CHAPTER XXVI COLLOIDS 155. Preparation of colloids. a. Prepare 4 or 5 cc. of a saturated solution of sulfur in alcohol and pour it into 100 cc. of water. The sulfur forms a fine white dispersion that will not settle. Filter the mixture (?). 2 b. To 10 cc. of water in a test tube add 3 drops of a dilute solution of gold chloride (6 g. crystallized HAuCl · 3H2O per liter). Heat to boiling and add 1 or 2 cc. of a dilute solution of formalin as a reducing agent (3 cc. commercial formalin per liter of water). The solution assumes a purplish color due to metallic gold. Compare the appearance viewed by transmitted light with that by reflected light. c. To 100 cc. of water add 2 or 3 drops of the solution of gold chloride. After thorough shaking, add a few drops of a dilute solution of tannin (about 0.1 g. per liter) and heat to near boiling. If the color is too faint repeat the addition of the two reagents. How does the color compare with that obtained in b? How do you account for the difference? The tannin acts both as a reducing agent and a protecting colloid. To the red solution add 1 cc. of a concentrated solution of sodium chloride. What change do you notice? 156. Preparation of colloids by double decomposition. a. Prepare a little antimony sulfide by dissolving a small grain of antimony oxide or antimony chloride in 5 cc. of dilute hydrochloric acid and adding a solution of hydrogen sulfide. The orange-colored precipitate has the composition Sb,S,. When the precipitate has settled, is the liquid colored? Set the tube containing it aside for reference. b. Dissolve a little tartar emetic (which contains antimony as a salt of a weak acid), not exceeding a wheat grain in bulk, in about 100 cc. of water, and add a few drops of a solution of hydrogen sulfide, taking care to avoid an excess. (The resulting solution should not have the odor of hydrogen sulfide.) What change do you notice? Filter a little of the solution. Is the filtrate colorless? Is a precipitate left on the filter? Boil 2 or 3 cc. of the filtrate and again filter. Does boiling cause the antimony sulfide to separate? Reserve the remainder of the solution for §§ 158, 159, 160. 157. Preparation of colloids by hydrolysis. a. To 100 cc. of water in a beaker add enough of a solution of ferric chloride to give a decidedly yellow color. Slowly heat the solution to boiling and note the change in color. The salt is hydrolyzed with the formation of colloidal ferric oxide. View the color by reflected and then by transmitted light. Compare the latter with the color of ferric oxide precipitated from dilute ferric chloride by ammonium hydroxide. Save the solution for §§ 157, 158, 159. b. Dissolve 2 or 3 g. of sodium acetate in 50 cc. of water. Now add 1 cc. of a solution of ferric chloride. What new salts should be formed? Should either of these undergo extensive hydrolysis? Do you obtain a precipitate? What is the color of the solution? This is largely due to the presence of colloidal ferric oxide. Save this solution for §§ 158, 159, 160. 158. Coagulation of colloids. Heat to boiling 5 cc. of the colloidal solutions obtained in 156, b, and 157, a and b. Is the colloid precipitated? To 5 cc. of each add about half a gram of ammonium chloride or calcium chloride and shake the solution vigorously. Do any of the solutions give a precipitate? To 5 cc. of each add 1 drop of concentrated sulfuric acid. Would you expect this reagent to precipitate ferric oxide? Explain. Remembering that fermentation often produces acids, how can you explain the curdling of milk on souring? 159. Protective colloids. Dissolve a few small pieces of agar (a colloidal material resembling gelatin) in about 10 cc. of |