Garrv A. Rechnitz State University of New York Buffalo, New York 14214
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Analytical Chemistry: A Kinetic Viewpoint A new approach
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undergraduate instruction
For several decades, the equilibrium view of chemical reagents and reactions has completely dominated the teaching of analytical chemistry. While this approach has been highly successful for the description and solution of many classical analytical problems, it does not fully meet the stringent demands of more modern techniques involving rapid instrumental measurements, highly selective reagents, and automated analyses. Furthermore, several novel analytical techniques based on kinetic principles, such as catalytic and differential rate methods, are rapidly gaining popularity among practicing analytical chemists (1). If this apparent trend represents a significant change in the natnre of modern analytical chemistry, then it is important that undergraduate instruction in analytical chemistry give some emphasis to the kinetic as well as the equilibrium viewpoint. This paper is an effort to suggest one possible means of accomplishing this goal. Several recent textbooks intended for undergraduate analytical chemistry courses have sought to meet the need for kinetic material by the addition, to the usual body of classical topics, of one or more chapters dealing, typically, with elementary rate expressions, the resolution of two-component mixtures via rate methods, and, perhaps, a listing of some slow or catalyzed solution reactions. While this approach reflects an encouraging awareness of the problem, it does not provide a wholly satisfactory solution. 1 feel very strongly that novel kinetic material should be integrated with and not merely added to the conventional chemical topics of analytical courses. Secondly, the kinetic material chosen should not simply repeat or borrow a classical physical chemical treatment, but should be so selected as to show how kinetic considerations influence analytical reactions and techniques and how the use of kinetic information can he of value in the choice of optimum conditions and the interpretation of analytical data. Finally, the theoretical aspects of kinetics must he coordinated with effective experiments designed to demonstrate the manipulation of kinetic variables and the use of such parameters in the solution of practical analytical problems. Important principles and examples dealing with chemical kinetics can be introduced quite logically in Presented a t the Joint Symposium on The Teaching of Analytical Chemistry a t the 152nd Meeting of the American Chemical Society, New York City, September, 1966.
connection with discussions of analytical reagents, stoichiometric reactions, and selected analytical methods. Table 1 lists some representative topics treated in my course under these main headings. Naturally, the selection of subject matter can be readily altered or expanded to suit individual needs and time available. It is not suggested that kinetic material substantially replace the classical analytical topics, rather that a balanced view of both be presented. Table 1.
Some Analytical Topics Suited for Kinetic Treatment
Reagents Stability, interaction with solvent, air oxidation, equilibrium and kinetic selectivity, thermal and photochemical deeomposition Analytical Reactions Deviation from stoichiometry, catalysis, solvent effects on rate, reaction mechanisms, relative rates Kinetic Methods of Analysis Rate and differential rate methods, catalytic and enzymatic analysis, resolution of multicomponent mixtures, continuous analysis
Because the effective assimilation of such material requires some knowledge of the fundamentals of reaction kinetics and associated techniques, it may be necessary to review some topics which should have been covered in prior physical chemistry courses; some of the more modern subtopics listed in Table 2 might be useful in any case. I n a one-semester analytical course it is, unfortunately, not possible to devote more than about three lectures to these topics, and students must be required to supplement the lecture material by fairly extensive outside reading. Fortunately, some excellent books and reviews are available for this purpose (3-4). A very large number of reactions and associated kinetic studies is available as a source of suitable examples and practical illustrations. Indeed, the Table 2.
Selected Aspects of Theory and Technique
Treatment of Rate Data Empirical rate expressions, diagnosis of reaction paths and correlation with mechxnisms, p H , and ionic strength effects Marcus Theory far Redox Reactions Prediction df reaction rate8 from exchange and equilibria data, relation of homogeneous and electrode reactions Special Experimental Methods Fast reaction techniques, flow and pulse methods, reactions of solvated electron, steady-state techniques Volume 44, Number 6, lune
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problem becomes one of choosing the most useful and effective case studies. Every instructor will have his own specific preferences, but examples ought to be chosen from systems which are of some analytical importance and from kinetic studies carried out under circumstances which approximate realistic laboratory conditions. Thus, under the heading of analytical reagents, the writer selects examples of oxidizing and reducing agents which find fairly common use in volumetric analysis. Table 3 lists some of these reagents and the specific aspects of their solution chemistry which are discussed. The selection need not be restricted to redox systems or even to inorganic reagents, of course. Table 3.
Lecture Examples-Reducing Aaents
and Oxidizing
Chromium(I1) Reaotion with solvent; primary products in one or two ereactions; chromium(1V) as intermediate; ligand bridging and catalysis; generation from thermodynamic~llyunstable compounds Vsnsdium(I1) Reaction with H+, Oz, and C104-; exchange of ligands, eexchange Iron(I1) Predominant species and reaction paths; air oxidation; iron(1V) as reactive intermediate; ligand catalysis Silver(I1) Stabilization in aqueous media; formation of silver(II1); role as a. c%talystr control of reaction rates Cobalt(II1) Reaction with OH-; nature of solution species; dimeri~ation, and complex formation Cerium(1V) Predominant solution species and relative reaction rates; favored paths for one and two e- rertctians; interaotion with ~lvent.
