Silver Tree
To the Editor: For several years I, too, have been disturbed by class results of the silver tree experiment performed in conjunction with the CHEMS program. In 1965 we determined the si1ver:copper ratio, R, to he 1.89 and in 1966, 1.91. This year students were instructed to use the new procedure described by Carmody and Wiersma [J. CHEM. EDUC.,44, 417 (1967)l. Herein are listed the values of R obtained by fifty-five students each performing the experiment individually.
The average valne of R is 1.94. If one considers the middle two-thirds of the data, as I did in obtaining R(1965) and R(1966), the averagevalue of R becomes 1.95. Students performed all weighings on centigram halances. The loss of copper fines was further minimized by pressing a spatula against the heaker during decant* tion. The wash water flowed out beneath the spatula, and the fines, floating on the surface, moved toward the rear of the heaker after bouncing off the spatula. The opportunity to witness this reaction in progress generated significant student interest. The average value of 1.95 compares favorably with the expected result of 2.00. This new procedure is certainly superior to the old one and should he highly recommended for general use.
This vessel allows the use of small volume while utilizing the kit electrodes in a convenient configuration. The platinum and calomel electrodes are placed in the two arms of the vessel, and the DME and Nz are put through a stopper. The students construct the DME from capillary tubing, a leveling bulb, and glass and tygon tubing. A deaereating tube is made in a T configuration to overcome the slow and somewhat inadequate deaereation and mixing in the vessel arms that occurred with a conventional gas inlet tube. This tuheis made in two parts for easy insertion. The cell is connected to a Heath R polarographic module in our laboratory, and can he used for both two- and three-electrode work. The internal resistance of the calomel cell is relatively low. For amperometric titrations with the DME that the Kontes kit supplied 125- or 250-ml vessels are used.
Present Address: Astronomy Program, Space Sciences Bldg., Univemity of Maryland, College Park, Md. 20742
Modifled Electrode Kit
T o the Editor: We have found the Kontes R electrode kit, designed by William C. Oelkeof Grinnell College, useful in teaching electrochemistry to students of analytical chemistry. The kit electrodes have the educational virtues of requiring the student to prepare the materials and assemble the electrode, and they work well when completed. The kit calomel cell is very much more reliable than those constructed by students from scratch. We have extended their use to two- and three-electrode polarographic work by the use of the diagrammed vessel (see fig.) produced from our design by Kontes. Volume 45, Number 4, April 1968
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