Colloids & The Tyndall Effect from General Chemistry by N. Glinka, Peace Publishers, Moscow, circa 1960s
188. Crystalline and Colloid States of Substances. If a solution of sodium silicate is added to concentrated hydrochloric acid, the resulting silicic acid does not separate out as precipitate but remains in solution together with the sodium chloride formed during the reaction. The hydrochloric acid and sodium chloride can be removed from the solution in the following way. The solution is placed in a bottomless cylinder with a membrane of parchment paper or an animal bladder bound over its end. The cylinder is submerged in a wider vessel containing water which is continuously renewed. The sodium chloride and hydrochloric acid diffuse freely through the membrane into the outer vessel, but the silicic acid cannot penetrate the membrane and remains in solution. As a result, the cylinder will be found after some time to contain a pure solution of silicic acid. The method of separating dissolved substances based on the fact that one of them will not diffuse through a membrane, is known as dialysis, and the apparatus described is called a dialyzer. Many other dissolved substances, besides silicic acid, viz., glue, gelatin, egg albumen, etc., cannot penetrate a membrane of parchment paper or bladder. In the sixties of the last century the diffusion of dissolved substances through vegetable and animal membranes was studied in detail by the English chemist Graham. Graham found that all substances capable of diffusing in solution are crystalline in the solid state. On the contrary, substances which could not diffuse through membranes were found to be amorphous, and formed shapeless (and to a certain degree plastic) masses when isolated from solution. On this basis Graham called the former crystalloids and the latter colloids (from the Greek "colla"-- glue). However, as early as 1869, the Russian botanist I. Borshchov put forth the assumption that the particles of certain colloids may also be of crystalline structure. Further investigations confirmed this assumption and led to the conclusion that Graham's division of substances into crystalloids and colloids should be rejected, as not only such typical colloids as albumen could be obtained in the form of crystals, but many indisputable crystalloids, such as common salt, could be obtained in the form of colloids. Finally, it was proved that the same substance could behave like a colloid in some solvents and like a crystalloid in others. For instances ordinary soap dissolved in water diffuses very slowly and cannot penetrate a membrane, showing it to be a colloid; but in alcohol solution the same soap possesses the properties of a crystalloid. Thus, the sharp demarcation line between crystalloids and colloids gradually disappeared, and at present we can speak only of the crystalloid or colloid states of substances, just as we have spoken above of their solid and liquid states. The colloid state of substances plays a very important part not only in chemistry, but also in biology, medicine, technology and agriculture, and therefore we shall dwell on it in some detail. 189. Dispersed Systems. If a fine powder of any insoluble substance, say clay, is shaken with water, the larger particles will soon settle at the bottom while the finest will remain in a "suspended" state in the water for a considerable length of time, so that the liquid may remain turbid sometimes for weeks. Liquids with particles of a solid substance suspended in them are called suspensions. If minute drops of a liquid are suspended in another liquid the system is called an emulsion. An emulsion can easily be obtained by shaking an oil vigorously with water in the presence of substances capable of lowering the surface tension of the oil. Ordinary milk is an emulsion of minute drops of butter fat in water. Particles suspended in liquids can be separated from them by filtration. Ordinary filter paper will detain particles down to 5 microns, i.e., 0.005 mm., in diameter, specially prepared filter paper down to 1 micron, while clay filters detain particles as small as 0.2 microns. As long as particles above 0.1 micron in diameter are present in a liquid, it will not seem quite transparent, and the suspended particles can be detected in a drop of the liquid with the aid of an ordinary microscope. (continued in part 2) -- The silver-list is a moderated forum for discussion of colloidal silver. To join or quit silver-list or silver-digest send an e-mail message to: [email protected] -or- [email protected] with the word subscribe or unsubscribe in the SUBJECT line. To post, address your message to: [email protected] Silver-list archive: http://escribe.com/health/thesilverlist/index.html List maintainer: Mike Devour <[email protected]>
