MIRRORS AND B-BATTERIES AS STUDENT PROJECTS
Project work creates a great deal of interest in chemistry. Two projects which have been found highly satisfactory (except for the fact that they are both rather expensive) are the silvering of glass and the preparation of Bbatteries. Probably the greatest difficulty in the making of mirrors is in getting the glass clean. If there is the slightest amount of grease or dirt on the surface to be silvered, it will show in the final product. If the glass is extremely greasy, clean first with potassium hydroxide. If is it not very greasy, rinse with alcohol and then with ether. Then clean with nitric acid and lastly with warm, distilled water. In some cases stannous chloride is used after the nitric acid treatment. Then distilled water is used. At no time should the glass become dry from the time the cleaning process is started until the coating of silver is deposited on the glass. In case it should at any stage become dry, it is best to start the whole process over. Be sure that all chlorine, free or combined, is removed from the glass. The silvering is conveniently done in a shallow tray only slightly larger than the surface to be silvered. Care should be taken to avoid touching the solution, not only to protect the hands, but also to prevent contamination of the solution. AU chemicals used in the preparation of solutions should be C.P. materials. Distilled water should be used. There are a number of formulas to choose from, but the two easiest to carry out are: (1) the Rochelle'salt process, and (2) the formaldehyde process. Of these two, the formaldehydeprocess is the easier. These methods may be found in almost any book of chemical formulas or in a Bureau of Standards circular, "Methods of Silvering Glass." The making of the B-battery is of particular interest to the pupil who has an old radio set, for he can actually get results from the product of his work. The B-battery may be made as an Edison storage battery. This may be prepared by taking a number of test tubes fitted with two-hole stoppers. In the battery exhibited at the Academy of Science meeting last May forty-five tubes were used-five rows with nine cells in a row. Through each stopper was inserted a strip of iron and a strip of nickel. These were connected in a series, and clamped together in order to insure good connections. Each tube was then partly filled with the electrolyte, a potassium hydroxide solution with a specific gravity of 1.25. Then the stoppers with the iron and nickel strips were placed in the tubes making certain that the strips did not touch within the cells. Then by means of a medicine dropper the electrolyte was brought up to the proper level by adding the solution through the holes in the stopper. A drop of oil was placed on the (Continued on page 1653) 1648
VOL.9, NO. 9 IMPROVEMENT ON CHLORAMINE-T METHOD
1653
This entire procedure was repeated with the bromide solution. Only a faint trace of color could be detected in the third test tube a t the 1 :lo0 dilution, while the carbon tetrachloride and carbon disulfide were light yellow. The thud tube was colorless a t the 1:ZOO dilution but the carbon disulfide was still a light yellow and the carbon tetrachloride was a faint yellow. In both cases the carbon disulfide proved a little more sensitive than the carbon tetrachloride. This improvement can be applied to the method suggested by Gerstenzang2by drawing off in a test tube three or four cc. of the solution a t each point in the procedure where the chloramine-T is to be added. To this sample is added half a cc. of carbon disulfide or carbon tetrachloride, three or four drops of a 5% solution of chloramine-T are slowly added, and the tube is thoroughly shaken. Whether the result of the test is positive or negative, the procedure suggested2 after such a result is carried out on the main portion of the solution.
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top of the electrolyte to act as a valve. It would 2low the gases produced in charging to be given off but would not allow the carbon dioxide of the air to react with the ferrous compound and destroy the battery. The nickel is the positive plate, and the iron the negative. When the cell is charged the active material on the positive plate is nickelic oxide and nickelic peroxide. On the negative plate, when charged, it is iron; and when discharged, is ferrous hydroxide. The plates are formed by repeated cycles of charge and discharge. Each cell should develop 1.2 volts. By means of movable clamps the desired voltage may be obtained.
