Microscale Redox Titrations Using Poly-'N'-phenyl Anthranilic Acid Fluorescent Turn-Off Indicator for Undergraduate Analytical Chemistry Lab

For the present lab demonstration, we have developed fluorescent indicators to detect the end point in microscale redox titration of ferrous ions in permanganometry, cerimetry, and dichrometry. Microscale redox titrations were carried out effectively in micromolar(µM) concentrations and semi-microvolumes of the analytes. At the end point, fluorescent indicating action was confirmed successfully by the quenching of bluish-white fluorescence of poly-N-phenyl anthranilic acid (PNPA-H) and N-phenyl anthranilic acid (NPA-H). The fluorescence turn-off mechanism was due to the oxidation of protonated diphenyl benzidine dicarboxylic acid units (fluorescent) to diphenyl diquinoid dicarboxylicacid units (nonfluorescent) of PNPA-H and NPA-H. Standard distribution curves and statistical methods have revealed that fluorescent indicators have good precision and accuracy for the estimated masses. Students' responses to micromolar titration and semi-microvolume titrations using fluorescent indicators were also analyzed. Fluorescent indicators like PNPA-H and NPA-H provide an excellent opportunity for microscale volumetric estimation of ferrous ions, which is otherwise difficult using conventional indicators. The indicating action of fluorescent indicators for microscale redox titrations was beneficial in reducing the time of experiment and chemicals used. Estimating microscale quantities of mass in the lab via a simple microscale redox titration would encourage students to practice green chemistry.