PHOTOMICROGRAPHY AS A TOOL FOR TEACHING COLLOID CHEMISTRY

ERNST A. HAUSER Massachusetts Institute of Technology, Cambridge, Massachusetts, and Worcester Polytechnic Institute, Worcester, Massachusetts HORACIO E. BERGNA L.E.M.I.T. and University of La Plata, La Plata, Argentina

ONE of the greatest problems faced by an educator who is responsible for imbuing college or university students or students of technological instit,utes with the funda- mentals of colloid science is t,he need for offering as many visual demonstrations of the colloidal state of matter as possible. Rased on many years of experience of one of us (E.A.H.) in teaching colloid chemistry, i t must be admitted that the most effective way is to apply Samuel John Stone's words, "What I can't see, I never will believe in," to such courses. This is of primary importance in trying to explain that what we today term "colloids" does not refer to specific compounds but to a specific state of matter.

Thomas Graham, who coined the word "colloids," differentiated het,rveen them and crystalloids according to the diffusivity of their solutions.' A. Mueller2 pro- posed a division into suspensions of fine particles and solutions of high molecular compounds. H. Bechhold3 even differentiated between organic and inorganic colloids. Depending on the point of view taken by various authors, colloids were considered as belonging either to the group of mechanical suspensions or to molecular solutions. A typical dualistic point of view resulted. Not until 1906 was the "disperse state of matter" as the general point of view introduced by Wolfgang Ostwald,' the colloidal st,ate being one step in the series extending from matter in the coarse stat,e through microscopically detectable dispersions to the colloidal state and further down to truly molecular solutions. In his book, "The World of Neglected dimension^,"^ Ostwald summed up his point of view in the following statement: "Modern colloid chemistry t,eaches that there are no sharp differences between mechanical suspensions, colloid solutions and molecular solutions. There is gradual and continuous transition from the first through the second to the third. Me- chanical suspensions, colloidal solutions and molecular or true solutions have the dispersed state in coinmon."

Entirely independent of Ostwald's primarily theo- retical deductions, the Russian scientist, P. P. Ton Weimarn, offered the necessary experimental proof for Ostwald's statement during 1905 to 1907.' Studying the reaction

M~SOI + Br(CSS)z + q. = Mn(CNS)* + BaSO, + aq. he found that the consistency in which the insoluble harium sulfate precipitated depended entirely on the concentration of the reacting solutions.

On the basis of over 200 experiments he found that it mas always possible to obtain extremely fine crystals. coarse flocks, or jellies, which upon dilution formed colloidal sols, depending only on the conrentration of the reacting substances used. Iron Weimarrl summed up this first important experimental contribu- tion t,o colloid chemist,ry in the following statements: "The so-called colloidal, amorphous and crystalloidal states are all t,ogether universal (possible) properties of matter. . . . Generally speaking, it follows from these investigations that colloids and crystalloids are by no means t,wo special worlds but that t,here exist close re- lations between themselves as well as between them and the gaseous and liquid states of matter."=

This contribution is today known as "von Weimarn's Law of Precipitation." I t is beyond any question of a doubt one of the greatest contributions to the develop- ment of modern colloid science and t,herefore its teach- ing deserves special attention.

Our experience has proved that classroom demonstra- tion of the von Weimarn Law of Precipit,ation carries with it serious drawbacks. The proredure is time- consuming. I t is necessary to set up three filtration funnels equipped with quantitative filter paper, make up three solutions with different concentrations of reacting chemicals and then subject the formed dis- persions to filtration. All equipment must be perfectly clean and the water used for making up the solutions absolutely electrolyte-free. But even with the greatest

GRAHAM, TII., Phil. Trans. Roy. Soc., 151,183 (1861); Pm. precautions one never can obtain as clear-cut a differ- D^_. ".. 1 3 ""= ..oe*\ 'buy. ow., '2, o*., \lo"'+,.

MUELLER, A., Za't. anorg. Chern., 36,340 (1903). entiation as one would wish between thereaction prod- ' BECHHOLD, H., Zeit. physikal. Chem., 48, 385 (1904). uct obtained with the greatest dilutions and that with 4 HAWSER, E. A., "Colloidal Phenomena," McGraw-Hill Book the highest concentration. Although the residues left - -

Co., Inc., New York, 1939, pp. 1-25. O~TWALD, WO., Koll. Zeit., 1,291,331 (1906); "An Introduc- "ON WEIMICRN, P. P., J. Rus8. Physik+chem. Ges., 37, 949

tion to Theoretical and Applied Colloid Chemistry,'' 2nd ed., (1905); Koll. Za'l., 2, 76, 128, 190, 230 (1907); 3, 282 (1908); John Wiley & Sons, Ino., Nen. York, 1922, p. 14. 4, 2i, 123, 198, 252, 315 (1909); 5,62, 117, 150, 212 (1910).

618

DECEMBER, 1952

The "on Weirnarn Law of procip-

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The van Weimarn Law of precip-

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620 JOURNAL OF CHEMICAL EDUCATION

on the filter papers show a marked difference in re- tained material between the low, medium and high con- centrations, thereby offering visual proof that the colloidal range of dimensions cannot be set off by a straight line of demarcation, the experiment is not so convincing to the newcomer as it should be. Un- questionably the greatest disadvantage, however, is the time lost while this experiment is in progress; under our prevailing educational setup every minute at our disposal must be used to the best advantage.

We therefore decided to offer a visual demonstration of the von Weimarn Law of Precipitation by the use of ultra- and electron-microscopy. The electron micro- scope hes already been applied successfully in the teach- ing of science," providing direct observation of phe- nomena previously known only by indirect means. One of us (H.E.B.), jointly with Prof. Solari and Prof. Catoggio of the University of La Plata in Argentina, has already successfully used Fischer's electron photo- micrographs in teaching quantitative chemical analysis. By the use of the electron microscope and the Ultropak

7 FISCHER, R. B., J. CHEM. EDUC., 24, 484 (1947).

ultramicroscope we have now been able to offer a very simple and graphical demonstration of the von Weimarn law. Although the results do not reveal any new information they nevertheless offer visual proof of the diierence in particle size obtained when reacting the chemicals at various concentrations. Figure 1 shows a plot of the concentration vs. particle size of the precipitate and a t the same time ultraphoto- micrographs of these precipitates. Although the differ- ence in particle sizes obtained when using various con- centrations of the reacting chemicals is clearly evident, we also subjected the precipitates to electron-photo- micrography. Figure 2 shows the results of the elec- tron-microscope studies.

We feel that showing these two figures by projection on a screen will give the teacher a much better basis for explaining to his students what the von Weimarn Law of Precipitation stands for than to carry out the time-consuming experiments previously referred to. Besides this, the student may now even see the struc- ture of the precipitated particle, something which has hitherto not been practical in classroom demonstra- tions.