ELECTRODE POTENTIALS Submitted by: Donald J . Shoiubert, Douglass College, RutgersUniversity, New Brunswick, N. J. Checked by: David B Moss, Hiram College, Hiram, Ohio
PREPARATION
Provide two conventional Daniel1 cells consisting of zinc bars in 1 M .%SO4 and copper bars in 1 M CuS04 (the solutions may be in beakers connected by salt bridges or porous cups may be used), conc NH40H, conc NaOH, and a voltmeter with a large scale (such as the KCA Senior (Voltchmyst) which has a 7-in. scale). DEMONSTRATION
Assemble the cells and show that each has a potential of 1.1 volts Pour conc NaOH into the zinc com-
partment of one cell and note the voltage. Next add conc NH40H to the copper compartment of the other cell and read the voltage of this cell. REMARKS
The observed shifts in the voltages may be discussed in terms of the ion concentration changes according to the appropriate chemical reactions: Zn++ (aq) Cu++ (aq)
+ 40H-
i=t
Zn(OH),--(aq)
+ 4NHs r Cu(NHA++( a d
m a 1
of Chemical Education, ' March 1965
PROJECTION MANOMETER Submtted b y William H. Eberhardt, Georgia Institute of Technology, Atlanta, Ga. Checked by C, Francis Egan and L Goldthwait, NSF Chemistry Institute, Montana State College, Bozeman
SCREEN
PREPARATION
Obtain a closed-end mercury manometer about 6 in. high, a projection optical bench, two mirrors, screen, benzene, toluene, and a hypodermic needle. DEMONSTRATION
Mount the manometer close to the large (condensing) lens, place the projection lens near the end of the optical bench, and arrange the mirrors as indicated in the figure. This arrangement moves the image high enough to be seen readily and also makes possible a magnification of 10 to 1. The image is inverted, but experience indicates this is not a serious drawback. The device may be operated by coupling the manometer with a vacuum system and injecting a sample of liquid with a syringe or hypodermic needle appropriately bent to reach part way around the bottom of the U. REMARKS
Potential uses include: (1) Measurement of the vapor pressure of a liquid such as benzene. ( 2 ) Investigation of Raoult's Law: (a) Measure the pressure of pure benzene p mand then pump out the system; (b) Measure pa of a known sampleof toluene (1 ml), i c ) Add known volumes of benzene and measure the p for eachmixture. Jow-nal
of Chemical
Education
March 1965
