Continuous-flow analytical systems, diffusion

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 stream, where its mixing and reaction with reagents in the stream are controlled by the kinetic processes of convection and diffusion. 

Hyphenation of automatic continuous flow systems (such as SPE, dialysis, gas diffusion, evaporation, direct leaching, etc.) to CE and the coupling of automatic sample preparation devices into commercial CE equipments have been devised as a means to simplification and miniaturization of analytical procedures. An automatic online SPE device for the multiresidue extraction of seven pesticides has been described. Four river samples were spiked with the test mixture at three different levels presenting recoveries Irom 90% to 114%. 

An approximate analytical solution has been developed to calculate the exit concentration from a continuously recirculating facilitated transport liquid membrane system. The system is modeled as a series of SLM-CSTR pairs. The solution allows for two-dimensional transport (axial convective and radial diffusive) and laminar flow. The solution allows one to estimate the effect of a change in system variables on the operating performance. Comparison with experimental data was very good. 

The most important use of residence time theory is its application to equipment that is already bnilt and operating. It is usually possible to find a tracer together with injection and detection methods that will be acceptable to a plant manager. The RTD is measnred and then analyzed to understand system performance. In this section we focns on such uses. The washout function is assumed to have an experimental basis. Calculations using it will be numerical in nature or will be analytical procednres applied to a model that reproduces the data accurately. Data fitting is best done by nonlinear least squares using untransformed experimental measurements of W(t), F(t), or f(t) versus time, t. Eddy diffusion in a turbulent system justifies exponential extrapolation of the integrals that define the moments in Table 1-2. For laminar flow systems, washout experiments should be continued until at least five times the estimated valne for t. The dimensionless variance has limited usefnlness in laminar flow systems. 

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