is used in the titration. This procedure eliminates the need for pipeting 203 ml of treated sample. B.O.D. bottles are made to contain 300 ml *3, and t h i s 1% precision is in line with the analytical precision cited by Professor Stagg in the article. In addition, reference is made t o the 12th Edition of "Standard hlethods for the Examination of Water and Wastewater." This edition was superseded in 1971 by the 13th Edition, although the revisions did not affect the standard procedure for the dissolved oxygen det,ermination.
TOBY E. BERCER GEOGRAPHY BARNARD COLLI:GI: C o ~ u ~ nUNIVI:RSITY, r.~ NEW YORK10027 ~ ~ : P . \ R T M I I N OF T
Relative Reactivities in Free Radical Systems
To the Editov: I have one comment to make relative t o the otherwise finc article by Hutchinson and hIosher [J. CHEMEDUC., 48,629 (1971) 1. Their cqn. (9), which t h ~ ystatc, "could be derived t,o relate t,he ratio of rate constants to the formation of products formed . . . " is not consistent u i t h their eqn. (8), nor is it in fact generally true. Under certain conditions eqn. (9) may bc useahle, but certainly not with the initial conditions used by the authors, i.e., 0.03 mole of each hydrocarbon and 0.04 mole of chlorinating agent. Use of eqn. (9) would be valid if one begins the reaction with equal concentrations of hydrocarbon, but in large excess compared to chlorinating agent-in which case the concentrations of the former would remain relatively constant (and hence nearly equal) and the corresponding yields of product would be proportional to the reaction rates and the rate constants.
PETER J. HANSEN 7 i o n ~ w m ; s ~COI,LI:GI,: m~ OH.INGI;CITY, Io\v.\ :1041
Commercial Gak Replaces Hydrogen in General Chemistry Laboratory
Perhaps your readers would be interested in the following: Recently published general chemistry laboratory manuals indicate that it is not widely recognized that commercial combustible gas, now largely methanc in most metropolitan distribution systems, can bo used in placa of hydrogm to reduce metal oxides. Not only is natural gas cheaper but its explosive limits an, con~iderablynarrower. Furthermore, our experience and thcrmochrmical calculations show CHa is a t least as good as a rcducing agent.
162 /Journal of Chemical Education
