Development of algorithms for automated spectrophotometric titrations

A versatile method for evaluation of equilibrium constants from automated spectrophotometric titrations is described. General algorithms were derived for normalized titration curves of reactions having various stoichiometric ratios. Normalization of data involve converting it from absorption versus time to product formed (0 to 1) versus equivalents of titrant added. Relevance of derived algorithms was assessed by the quality of their fit to real data. The demonstration system chosen for this investigation was the 1:3 complexation reaction of Fe(II) and 1,10-phenantroline (o-Phen). Titrations were performed at three distinct infusion rates under controlled pH of 5.0 and at 25.0 °C, with sets produced using both Fe(II) and o-Phen as the titrant. Direct fitting of titration curves yielded values for overall equilibrium constants. Furthermore, resulting curves revealed reproducible initial deviation, consistent with step-wise reaction, when titrating Fe(II) with o-Phen. No unusual deviations were evident anywhere in the titration curves when Fe(II) was the titrant. Overall, our approach facilitates adoption of automated titrimetry and data analysis methodologies for the determination of stoichiometries and equilibrium constants of many UV/Vis active reactions.