A USEFUL CHART FOR QUALITATIVE ANALYSIS L. P. BIEFELD Purdue University, Lafayette, Indiana
CHARTS displaying flow diagrams, curves, tables, and apparatus sketches are valuable aids to lectures and class discussions in qualitative chemical analysis. Satisfactory displays are drawn on 2' X 3' white Bristo1 board with black and red India inks. The drawings are not substitutes for lantern slides and demonstration experiments but are used in con-
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FIGURE1.--SOLUBILITY PRODUCT VALUESOF THE SULFIDES OF GROUPSI1 AND I11
junction with them. Charts have advantages over slides. No special projection or light control equipment is required. A series of charts can be exhibited and discussed simultaneously. Being able to refer rapidly back and forth from one drawing to another facilitates comparisons, contrasts, and explanations of visual material presented. ~ i g & e1 shows a chart helpful for discussion of solubility product, solubility, and controlled precipitation as applied to separation of the acid sulfide from the alkaline sulfide group. The curve is obtained by plotting the negative logarithms of the solubility products of the sulfides against the cations involved. The cations are in order of increasing solubility of their respective sulfides and are separated by an arbitrary, uniform distance. The solubility product values were procured from the class textbook (1). The values for the alpha modifications of cobalt and nickel sulfides were chosen because these forms presumably predominate when Group 111is precipitated. The solubility products of ferric and arsenious sulfides are omitted because the values in the literatwe are not closely correlated to the actual conditions of precipitation ( 2 ) . LITERATURE CITED
(1) MIDDLETON AND W n ~ ~ ~ ~ , " S e r n i mqualitativeanalysis." icro Prentice Hall, Inc., New York City, 1938, p. 111. (2) Ibid.,p. 107.
