tailieunhanh - Modern Analytical Cheymistry - Chapter 13

Kinetic Methods of Analysis Một hệ thống dưới sự kiểm soát nhiệt động lực học trong một trạng thái thăng bằng, và tín hiệu của nó có một giá trị không đổi, hoặc trạng thái ổn định (hình ). Khi một hệ thống kiểm soát động, tuy nhiên, những thay đổi tín hiệu với thời gian của mình (hình ) cho đến khi cân bằng được thành lập. Đo Như vậy đến nay, các kỹ thuật chúng tôi đã xem xét có liên quan đến thực hiện khi hệ thống ở trạng thái cân bằng. Bằng cách thay đổi thời gian. | Chapter 13 Kinetic Methods of Analysis . . system under thermodynamic control is in a state of equilibrium and its signal has a constant or steady-state value Figure . When a system is under kinetic control however its signal changes with time Figure until equilibrium is established. Thus far the techniques we have considered have involved measurements made when the system is at equilibrium. By changing the time at which measurements are made an analysis can be carried out under either thermodynamic control or kinetic control. For example one method for determining the concentration of NO2_ in groundwater involves the diazotization reaction shown in Figure The final product which is a reddish-purple azo dye absorbs visible light at a wavelength of 543 nm. Since the concentration of dye is determined by the amount of NO2_ in the original sample the solution s absorbance can be used to determine the concentration of NO2_. The reaction in the second step however is not instantaneous. To achieve a steady-state signal such as that in Figure the absorbance is measured following a 10-min delay. By measuring the signal during the 10-min development period information about the rate of the reaction is obtained. If the reaction s rate is a function of the concentration of NO2_ then the rate also can be used to determine its concentration in the There are many potential advantages to kinetic methods of analysis perhaps the most important of which is the ability to use chemical reactions that are slow to reach equilibrium. In this chapter we examine three techniques that rely on measurements made while the analytical system is under kinetic rather than thermodynamic control chemical kinetic techniques in which the rate of a chemical reaction is measured radiochemical techniques in which a radioactive element s rate of nuclear decay is measured and flow injection analysis in which the analyte is injected into a continuously flowing carrier .