96
JOURNAL OF CHEMICAL EDUCATION
JANUARY, 1928
LECTURE DEMONSTRATIONS FOR GENERAL CHEMISTRY E. ROGER W~smurw,UN~VERSITY OF NEBRASKA, LINCOLN, NEBRASKA* Cupric Ammonia Hydroxide-Artificial Silk One of the methods for the manufacture of artificial s i k offers an interesting and instructive experiment, well adapted to use as a lecture demonstration for general chemistry. The "Cuprammonia Process," making use of the precipitation of cellulose by sulfuric acid from its solution in cupric ammonia hydroxide, may be profitably shown in connection with the study of copper. In addition to giving an insight into an important industry, it serves to fix the attention of the student upon the peculiar class of complex ammonia compounds by showing the practical importance of one of its members. The material may he prepared, and the experiment carried out as follows: Copper hydroxide is precipitated from a solution of copper sulfate by the addition of the proper amount of ammonium hydroxide. The solid copper hydroxide is then filtered and washed with water until practically free from ammonium sulfate when it is ready to be dissolved in ammonium hydroxide, forming the deep blue cupric ammonia hydroxide. Seven grams of the solid to every 100 cc. of 28% ammonium hydroxide, forms an effective solvent for cellulose; considerable variation from this concentration, however, will not seriously affect the success of the experiment as a demonstration. For the cellulose one may use either filter paper or ordinary cotton. Filter paper, torn into bits, will usually dissolve more readily than cotton, which has a tendency to form lumps, the inner portions of which are protected from the solvent action. Only a few minutes are required to dissolve two or three pieces of nine cm. paper in about 50 cc. of the solvent. The solution is somewhat viscous; if too much so it may be thinned by the addition of ammonium hydroxide. I t has a tendency to crust over if exposed to the air but may he preserved indefinitely in a stoppered bottle. In performing the experiment it is well to have a large beaker containing 600 to 800 cc. of dilute sulfuric acid; 6 N acid works very well. An ordinary pipet is then filled with the solution of the cellulose, which is next blown out into the acid through the pointed end of the pipet held just under the surface of the acid. Since the cellulose hardens as soon as it comes in contact with the acid it is well to start blowing as soon as the end touches the solution, the cellulose on emerging should form a long, deep blue thread or string, easily visible throughout a large room. After a few minutes the string will lose its blue color due to the removal of the ammonia from the complex by the acid, forming ammonium sulfate,
* Much of the work which led to the development of these demonstrations has been carried out by Myron Johnson and CliffordCam, students in the University of Nebraska.
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copper sulfate, and water. Sometimes a sack tends to form over the end of the pipet and further blowing only serves to enlarge the sack, a violent shake or two will usually jar this off and the rest of the cellulose will issue as a string. The tensile strength of the material is not great, especially while it is wet. If it is desired to f o m stronger threads they should he removed from the solution of acid as soon as possible after the blue color has disappeared and washed free' from the acid in several changes of water and carefully dried under tension in warm air. Froth Flotation I t is unnecessary to go into detail in regard to the importance of the froth flotation process for the concentration of low-grade copper ores. Let it suffice to call attention to the fact that more copper has been made available for use by the discovery of this process than has been made available through discoveries by prospectors in the last quarter of a century. In spite of this importance, the method, so dierent from ordinary laboratory procedure, is apt to mean but little to the average student of first-year chemistry. Yet it is possible to demonstrate the essential phenomenon by an experiment requiring only common reagents and a few minutes of time, so simple and convincing that the observer can hardly fail to gain an understanding of the method even though the underlying theory be beyond his present range of comprehension. The required materials are powdered copper, fine clean sand, cottonseed oil, kerosene, and sulfuric acid. Because the relative quantities of the dierent ingredients and the method of procedure must vary somewhat with different samples of sand and copper, it is not possible to give directions which will work equally well in all cases. The commercial procedure varies in different parts of the country and with different ores. The following procedure has been found satisfactory with a number of different mixtures and it is presented with the belief that it will a t least offer suggestions for a successful experiment with almost any mixture of sand with copper, copper oxide, or copper sulfide. A small amount (0.3-0.4 gm.) of the finely powdered copper is mixed in a test tube with ten or fifteen times its weight of clean sand. The tube is then half fdled with water and a few drops of sulfuric acid are added and mixed. Cotton-seed oil is next poured into the tube forming a layer from 0.5 to 1.0 cm. thick, dependmg on the amount of copper present, a drop or two of kerosene is then added and the whole shaken vigorously. On being allowed to settle the sand will fall to the bottom and the copper-coated oil bubbles will rise to the top, leaving relatively clear water in between. If it is desired to devote more time to the demonstration one may
