A robust multisyringe system for process flow analysis Part II. A multi-commuted injection system applied to the photometric determination of free acidity and iron(III) in metallurgical solutions

A new software-controlled volume-based system for sample introduction in pr ocess flow injection analysis was developed. By using a multi-syringe buret te coupled with one or two additional commutation valves, the multi-commute d injection of precise sample volumes was accomplished. Characteristics and performance of the injection system were studied by injecting an indicator in a buffered carrier. Three configurations were implemented in order to a chieve two different tasks: the single injection of a sample in a two- or t hree-channels manifold, and the dual injection into different streams. The two channel flow system using the single injection was applied to the deter mination of free acidity in diluted samples containing high levels of iron( iii), by employing the single point titration methodology. The precipitatio n of ferric hydroxide was prevented using the ammonium and sodium salts of oxalate and acetate as buffer titrant. Methyl Red was employed as indicator . The procedure allows determination of acid concentration in solutions wit h a Fe(iii)/H+ molar ratio up to 0.2. Samples with higher Fe(iii)/H+ molar ratios were spiked with a known strong acid at dilution. The three-channel configuration was applied to the determination of ferric ions, using, as re agent, a merging mixture of sulfuric acid and potassium thiocyanate. The do uble injection system was implemented in series in a single (three-channel) manifold in such a way that a different injection volume and a changed rea gent were used for each analyte. It was applied to the separated or sequent ial determination of free acidity and ferric ions. In this configuration, i ron(iii) was determined using 0.5-0.7% (w/v) sodium salicylate solution as reagent. The systems can operate at up to 100, 84 and 78 injections per hou r, respectively. Determinations on synthetic and process samples compared w ell with the reference values and procedures. Recoveries of 95-102% with a maximum RSD value of 5.4% were found for acidity. The respective values obt ained for iron determinations were 96-105% and 4.3%.