Table V. Kinetics of Hydrolysis of Cinnamic Anhydride at 25 "C PH lOakob~s~-')" 8.72 1.44 9.16 3.09 9.64 6.87 9.84 10.49 9.95 13.87 In aqueous borate buffers containing 4% acetonitrile. Total borate concentration 0.0125M; ionic strength ranged from 0.042 to 0.025.
their basicity constants would indicate. Part of the deviant behavior may be a consequence of the use of aqueous pK, values, but probably steric effects account for much of it. The minimum acylation reaction time is readily calculated with the data in Table IV and the rate equation, which is rate = k z (anhydride) (amine). The initial anhydride concentration in the analytical procedure is about 0.009M. While this is not always a large excess over the amine concentration, it often will be, and it will be assumed that pseudo-first-order
conditions apply. Then, roughly, taking n-butylamine as an example, k,,, = (158.3 M-lsec-') (0.009M) = 1.42 sec-1, so t l p = 0.693/kOas = 0.5 sec. Reaction is essentially completeLe., 99.9z-in ten half-lives, or about five seconds. A considerable latitude is then granted to allow for thorough mixing of the solutions, as well as for the expected rate decrease if the anhydride concentration decreases significantly during reaction. The alkaline hydrolysis of cinnamic anhydride is first-order in anhydride and in hydroxide, The apparent first-order rate constants are listed in Table V. A plot of log kobsagainst pH is slightly curved at the lower pH end, indicating marked incursion of the uncatalyzed "water" reaction, so that the firstorder constant can be related to hydroxide activity by: kobs= kH,o koHaoH (though it is possible that the buffer components may be involved in the rate equation.) koH has the value 155 M-l sec-l.
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RECEIVED for review March 14, 1968. Accepted May 10, 1968. Work supported by a grant-in-aid from the Research Committee of the University of Wisconsin. Financial support was also provided by the Upjohn Company.
Spectrophotometric Method for Hydrofluoric Acid in Stainless Steel Pickling Baths J. P. McKaveney Crucible Steel Company, Research Laboratory, P . 0.Box 988, Pittsburgh, Pa. 15230
The procedure utilizes the bleaching of the red-colored ferric acetylacetonate complex at 470 mp in a 0 . 1 0 ~ "0, medium for the measurement of HF. The effect of variations in total acid of the original sample, as well as acetylacetone concentrations, is examined. A variety of synthetic and plant samples were analyzed to evaluate the method. The technique i s fairly rapid (
