EVALUATION OF THE PRECISION OF THE FLOW-INJECTION DOUBLET PEAK METHOD

The factors affecting the precision of the Bow injection doublet peak method are discussed with particular reference to the contribution fro m the uncertainty associated with the calibration procedure. A single- line Bow injection manifold was used in which the concentration gradie nts were generated by alternating helical reactors and the reaction pr oducts were monitored by a simple detector incorporating diodes as lig ht source and intensity transducer. The reactions between lanthanum an d Methyl Thymol Blue and between sodium hydroxide and hydrochloric aci d in the presence of Bromothymol Blue were used. Optimization studies involved the method of steepest ascent, two figures of merit (the slop e of the calibration and the standard deviation of a measured concentr ation) and two parameters (pump speed setting and the number of segmen ts in the alternating helical reactor). The resulting minimum uncertai nties, which ranged from 3 to 9%, were found to be insensitive to chan ges in experimental parameters. The major contribution to the uncertai nty in a determined concentration was found to be the scatter of the p oints about the regression line fitted to the calibration data. Altern ative approaches to Bow injection peak width methods are discussed and it is pointed out that the doublet peak method has the unique feature of giving rise to an inherently linear calibration function, because the reference points at which the time interval is measured are not fi xed at a particular value of the detector response for all concentrati ons.