A Convenient Method To Recover the Solvent from Halogen-CC14 Mixtures Due to environmental and safety considerations, the use of halogenated solvents in undergraduate chemistry teaching laboratories has heen reduced considerably. However, many general chemistry laboratories still use CCldH20 to separate haloeenhalide unknown mixtures. Although only a few milliliters are used per student, a typical freshman class will generate several liters of halogen-CCL waste eachyear. Also, we believe the h~mogeneausequilibrium
remains a useful part ofthe undergraduate physical chemistry laboralory.' Solutionsof known l r cuncentration are used to measure the distribution coefficient, k = [I?]H [I:],U,, which is, in turn, wed tu determine 1111~ of "unknown" solutions. The ~roceduresuaaeated in ref I calls fur at least 600 ml. of CCI, per experiment. The solution volumes could he halved without'too much tr%hle, hut beyond that the preparation and anilysis of the samples becomes difficult. Even if the students work in small groups, a physical chemistry laboratory of a dozen or so students will aLso generate several liters of IZ-CC14waste each year. Storage of large containers of halogenated solutions or disposal in a toxic waste site are unacceptable alternatives if the solvent can be recovered and reused. easily contaminates Ordinarily distillation could be employed to recover the solvent, hut the high vapor pressure of 1% the distillate. Although some separation occurs, a very efficient column is required to completely separate the IZfrom the CC14. Many undergraduate laboratories do not have the appropriate resources for that type of separation. We present an alternative to disposal and distillation which is easy to perform, allowing the solvent to be recovered and stored within one hour after the completion of the experiment. During the laboratoryexperiment all mixtures containing CClr are collected in an appropriately sized waste container. After all solutions are collected, as much of the aqueous layer (containing the halides) as possible is decanted. A dilute aqueous solution (e.g., 0.1 mol/L) of sodium borohydride is then added to reduce the iodide to moleculariodine, Iz. (NaBHn is frequently used to reduce heavy metals in solution, allowing the resulting sludge to be collected and processed.) Even though the NaBH, is highly polar and only reduces the Iz(aq), this will force Iz out of the CCI, phase in an attempt to regain equilibrium. Periodic shaking enlarges the phase boundary and speedsup the process; the use of a magnetic stirrer will help hut is not necessary. The disappearance of the Ip in the CC4 phase is readily observable. Caution: Large amounts of hydrogen gas are evolved and precautions for releasing and venting the hydrogen are required. After all of the Iz has been reduced, the CCld is separated, dried over MgS04, and filtered. The dried CCll can then be distilled or stared for reuse directly.
,,
' Shoemaker.D. P.; Garland. C. W.: Nlbler. J. W. ExperImentsInPhysicalChemis~,5th ad.; McGraw-Hill: New Yark. 1989: pp 211-218. Joreph E. Sabol David W. Kurtz Ohio Norlhern University Ada. Oll45810
532
Journal
of Chemical Education
