the commercially supplied grade of the pesticide "methoxychlor," which has widely replaced DDT in use, contains about U% of an imnuritv (tetrakis-D-anisvlethvlene)which develops a bright 6lue color in the'presence df bromine in acetic acid. Recrvstallization of crude methoxvchlor from ethanol removes this impurity and the resultant pure methoxychlor no longer gives the "hlue color test." The melting points of the samples, however, only differ slightly. The methoxychlor used was supplied, for example, by Sigma Chemical Company, U.S.A. (technical grade). Retail "methoxychlor" contains a further 50% inert diluent and is unsuitable for direct use. The advantage of this experiment over an ordinary recrystallization is two-fold: (a) The compounds are relevant even though complex and point out that evaluation of environmental effects of pesticides must take into account impurities that are incorporated during manufacture. (h) The student can immediately see by means of the "hlue color test" whether he has been successful. Full information and experimental details can be ohtained from the authors.
CCl, Methoxychlor
the reaction progressed. Thus, the progress of the reaction could then be monitored visually from the appearance of the solution, and this confirmed quantitatively using a colorimeter. Such reactions occurring on a time scale convenient to monitor, and not subject t o complications, appear t o be surprisingly few. A reaction which does fulfill these criteria and has been adapted for such a kinetics experiment is the oxidation of iodide ion hy persulfate.
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+ s2oa2- I:! + 2s02Unon mixing- the reaeents. - . the initiallv colorless solution 21-
becomes a progressively darker orange-brown color as the reaction oroceeds.. eeneratine iodine (existine - in solution as the 13- complex). In order to investigate the effect of concentration upon the reaction rate, several solutions differing in the concentration of one reagent are allowed to react for a given period of time (10 min.), whereupon the color is compared visually and with the colorimeter. The variation of reaction rate with temperature can similarly he determined by mixing solutions of identical concentrations in test tubes a t 0°, 25O and 50°C. and comparing the absorbance of the solutions after 10 min. A write-up of the experiment may he obtained by writing to the author or Prof. R. E. Kirby a t Queens College.
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Compact Compacts ~ C I ~
DDT A Simple General Chemistry Kinetics Experiment Martin Gellender Queens College of C.U.N.Y. Flushing, New York, I1367 In order to clearly illustrate principles of chemical kinetics, a chemical reaction was sought which would result in a gradual and clearly distinguishable appearance of color as
806 / Journal of ChemicalEducation
R. F. Evilia University of New Orleans New Orleans, Louisiana 70122
A recent note IShen, S. Y., J. CHEM. EDUC., 51, 784 (1974)l reports a'new method for determining molecular weights of acids. Unfortunatelv, this method rewires unreafistically accurate p H meas&emeuts to yield acceptable results. For example, using this method to determine the molecular weight of acetic acid yields a value of 60.20 if p H values of 2.373 and 2.875 are used for 1M and 0.1 M acetic acid, respectively. A molecular weight of 296 is obtained, however, if the experimentally more reasonable values of 2.37 and 2.88 are used for the pH's.
