stage. As an introduction to the behavior of ions in solution, it seems advisable to explore some of the structures found among inorganic substances, and to attempt
some explanation of why certain compounds are soluble and others are not. This approach requires an elementary analysis of those forces which hold atoms, ions, and molecules together. Such a discussion should be an expansion of what was presented in general chemistry, and should include the electrostatic forces operating between ions, the covalent bonding occurring between atoms, and the dipole forces which may bind atoms, ions or molecules. In connection with the forces producing bonds between atoms and molecules it is desirable to describe the differences and similarities among ionic, covalent, and polar compounds, as well as certain conditions necessary for the formation of each boud-type. The problem of solubility also embraces the role of water as a solvent, and calls for a simplified discussion of the energy factors involved in bringing a substance into solution. Most of the theories of qualitative analysis are concerned with the physical and chemical properties of solutions. Among the physical characteristics which should be described are the electrical and colligative properties. By means of these properties it appears logical to emphasize the differences in compounds resulting from the various modes of bonding. Other properties of solutions provide an introduction t o certain well-known theories such as the Arrhenius theory of ionization with its shortcomings, the DebyeHiickel theory of complete ionization and interionic attraction, as well as the concept of activity which arises from this theory, and the Br$nsted definitions of acids and bases. AQUEOUS SOLUTIONS EQUILIBRIA
Important in the study of qualitative analysis are the laws and theories which pertain to the various forms of equilibria that may exist in aqueous solution. Among these equilibria are those that apply to ionization, solubility, complex formation, and oxidation-reduction, Unfortunately, the law of mass action is sometimes used rather loosely in describing systems in equilibrium. The law of mass action, which deals with reaction rates, is not of itself particularly important in its application to ionic reactions, since such reactions take place quite rapidly. However, a mathematical interpretation of this law serves as an excellent introduction to the presentation of equilibrium constants for the various forms of chemical equilibrium. The applications of ionic equilibria are usually classified into two types based upon the homogeneity of the system in equilibrium. For a heterogeneous system, the solubility product principle not only provides a means of relating solubility to solubility product constants, but it may also be used to identify other factors affecting solubility such as temperature, common ion effect, particle size, and salt effect. Furthermore, this principle offers an opportunity to correlate classroom theory with laboratory exercises. Topics of this nature should include fractional precipitation, and the relation of pH to solubility. The application of the law of chemical equilibrium to the ionization reactions of weak electrolytes is fairly well standardized in most textbooks. This is not true for the topic usually designated as hydrolysis. A modern approach to the topic of hydrolysis will indicate that hydrolysis constants are equivalent to ioniza-
tion constants, and that there is no fundamental difference between the K , of acetic acid and the K g of the acetate ion. Hydrated ions may also he classified as weak acids, inasmuch as these ions, when strongly hydrated, exhibit acidic properties. The topic of complex ions offers addit,ional opportunities to amplify upon the structure of matter and the nature of chemical binding. I n t,his connection, the charge and size of the central ion can he related to its coordination number as well as the st,rength of the bond, or bonds, in the complex. These relations may also be used to predict the extent. of hydration, and the magnitude of acid constants of hydrated cations. It is not beyond the scope of qualitative analysis to show that magnetic moments of various complex ions in aqueous solution may serve as a means for differentiating between ionic and covalent bonding. The explanation of soluble hydroxide complexes is more plausible than the older concept of amphoteric hydroxides. For example, the reaction of the zinc ion with excess hydroxide ions to form a soluble hydroxide complex is a more logical approach than the older concept of zinc hydroxide acting as an amphoteric hydroxide. If organic reagents are used in the analyt,ical schemes, the discussion of complex ions may include some explanation of chelate formation. Homever, t,he topic should be touched only lightly since students in qualitative analysis have very little knowledge of organic chemistry, and it is beyond the scope of this course to attempt an addition to such knowledge. Although the topic of colloids cannot he omkted, the discussion of this branch of chemistry should be largely confined to the properties of sols, and especially those properties relating to adsorption. important explanations which are relevant to this topic are the adsorption of ions by electrovalent sols, the formation and growth of crystals, and the various methods for coagulating colloids. A simplified approach to the theory of oxidation and reduction, which may be effectively used in qualitative analysis, is yet to he devised. This topic should certainly contain more than exercises in balancing equations. Many textbooks offer an elementary approach through a discussion of single electrode potentials, galvanic cells, and the electromotive force which may be derived from a combination of half-cells. The advisability of this approach is debatable. CONCLUSION
The topics which should make up the content of the theoretical portion of qualitative analysis are those which offer an integrated picture of recent concepts concerning the behavior of ions in aqueous solution. These topics should be developed in an orderly sequence regardless of any expediency within the experimental procedures. This does not imply that timehonored schemes of analyses should be abandoned since they are the most powerful tools for imparting a knowledge of the chemical properties of inorganic substances to students. However, in order to survive, qualitative analysis must come of age. This means that the theoretical aspects of the course must be so designed as to keep step with advancing chemical thought. JOURNAL OF CHEMICAL EDUCATION
