Physico-chemical studies in flow analysis
thesisposted on 27.07.2017 by Francis E. Powell
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The first part of this study was the characterisation of an impinging jet electrode in an amperometric detector; the device having found extensive application in flow injection analysis. A voltammetric study of the detector in the stopped flow mode was carried out and evidence is presented of restricted diffusion imposed by the shallow depth of the cell. In hydrodynamic voltammetry, the detector exhibited a gradual progression from thin-layer to wall jet behaviour as the flow rate increased. This gradation is discussed in terms of a model in which flow in the electrode chamber forms concentric rings, the streamlines being successively perpendicular, oblique and parallel to the electrode. The response of the detector and its associated equipment was studied by two experiments. Firstly, the fidelity of the electrochemical instrumentation and recording system was ascertained from its electronic response to a RC (resistance-capacitance) circuit functioning as a flow injection transport analogue. Secondly, the dynamic response of the electrochemical cell was established from an experiment using a concentration step input delivered through a short, straight manifold. The results indicated that laminar flow in the delivery tube was modified by mixing stages in the cell channel and its connections to produce a final dispersion which defines an effective detection volume of only 7µL. The electronic and cell responses indicate that the total detection system would impose little extra dispersion in a practical flow injection line. In the second part of this study, photometric titrations were carried out in a stirred tank reactor in which the volume changed linearly with time. The general relation for the concentration gradient when the tank is used as a mixing device was examined experimentally under various flow conditions. In particular, the precision of linear concentration gradients was ascertained when peristaltic pumping was employed. These gradients were utilised to titrate analyte within the tank by means of titrant delivered by pump flow with photometric detection in an exit stream. Self-indicating titrations, following changes in the absorbance of analyte, titrant or reaction product, were performed each conforming to theoretical prediction. Due to the external detection system employed, dispersion and transportation lag effects were observed and accounted for.