364
JOURNAL OF
CHEMICAL EDUCATION
. FEBR~ARY, 1932
IMPROVISED SPOT PLATES DEAREDITOR: In teaching qualitative analysis, I have developed the following simple idea, which may be of some interest to your readers. In determining the pH of a solution with methyl violet, or any like experiment, we have been accustomed to place drops on smooth porcelain plates, as the more expensive ones, with depressions, are not available to us. To prevent the drops from running together, a substitute for the "depression plate" may be made from old crucible covers. Every laboratory, however poor, seems to have an abundance of these. A slit is made in a two-inch cork and the knob of the cover placed in it, which provides a steady, supported porcelain dish. Sincerely yours, (MRS.)LOUISEP. MITCHELL WOMEN'SCOLLEGE UNIVERSITY OF DELAWARE NEWARK, DELAWARE
AN EQUILIBRIUM EXPERIMENT FOR AN INTRODUCTORY LABORATORY COURSE IN PHYSICAL CHEMISTRY DEAREDITOR: C. V. King's criticism [J. CHEM.EDUC.,9, 15Wl (Jan., 1932)] of our extrapolation [J. CHEM.EDUC.,8; 2065 (Oct., 1931)] of the equilibrium constant of the reaction Ag Fe+++ i Ag+ Fe++ is theoretically valid, and leads to a decidedly higher value of the equilibrium constant than the one we obtained. However, since the extrapolation of the log K, -v$ curve is carried out over such a long range it might he well to attempt to verify the result, using the available activity coefficients at the various ionic strengths used. (Here again we shall use the activity coefficients found in Lewis & Randall's "Thermodynamics," though the assumption of anything like exactness in the values for Fe++ and especially Fe+++ is certainly not warranted.) The values so obtained are given in the table.
+
"
0.0335 0.0506 0.1024
Here KO = K fA'ifFe++.
f~.+++
+
. . . . ... . . . ..
KO 0.158 0.15J 0.144
The tendency toward higher values with in-
creasing dilution indicates the probable direction of the value of the true constant and evidences the inaccuracy of the activity coefficients used. The difference between this value and the values of 0.114 obtained electrometrically by ourselves and 0.1306 obtained indirectly from an analytical
