Determining an Endpoint by the Appearance of Precipitate One of the more interesting experiments on the study of the solubility product concept that has appeared recently1 involves the titration of various concentrations of iodide solutions with a lead(I1) solution. The endpoint of the titrations is taken as that point where the faint yellow lead iodide precipitate persists in solution. It is the difficulty in consistently reproducing this physical change in the appearance of the solution that causes the students frustration, anxiety, and considerable eye strain. In an effort to eliminate this singularly detracting aspect of the experiment, we have found a simple and quite dependable way of "identifying" the endpoint. Working in a semi-darkened laboratory, the equipment is arranged as depicted .,,.,. " ~ , in the figure. All the apparatus is in accordance with the published directions except far the addition of a high intensity pencil-beam light source, such as the Bausch & Lomb Nicholas Illuminator. The light beam is foolled so that it shiner through the edge of the flask and a small portion of the solution contained therein. When the end.,,""." point is reached, there is a very definite contrast, which persists after thorough mixing, between the beam region (illuminated lead iodide particles) and the rest of the solution. By employing this teehnique, we have not only achieved consistently proper trends in the data, hut have sacrificed surprisingly little aecuracy in the final extrapolated estimation of the solubility product.
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'Goodman, R. C., and Petrucci, R. H., J . CHEM. EDUC., 42, 104 (1965); Mantei, K. A,, and Petrucci, R. H., "Solubility Product Constant of Lead (11) Iodide by Data Extrapolation," EQUL-067, Willard Grant Press, Boston, Mass., 1972. George T. Coehran Rollins College Winter P a r k , Florida 32789
David E. Hobart P S C Box ~ 2 1 3 6 APO S a n Francisco, California 96304
Volume 52. Number 7, July 1975 / 459
