Evaluation of Conversion by Stopped-Flow FIA

Since any FIA response curve is a result of physical dispersion and chemical conversion one may write  

Let us consider first a very fast chemical reaction, which in a FIA system yields a response curve as the one obtained with the product, if injected as such (Fig. 2.32 , curve A). This is a response analogous to the familiar dispersion curves discussed in Sections 2.2, 2.3, 2.4, and 2.5, the only difference being that a curve obtained with the injected product P truly reflects a response of the detector to the converted analyte in question (rather than to a tracer dye—as discussed previously), and, therefore, the C%K value becomes incorporated into the peak-height measurement. 

It would be convenient to assume that a large excess of reagent R originally present (C ) is instantaneously mixed with sample material (A), so that the pseudo-first-order-reaction conditions prevailed and, hence, rate equation (2.47) could be used, but this is seldom the case in a real FIA system. [If it were the case, then the position of the maximum t/(response)/i/L = 0 in Fig. 2.29 would not change with the concentration of the injected sample ] Thus, for theoretical prediction of the fractional conversion in a FIA system, a correct reaction rate equation must be selected, and the value of the reaction constant must be known. 

However, the stopped-flow FIA technique allows the fractional conversion to be measured experimentally for any FIA system by resolving the contribution of the dispersion process and of chemical kinetics. By stopping the flow (cf. Sections 2.4.3 and 4.3) any element of the dispersed sample zone can be selected and arrested in the flow cell, where the change of response with time [i.e., rf(response)/ /t] can be monitored continuously. 

For a fast chemical reaction, such as the determination of calcium by o-cresolphthalein using colorimetric detection at 580 nm [253], shown in Fig. 2.32b, the product formation is so rapid that the response curve is identical with the dispersion curve obtained if product were injected as such. This is readily seen from the zero slope (dRIdt = 0) of the reaction rate curve A = B recorded with the element of fluid at the peak top (at CT , using delay time tdi). The same stopped-flow experiment repeated at the peak gradient (at Ca, using delay time r z) again has a zero slope 

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