The discussion of analytical reagents leads quite naturally to a consideration of complete cross-reactions. Again, examples can readily be chosen from practical analytical systems and should be selected to illustrate various important principles such as kinetic limits of stoiehiometry, catalysis and inhibition, suppression of side reactions, etc. It is especially desirable to demonstrate here how changes in reaction mechanisms, e.g., by stabilization of intermediates, can produce desirable or undesirable analytical consequences and how control of kinetic variables can be of value in the attainment of optimum reaction conditions for specific analytical purposes. Some typical examples of illustrative systems are given in Table 4; others have been proposed by Schenk (6). Table 4.
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Perhaps the most important means of encouraging students to think in kinetic as well as equilibrium terms is the use of interesting and effective laboratory experiments. To be effective for this purpose, the experiments should be challenging hut to the point, so that the relationship between kinetic parameters and analytical measurements is readily apparent; practical considerations may l i t the duration of such experiments and restrict the techniques employed to the use of instrumentation commonly available for undergraduate analytical courses. A realistic balance must be found between the amount of time used to gather experimental data and that spent in calculations and interpretation of results; clearly, both kinds of operrttions are important. Because most of the lecture examples in my course are drawn from oxidation-reduction systems, the laboratory experiments used are purposely chosen to afford some exposure to acid-base, complexation, and precipitation reactions. Table 5 gives an outline of three such experiments and their objectives; more extensive discussions of the chemistry and measure ments involved may be found in the literature (6-8). Schenk (6) has also proposed some excellent studies, well suited to this purpose, based on analytical redox reactions. Table 5.
Laboratow Ex~eriments
Formation of Cbromium(II1)-EDTA Complex (Ref. 8) Technique: Optical (follow complex a t 545 ma), point by point Data taken: Absorbance as function of time a t several pH values (3.5-5.5); final ahsorbmce (-4 6r) Desired results: Reaction orders with respect to chromium(111) and H + Emohasis: Handline of rate exoressions and d a b . use of 'Gseudo" rate constants; d~monstrates that l&e K f (log K f 23) Z fast rate
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Crystal Violet Dye Reaction (Ref. 7) Technique: Optical (follow dye a t 590 mp), continuous monitoring Data taken: Initial absorbance, absorbance as function of time with variations in oH and ionic strength (at constant Specific rate C O ~ Srrwtion ~., order for On-, I ) ~ i n wiultd: l ionir. ~Irrnglheflrvr on rate m l : 1),4,ye-lliwkrl law, nature of nctivntpd complex Kinetics of Tetrsphenylhorste Precipitation Reactions (Ref. 8) Technique: Potentiometric using cation-sensitive glass electrodes, continuous monitoring a t constant temperature Data taken: emf vs. time at varying initial concentrations of K t , Rh+, and mixtures of the two Desired results: Specific rate constants, test of rate expression, resolution of cation mixture on basis of rate differences Emphasis: Selection of "correct" mechanism, induction effects, use and limitations of differential rate method
Lecture Exam~les-Selected Cross Reactions
C ~ ~ ~ ( I V ) - I ~ OReaction ~(II) Effect of solvent on reaction rate; deviation from stoichiometry in presence of organic acids; test of Marcus theory as function of complexation Chromium(VI)-Arsenic(II1) Reaction Catalysis by I-, heavy metals; formation of chromium(1V) and (V) and arsenic(1V) as intermediates; induced reactions Cerium(IV!-Maneanese(I1) Reaotion
318
Labomtory Experiments
Journal of Chemical Education
The modified analytical course outlined here has been in operation for only two semesters; thus, any attempt at an assessment of its effectiveness may be memature. It can be re~orted.however, that student interest in the kinetic material is high and that results obtained in the laboratory have been sufficiently reliable to meet the illustrative requirements of the experiments (see Hedrick (6) for summary of student results). Questionnaires filled out by students after
completion of the course are overwhelmingly in favor of more kinetic material for both lecture and laboratory. More than half of the students involved chose some kinetic problem as their topic for a special project performed in the latter portion of the course. Naturally, some difficulties have also been encountered, conservative coueafles what reaction mechanisms have to do with chemical analysis, some students lack the necessary background in physical chemistry to benefit fully from the lecture material, and laboratory experiments sometimes fail due to seemingly inexplicable causes; nevertheless, I that will be interested experiment along these lines and that, eventually, some
kinetic material will find its way into the content of undergraduate analytical courses. Literature Cited (1) RECHNITZ, G. A., Anal. Chern., 36, 453R (1964); 38, 513R (1966).
(2) YATSIMIRSKI, K. B.,"Kinetic Methods of Analysis," Pergamon Press, London, 1966. (3) CALDIN, E, F,, Reactions in Solution,,, John Sons, Inc., New York, 1964. (4) MARCUS, R. A,, Ann. Rev. Phys. Chem., 15,155 (1964). ( 5 ) SCHEW G. H., J. CDM. EDuc.8 41.32 (1964). (6) HEDRICK, C. E., J. CHEM.EDUC.,42, 479 (1965). (,) CoRsARo,G,, J. CDM. EDuc,, 41, 48 (1964), (8) M c C L ~ EJ., E., A N D R E C H NG. ~ ,A., And. Chem., 38,136 (1966).
Volume 44, Number 6, June 1967
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