Recycling Lead[ll] Halides from Solubility Experiments Charles W. J. Scaife and Chadlee D. Hall Union College, Schenectady, NY 12308 Several experiments have been published in which K,,'s of lead(I1) halides are determined. In one, a saturated solution of PbC12 is prepared a t a single temperature, and Pb2+from duplicate 100-mL aliquots is precipitated as PbS, filtered, dried, and weighed so that K,, can be determined from the mass of PbS formed ( I ) . In a second, excess PbC12 is mixed with 25 mL of deionized water, 0.100 M NaCI, and 0.100 M NaN03 solution, respectively, a t a single temperature, and undissolved PhC12 is filtered, dried, and weighed so that K,,'s can be calculated from dissolved PbCI2 determined by difference (2). In a third, standard solutions of P b ( N 0 3 ) ~ and KI in 0.2 M K N 0 3 are mixed in different proportions a t a single temperature to precipitate PbIz. Remaining [I-] is determined spectrophotometrically after oxidation of I- to I2 by KN02, and excess p b 2 + j is calculated from initial [PbZ+].The PbIz is then washed free of ions and usedto form a saturated solution. The [I-] is again determined spectrophotometrically, [Pb2+]is calculated from [I-], and K,, is then calculated two ways (3). In a fourth, four 25-mL aliquots are removed from a saturated solution of PhC12 at temperatures from 90 to 25 OC, and [Cl-] is determined by a Mohr titration involving addition of NanCrOa followed by titratiou with a standardized AgN03 solution (4). Solid PbCr04 and AgCl are formed, and excess Cr0d2- remains. Thermodynamic functions are calculated from the temperature-deuendent K,'s. No methods are'given in these experiments to recycle or disoose of either the solids or solutions containina Aa. Cr, or ~ b Salts : of these elements are specifically re&l&edby OSHA, and the cations have specific water quality limits. Solids cannot be discarded as normal trash, and solutions cannot be poured down the drain. Costs of chemicals and waste disposal make recovery and reuse almost mandatory. The fourth experiment ( 4 ) has been scaled down and modified so that students obtain data for calculatine [Pb2+1. [CI-1,and K,, at four different temperatures betwein'75and 5 "C. as well as AHo and ASo from theslooe and interceot of a pldt of In K,, versus 1/T and AGO from AHD - TAS'O. In addition, students recover almost 90% of the original PbC12, thus saving on costs of starting chemicals and disposal. This is accomplished by replacing the Mohr titration withagravimetric method. Specifically, an optimalamount of 6 M NaCl provides a common ion so that PbC12is precipitated quantitatively when the several saturated solutions are chilled to near 0 OC. The PbC12 is then filtered, dried, and weighed t o obtain data for determining K,,'s. This eliminates the need for chemicals other than PbC12. Very small amounts of Pb2+ ~~
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remaining in filtrates can be precipitated as PbS (5) and disposed of inexpensively as waste. Experimental Prepare an ice-water bath in a pneumatic trough, and obtain four 50-mL or larger beakers numhered 1,2,3, and 4, respectively, and 150 mL of distilled water in a 250-mL beaker. Weigh 3.0 g of PbC12 into a 400-mL beaker numbered 5, andadd 125mL of distilled water. Caution:PbC12is poisonous; wash your hands thorouehlv .. . after . use. Heat the mixture to a h o u t k ~"C, stirring to ensure the formation of a saturated solution. Caution: Do nor .stir the solution with your thermometer. As soon as the mixture reaches 85 OC, place the beaker on a hot pad, and allow the undissolved PbC12 to settle. While the PbClz solution cools, heat the 250mL beaker of distilled water, and warm a 100-mL graduated cylinder by pouring hot distilled water from the 250-mL beaker into it. After the graduated cylinder is warm, drain the water completely. When the PbClz solution drops to 75 OC, record the actual temperature, use a "hot hand" or cloth to decant 25.0 mL of clear PbC12 solution into the warm graduated cylinder, and quickly transfer i t to beaker 1. (Hold a curved piece of filter paper down in the solution near the lip of the beaker to prevent any floating solid from getting into the graduated cylinder.) Rinse the graduated cylinder with 5 mL of hot distilled water, and add it to beaker 1. Repeat the procedure of the previous paragraph to transfer 25.0-mL portions a t about 50 "C to beaker 2, 25 OC to beaker 3, and 5 OC t o beaker 4. Use the ice water bath to help you reach the lower temperatures. Add 4.0 mL of 6 M NaCl to each numbered beaker. Chill each beaker to near 0 OC in the ice water bath. Swirl the contents of the beakers periodically to ensure that temperature equilibrium is achieved. While you are waiting for the solutions tocool, weigh five pieces of filter paper marked 1,2, 3,4, and 5, respectively. Filter the contents of beakers 1-5 through the filter papers. Use a rubber policeman to scrape solid remainingin the beakers onto the filter papers. Dry the filter papers in an oven or under a heat lamp for 45 min. Empty the contents of the filter flask into a beaker. Add a chip of Na2S to the beaker to precipitate any remaining Pb2+. Filter the PbS, and scrape it into a beaker marked SOLID PbS. Weigh each filter paper after cooling, and scrape the solid PbCI! into a beaker marked SOLID PbCI..
Volume 67
Number 7 July 1990
605
