A swept electron beam rapid scan spectrophotometer has been built in this laboratory. This device utilizes techniques which were originally intended for video applications.
b An instrumental technique is applied to a kinetic study of the cystine catalysis of the iodine-azide reaction.This reaction is essentially first-order in cystine, but minor pH-dependent side reactions appear to cause a higher-order effect a t high cystine concentrations. It is first-order in iodine and has an iodide dependency between -0.5 and -1.0 order.The azide dependency ranges from first-order at high azide (1.OM) and low iodide ( 1 X 10-3M) concentrations to almost second-order at low azide (0.1M) and high iodide concentrations ( 1 X 10-W). The reaction shows a maximum rate at p H 5.2 to 5.4 and a minimum rate a t pH 7.5. A reaction pathway is proposed which is in agreement with the kinetic data and reaction characteristics observed. Conclusions are drawn concerning certain features of disulfide compounds which influence their activities as catalysts for the iodine-azide reaction. ATALYSIS by sulfur compounds of C the oxidation of azide by iodine has attracted significant interest (7, IS, 14). This reaction has been investigated, taking advantage of a new instrumental method for reading numerical values of reaction rates automatically. I n this manner, high precision kinetic data have been obtained near zero reaction time with the result that a more complete understanding of these reactions is obtained. EXPERIMENTALThe concentration cell and measurement system have been described (9, IO).For purposes of determination of reaction order dependence on initial concentration of reagents, a combination of the isolation and differential methods ( I ) was applied. Isolation was achieved by adjusting the reagent concentrations to obtain a reaction which was pseudo-first-order in iodine. Dependencies were then observed by varying individual reagent Concentrations and observing the different rates of iodine consumption.The standard conditions upon which the relative rates of the reaction were Present address, Central Research Dept., Monsanto Co., St. Louis, Mo. based were as follows: 4.92 X 10-4X Iz, 5.0 X 10-3M I-, 0.9iM NaN3, 6.65 X 10-5Akf catalyst, p H of 6.0, and temperature of 25' + 0.05' C.Under these arbitrary conditions with cystine as the catalyst, the initial rate of iodine consumption was determined by calibrating the potentiometric response. This initial rate, -dIz/dt = 1.1 X moles 1 2 per liter-second, was assigned a relative value of 1.00. Rates for cystine catalysis measured under other conditions were assigned relative values to this. A number of other catalytic species were investigated under the standard conditions and assigned catalytic activity relative to that of cystine. RESULTSThe relative rates reported in this study are averages of a t least three runs per test. Relative standard deviations less than 1% were achieved in these measurements. All rate data for cystine have precision and accuracy comparable to those reported previously (9).The electrodes of the concentration cell exhibited Nernstian behavior. dlthough the cell potential became more positive with decreasing azide concentration a...
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