Richard G. Yalman
Antioch College Yellow Springs, Ohio
Teaching Colloid Chemistry in the Analytical Chemistry Course
Four years ago the chemistry department a t Antioch College undertook a revision of its offerings in physical and analytical chemistry including instrumentation (1). The purposes of this revision mere to eliminate overlapping of course work in the areas of physical and analytical chemistry, to make more use of the student's experience in analytical chemistry in the cooperative job program, and to strengthen the training in physical chemistry. There Tvere two major resnlts of this revision. First, the formal course in instrumentation was eliminated and more time was made available for the physical chemistry sequence. Second, physical chemistry became a course in mathematical chemistry and the introductory quarter of thermodynamics and kinetics was given by the physics department. The inherent dangers of a mathematical course were recognized. The most obvious is that the more factual descriptive material, including much of classical colloid and surface chemistry, wonld be virtually eliminated. Furthermore the material retained would be scattered over the various units of thermodynamics, kinetics, and electrochemistry. Finally the amount of Laboratory time devoted t o experiments in colloid Presented as part of the Symposium on the Tesching of Colloid nod Surface Chemistry before the Divisions of Colloid and S u c face Chemistry and Chemical Education at the 140th ACS Meeting, Chicago, September, 1961.
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and surface chemistry would be drastically reduced. As a result of these changes, there would be so much less unity among the topics comprising surface chemistry that the student might well wonder whether snrface chemistry was in fact a branch of chemistry. Most important, any motivation a student might have to learn surface chemistry might be destroyed. On the other hand these dangers could be minimized and perhaps surmounted by increasing the time available for teaching colloid chemistry in the second term of the analytical chemistry course. This also seemed to be a rather logical choice in that every technique of gravimetrio analysis and volumetric precipitation was devised either to use or to counter surface effects. Time was made available in t,his term by eliminating the classical brass analysis and by using EDTA titrations for calcium and magnesium. Our plan was to build the second term of analytical chemistry mainly around the formation and properties of precipitates, beginning with a n introduction to crystal structure. This was to be followed by rather standard experiments in gravimetric analysis, volumetric precipitation, and volumetric complex formation, using recently developed techniques. The latter part of the course was then to be devoted to colloid and surface chemistry with particular application to analytical chemistry.
There is nothing new in these ideas. Most analytical texts contain one or more chapters concerned with coprecipitation and adsorption phenomena and include a section on the ionic double layer. Recent books discuss nucleation and crystal growth, solid solutions, and homogeneous precipitation. The most notable example of this trend is the book by Laitenen (7), which devotes over 60 pages exclusively to the properties of precipitates and includes brief sections on the Freundlich and Langmnir adsorption isotherms, the OstwaldFreundlich solubility equat,ion, and the HelmholtzStern double layer. Selected Topics and Experiments
However, it seemed worthwhile to augment Professor Laitenen's material with references to other systems and to demonstrate to the student that the properties of the electrolytic colloid are specific examples of more general phenomena. The topics selected are outlined in Table 1. They are divided into three major categories: solubility and part,icle size, theories of adsorpt>ion,and the effects of adsorbed ions on t.he properties of solids. Table 1 also includes three laboratory experiments which have been chosen to demonstrate the colloidal propert,ies of analytically important mat,erials. Table 1.
Colloid Chemirtry in the Analytical Chemirtry Course
Solubility and particle sise Freundlich-Ostwnld equation Surface tension Sedimentation Adsorption isotherms Adsorption of wool violet by B~sO. ~ a n e m u i renuation Fre&dlioh k u a t i o n
Sedimentation rntc of RaSO+
Adsorption of acetir acid on cation resin
R -PT ianth~rrn - .- ...-....
