Iodine Coulometry of Various Reducing Agents Including Thiols with Online Photocell Detection Coupled to a Multifunctional Chemical Analysis Station To Eliminate Student End Point Detection by Eye

Abstract

Multifunctional chemical analysis (MCA) systems provide a viable alternative for large scale instruction while supporting a hands-on approach to more advanced instrumentation. These systems are robust and typically use student stations connected to a remote central computer for data collection, minimizing the need for computers at every student workspace. MCA networks offer multiple measurement capabilities; however, constant-current coulometry is still an uncommon MCA option. The characterization of an assembled teaching coulometry instrument for online data acquisition using an MCA system is described. A constantcurrent source connected to nonisolated Pt electrodes in a 150 mL beaker serving as the reaction cell caused the iodine titrant to be electrochemically generated for subsequent reaction with the analyte reducing agent. A photoresistor mounted on the stir plate below the beaker monitored the darkening of the solution due to formation of the starch−iodine complex. The change in voltage from an operational amplifier circuit permitted this titration curve to be stored by the MCA system for subsequent graphical analysis by Excel. Characterization of the instrument for direct analyte determination is performed with ascorbic acid, thiols, thiosulfate, and bisulfite. The number of electrons per mole of thiol for iodine titration of glutathione and N-acetylcysteine varied differently as a function of pH, indicating different reaction pathways. Ascorbic acid and the thiols are determined in dietary supplements with a recovery of 90−100%, and % RSDs of triplicate measurements ranged from 0.7% to 5%.

Coulometer from a Digitally Controlled Galvanostat with Photometric Endpoint Detection

“…In the last six years, some articles based on the use of coulometry, from the analytical point of view, were published describing the application to the detection and quantification of a variety of compounds from metals, pharmaceuticals, water, antioxidants, gases, and certified materials [ 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 ]. Furthermore, numerous papers on coulometry have been published in education journals due to their instructional character (see recent papers on coulometry for chemical education [ 19 , 20 , 21 ] and the references therein).…”