Abstract
This paper describes use of gradients of concentration generated in flow injection (FI) systems to perform determinations based on points where the concentration of titrant and analyte are at stoichiometric ratio. Two procedures were developed. In one procedure the titrant is injected in a FI manifold and merges with the sample which is continuously pumped towards the detector. In the other procedure the sample is injected and merged with the titrant which is continuously pumped. Both techniques make use of concentration gradients of the sample or titrant generated in FI manifolds that contain a mixing chamber. This gradient is calibrated employing only one standard solution (usually the titrant) in order to convert any detector signal, obtained in the elapsed time after injection, to instantaneous concentration values. The flow system is microcomputer controlled and data are treated to locate points where the concentration of titrant and analyte are at the stoichiometric ratio. These points are found in abrupt changes of the signal × concentration curves obtained in the presence of the reaction. The method has been evaluated for determination of Fe(II) and acetic acid by spectrophotometric and conductimetric detection, respectively. Results show a mean relative standard deviation lower than 1%, an average accuracy of 1% and a high sampling processing capability (40 to 60 samples per hour).
Highlights
When performed manually, titrimetric methods are cumbersome and slow
Requires individual flasks to contain each sample and, despite efforts to increase the speed of determination, the overall sampling processing is considered slow
This point is reached by keeping the flow rate of one of the solutions constant and making the other change linearly [3,4,5] or by keeping the flow rate of sample constant and by exploiting linear concentration gradients of titrant generated by an external gradient chamber [6] or by coulometry [7]
Summary
Titrimetric methods are cumbersome and slow. To overcome these drawbacks, automatic batch and continuous flow systems have been developed. For example, that after gradient calibration, the concentration of the injected solution passing the detection point at any instant, in absence of reaction, is known Both titration procedures proposed are based on the concentration profile established in the FI manifold after the introduction of a discrete volume of sample (in SIT) or titrant (in TIT). The same titrant solution is injected in the manifold, carried by water, merged with the line containing the same solution and with the other pumping lines, passed through the mixing chamber, and the peak-like concentration profile, observed, is obtained over the steady-state signal (Hss). Samples or titrant solutions (depending on the titration procedure chosen) are injected under computer control and the resulting signals, obtained in the presence of the reaction between the analyte and the titrant, are sampled in the same time intervals employed in the gradient calibration procedure.
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