Efiect of ions Charge on particles Sehultz-Hardy rule Langmuir kinetics, zero point potential, and solubility ~ r o d u c conrtant t Elketrokineties and eleetrocapillarity
Dialysis and flocculation of iron(II1) hydroxide sol
The relationship between the solubility of a particle and its size is given by the Freundlich-Ostwald equation
Although it has been difficult to demonstrate the quantitative application of this equation to solids, there has been wide acceptance of the Kelvin equation
relating the vapor pressure of small droplets to their radii. I n the analytical course these equations are not derived from thermodynamic principles. However, it is not difficult to demonstrate their relationship from kinetic considerations. The Freundlich-Ostwald equation then provides a useful qualitative explanation for the well known analytical maxim that "large crystals
grow a t the expense of small crystals." I t is also useful in discussing such phenomena as supersaturation, nucleat,ion, and seeding. The Freundlich-Ostwald equation also introduces the student to a new concept: the surface tension of a solid. The similarity of equations (1) and (2) indicates that the surface tension of a solid and that of a liquid are manifestations of the same phenomena. At this point surface tension of a liquid is discussed mit,h emphasis on its experimental determination. The application of these measurements to the occurrence and heights of meniscuses in volumetric ware and the amount of liquid in a pipet, etc., is then self-evident. The use of surface tension measurements as an analytical tool per so is also discussed. The section on adsorption begins with a study of the determination of t,he surface area of barium sulfate by the adsorption of wool violet (5). The results of these experiments are then compared with those from sedimentation rates. Kext the Langmuir adsorption isotherm is developed and this isotherm is applied to the data in the literature on the adsorption of eosin by silver bromide (S). Although some adsorption data can be treated brtt,er by the empirical Freundlich isotherm than by the Langmuir isotherm, there appears to be some hesitancy on the part of various authors to use the Freundlich equation. Since a form of this equation has been derived from a Langmuir treatment of heterogeneous smfaces (S), considerable at,tention is given to the Freundlich equation. Finally the Brunauer-Emmett-Teller isotherm is reviewed. I n the third unit the ionic adsorption a t the surface of an electrolytic precipitate is described in terms of the Helmholtz-Stern double layer. The relationship hetween the charge of the particle and the excess of the common ion is then apparent as is the Schultz-Hardy rule for the flocculation of sols. The significance of the zero point potential and the isoelectric point of other systems, including proteins, is discussed. The relationship between the concentration of ions in t,he bnlk solution in equilibrium with a surface of zero pot,ential and the solubility product constant are derived from Langmnir kinetics. The zeta pot,ential and electrokinetic phenomena are discussed. Finally as an application of both surface tension phenomena and the ionic double layer the effect of the zeta potential on the ele~trocapillarit~y of mercury is described. To as large an extent as possible the classroom material is taken from the original literature. The data of Fajans (S), on the adsorption of eosin by silver bromide, and the work of Kolthoff and iVIacNevin (5) on the adsorption of wool violet on barium sulfate have been ment,ioned above. Two additional papers of primary importance here are t,he study of the solubility of lead chromate as a function of particle size by May and Kolthoff (8) and the adsorption of silver and iodide ions on silver iodide by Kolthoff and Lingane (4). Similarly the laboratory experiments are concerned with materials handled in the middle part of the course on gravimetric analysis and volumetric precipitation. Thus the sedimentation experiment is performed on barium sulfate, and the flocculation experiments are performed on dialyzed iron (111) hydroxide sols. Cation resins are used in the experiments on adsorption. This is done for several reasons. I n the Volume 39, Number 4, April 1962
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first place the students have already used adsorption indicators in halide analysis, and they have treated the data of Fajans by the Langmiur isotherm. Secondly, adsorption studies with ion-exchange resins demonstrate the wide application of adsorption isotherms in chemical analysis. And third, experiments with ion-exchange resins are much more significant to students in analytical chemistry than, for example, the classical experiment on the adsorption of oxalic acid on charcoal. Additional experiments are performed as individual or group projects. It is apparent that the amount of surface chemistry included in the analytical course depends upon the available time, the teachers' interest, the students background, and the amount to be covered in the physical chemistry course. It must also be apparent that an increase in the emphasis on colloid and surface chemistry in the analytical chemistry course is not a panacea for whatever this symposium believes to be lacking in the undergraduate training in this important field. When the analytical course precedes the physical chemistry course, the thermodynamic development of surface phenomena obviously will be omitted. We have done this a t Antioch and have also omitted the study of surface films. Otherwise we fully cover the principlcs of kinetics and electrochemistry.
unwise however to use t,his definition in a course in analytical chemistry and then to proceed to discuss in detail the properties of larger crystals, the use of adsorption indicators, ion-exchange resins, etc. A second reason for redefining a colloid in analytical chemistry is based on the criterion of quantitative analysis itself. Thus the desired accuracy of quantitative technique is 0.1%. If only precipitating ions are adsorbed on an otherwise perfect crystal, then in order to achieve an accuracy of 0.1%, the crystal should be not smaller than several hundred mp, the upper limit of a colloidal particle. The uniqueness of classical electrolytic colloids is due to their large ratio of surface to mass and the corresponding importance of surface phenomena. The definition of a colloid should stress this importance. Hence, a colloid might be defined as a particle whose dimensions are such that its apparent properties are determined by surface phenomena.
A Redeflnition
(5) KOLTHOFF, I. M., AND MACNEVIN, W. M., J. Amer. Chem. Soe., 59, 163 (1937). (6) KRUYT,H. R., "Colloid Science," Vol. I, Elsevier Pub. Co., New York, 1952, p. 8. (7) LAITENEN, H. A,, "Analytical Chemistry," MeGraw-Hill, New York, 1960. (8) MAY. D. R.. AND KOLTHOFF. I. M.. J. Phm. a d Colloid
Literature Cited (1) Chem. Eng. News, June 8, 68 (1959). (2) FAJANS,R., "Radioelements and Isotopes," McGraw-Hill Book Co., New York, 1931, p. 98.
(3) HALSEY,G . D., in "Advances in Catalysis," Vol. IV, Academic Press. New York. 1952. D. 259. (4) KOLTHOFF, I. M., AND LINGANE, J. J., J. A m e ~ Chem. . Soc.,
..
58, 1531 (1936).
In conclusion, it might he worthwhile to redefine an electrolytic colloid. Although Kruyt (6) prefers to think of a colloid system, most definitions are concerned with particle size and set the lolver limit a t 10 mp and the upper limit a t 10W1000 mp. It seems
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