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BREDIG SOLS: A LECTURE DEMONSTRATION EMIL R. RIEGEL, ROBERT C. OSTHOFF, and DONALD 0. FLACH Unwers~tyof Buffalo, Buffalo, New York
THIS paper is presented in the hope that it may help teachers in high schools who are looking for a lecture demonstration in the colloidal field, and who are a t the same time seeking supplementary work for able students. For either purpose, the teacher is limited to the available facilities for such ext,ra experimental work. Given a source of direct current, only materials available in a high-school department of physics and chemistry are required. The directions given are precise, reducing the necessary number of trials to a minimum. Bredig ( 1 , 2) has described a scheme for the disintegration of metals by means of an electrical current in such a manner that a sol of the metal is produced. Essentially the method consists of striking an electric arc between metal electrodes below the surface of the dispersing medium. Under the conditions of the experiment the metal disintegrate to yield a sol consisting of particles of varying size. Under this treatment some metals yield sols of the metal alone, some yield oxide sols, and some yield mixtures of oxide and metal sols. The original method has been improved and extended by Svedherg (5) and others (4, h ) . Usually the apparatus described is rather elaborate. The use of a simple alternating-current type of apparatus for the production of ~ r e bsols i ~ is described by Thomas (6). The more s i m ~ l emet.hods for the use of iirect current 110 VOLTS D.C. that are described in the secondary literature are often lacking in detail. The simple method to be subsequently described may be employed to produce Bredig sols by means of a direct current. The procedure is suitable for use in high schools and college chemistry courses as the basis for a lecture demonstration in connection with the study of colloid chemistry. In the apparatus which was employed by the anthors a fixed and a movable electrode mere used, illustrated in the accompanying figure. These electrodes I consisted of pointed 3/ls
inch rods, A and B, of the desired metal which were bent as in the diagram. In the case of copper it was found that number 14 or 16 wire could be employed with equal success. The electrode holders were made from brass in order to minimize corrosion. The figure adequately illustrates the simple construction of the holders which may readily be reproduced in any manual training shop. The holdels may be fixed in position by any suitable insulating substance, i. e., C. The adjustment mechanism for the movable electrode consisted of a threaded shaft of '/,inch diameter, D. The end of this shaft pressed against the extremity of a brass rod, F, which had been countersunk and soldered to the movable electrode holder. The threaded shaft, D, was held in position by a short piece of brass rod, E, which had been drilled and tapped to conform with the threads on the shaft. The counterbalance for this device was supplied by a small spring as indicated in the figure. The source of power necessary to maintain an arc may be a two- or three-kilowatt motor-generator set or an equivalent source capable of delivering 100 to 120 volts direct current. The circuit details are shown in the figure. The re-
519
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Eredig Sol A p p u a t w
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JOURNAL OF CHEMICAL EDUCATION
sistor that was employed was rated a t 5 ohms and 12 amperes. While the resistor was overloaded 35 per cent, no difficulty was encountered due to the intermittent nature of the arcing process. A resistor of this type is, in general, more readily available than one of higher current rating. Cooling the resistor with a current of air is advisable when several sols are to be made in quick succession. In the preparation of a sol the electrodes are immersed in the dispersing medium. A suitable and convenient procedure is to place 250 to 300 ml. of the dispersing medium in a 400-ml. beaker and then to raise the beaker until the electrodes are immersed to a depth of 2 to 3 cm. below the surface of the liquid. Then, with the power on, the electrodes are brought into contact with each other and gently moved apart, thus striking the arc. Since the arc does not maintain itself over a prolonged period it is necessary to strike the arc periodically during t,he course of the experiment. In the case of the preparation of hydrosols the arcing time was found to be in the vicinity of 30 seconds, while the time reauired for the vrenaration of most alcosols was found to be about 15 seconds. certain precautions are necessary in cases in which wateris tobe employed as the dispersing medium. par example, the ordinary tap water of the city of Buffalo was found to be unsuitable for this type of work. This water contained about 165 parts per million of total dissolved solids and had a DHof about 8. On the other hand, the distilled water a t the University of Buffalo had a pH of about 5 and a specific oond11ctivity of 6 X lo-' reciprocal ohms per centimeter. The latter was found to be comoletelv suitable for the nreoaration of Bredig sols. In the event that no distilled water is a~~ailable, the use of ordinary Pyrex distilling equipment may afford a convenient means for the preparation of suitable water. The authors found that ordinary tap water that had been distilled through pyrex functioned satisfactorily as a dispersing medium. In any event a second distillation should yield a desirable ~roduct. When sols were prepared in water that had been distilled but once the arcing time to produce a sol of moderate concentration was somewhat longer than with the doubly distilled water. The authors prepared conductivity water in a moderate state of purity, as with a specific conductance of 0.5 X 10" reciprocal ohms. In this event the arcing time was found to be longer than that required in the distilled water of the university of ~ u f f a l b .The stability of the resulting sols was found to be somewhat A
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less than the stability of those which were produced in the distilled water that was available. Although a trace of alkali is generally regarded as aiding in the production of a sol that is very finely dispersed, it was found by the authors that a sol of greater stability resulted when all electrolytes were absent, in so far asthis is practical. However, it has already been mentioned that the stability of the sols that were produced in the conductivity water was lower than that of those formed in the ordinary distilled water. The results of the experiments conducted by the authors are summarized in the table that follows: Charge Metal
Copper Iron
Nickel Platinum
micellae
Green Yellow-green Green Dark green
Positive Positive Positive Negative Neeative
Pur~le
Gold
on
c0107 of 801
All of the above sols were stable for a period of at least a week, and no stabilizing agent was added. The particle size in these sols was observed to vary over a wide range. Those sols which possessed a positive charge probably contained some particles of the oxides of -.+,hew .---- -- ---- . par lecture demonstrations a, ~ ~ ~heam d ~willl show the colloidal nature of the suspension. The sols may be coagulated by electrolytes to illustrate floccu.. ~at~on. It should be further noted that several organic solvents may be employed as dispersing media for the Alcohols, carbon tetrachloride, and V ~ ~ ~ Ometals. U S several other organic solvents were employed successfully by the authors. In these cases the sols and the colors were the same but possessed somewhat less stabilitv than the hvdrosols.
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LITERATURE CITED
BREDIG, G., 2. Elektrocha., 4, 514 (1898). (2) BREDIG, G., Z . Phyaik. Chem., 32, 127 (1900). (3) SVEDBERG, T., "The Formation of Colloids," D. Van Nostrand Co., New York, 1921. (4) Bolu~sorr,J., AND T. SVEDBERG, ~ 0 1 l ~ iZhw., d 25, 154 (1)
(1919).
(5)
K~ .E --M,--E-E., R .. , AND T. SVEDBERG, J. Am. Chem.
sac., 46,
l Y X U (IY24).
A, W,, ,,Colloid Chemistry," McGraWWHillBook
Co., New York, 1934, p.
102.
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