ROBERT C. CROSSON, JR. HAROLD J. ABRAHAMS
by
the Bredis Method
requires little skill or care, although the product is apt to have a brief existence. Central High School, A method for preparing a colloidal dispersion of Philadelphia, Pennsylvania metals by the use of the electric current was discovered by Bredig in 1897. In this method the metal is first HIS paper presents the results of experiments with vaporized (from the cathode) by the heat of the arc the preparation of metallic colloids in nonaqneous produced and then condensed. If a trace of potasmedia by the Bredig method. Two classes of methods sium hydroxide is added to the water, the dispersed are in use for the preparation of colloidal dispersions, metal is stabilized. This method is a combination of coagulation or condensation and peptization or disper- dispersion and condensation. The procedure is: sion. In each method the aim is to produce a condition Two bars of the same metal are attached to a source in which the particles of the dispe~sedsubstance are in of electric current and serve as electrodes. These eleca state of very fine subdivision. These methods may trodes are immersed in water and brought into conbe summarized as follows: tact with each other for an instant to produce an arc, Coagulation (condensation) : This method is exempli- and then separated about 2 mm. A rattling noise genfied when a few drops of ferric chloride are added to erally results, and a colored discharge appears, showing boiling distilled water. The femc hydroxide molecules that some of the metal has been vaporized. These produced by hydrolysis coagulate and build up into metal particles are immediately chilled by thesnrroundcolloidal particles. ing water and do not therefore have time to assume an This is a chemical method and is widely applicable arrangement in which they would be too large and dense in obtaining colloidal dispersions of insoluble compounds to be capable of remaining suspended. Thus the elecwhich form as a result of (a) reduction (gold), (b) oxi- tric arc produces a colloidal dispersion of the metal dation (sulfur of H2S by SO2), or (c) metathesis (in- by snbdividingit. soluble chlorides, sulfates, phosphates, etc., hydroxides Bredig employed currents ranging from 5 to 10 amby hydrolysis, and sulfides by the use of hydrogen sul- peres a t 30 to 110 volts. He found that a gold disfide). persion thus prepared contained about 14 mg. of the Pefitizetion (dispersion): This method is subject to metal per 100ml. some variation, but in each variation the aim is the We have prepared the following colloids in nonsame-that is, to start with larger particles and tear aqueous dispersion using the Bredig method and a coil them down to such smallness that they will remain dis- from an ordinary household toaster (610 watt, 110 volt) persed and not sink. attached to a 115 volt a.-c. source. (a) One method of reducing the size of particles until Disoerring medium D i r ~ r r r r dmrlnl they are capable of passing into the colloidal state is to Hg A1 Sn,Fe, C Glycerin Methyl alehol ~i ~ d ~ n use a "colloid mill." In this method the material to be Mineral oil Hz, AL. F+ Zn,C dispersed into the colloidal state is merely ground with Turpentine Ca Z n Benzene Ca. Z n a liquid between two metal discs which rotate a t high Formaldehyde Hg, Fe,Z n speed in opposite directions, until the correct state Bars of the metal were attached to wires from the has been attained. (b) Another method for reduc&g particle size is to 110-v. a,-c. circuit with the coil of a household toaster subdivide drops (for example, an oil) into extremely in series, and an arc struck as described above. In the case of mercury the wires ended in a pair of fine droplets (by shaking with water) and then to precarbon electrodes which were dipped into two little pools vent the droplets from running together again into a pool by coating them with another substance. An of mercury, and the pools made to approach each other example is maydnnaise, in which case olive oil droplets closely enough to produce an arc. All of the colloids mentioned were found to yield a are suspended in vinegar and water, egg being used to Tyndall cone.' They remained stable in varying coat the oil droplets and thus prevent their coalescing. These methods of preparing colloidal dispersions pro- degrees for a t least three months, the supernatent vide methods which vary in applicability. Colloids of liquid still producing the Tyndall cone even after gold in a wide color range may be easily produced by the settling had taken place. Most of these colloidal disaddition of certain reducing agents to solutions of gold persions were gray in color, except those of tin and iron chloride. Some of these gold colloids are stable for which were brown. years but require care in their preparation. The use of a description of the apparatus used for this purpose, see a colloid mill is, of course, a commercial method. The H.1 For J. ABRAHAMS AND A. D u e ~ ~ nJOURNAL , OF CHEMICAL arc method, on the other hand, is rapid and simple and EDUCATION, 20,61-2 (1943).
T
