Time-temperature Indicators Based on the Acidic Permanganate-oxalate Reaction System

The kinetic behavior of acidified aqueous reaction systems based on the oxidation of sodium oxalate by potassium permanganate was investigated by UV-Vis absorbance spectrophotometry as part of a wider project to develop time-temperature indicators based on such systems. Reaction systems in various eutectic salt solutions as well as salt-free aqueous solutions were prepared and measured to establish the precision and accuracy of four different reaction systems designed to run for specified periods of time at 25°C. Reaction progress was also monitored in reaction systems previously subjected to prolonged periods of storage at −80°C, as well as systems subjected to repeated freezing and thawing, to determine the effect of preliminary freeze time or multiple freeze-thaw cycles on reaction run time. Eutectic-free aqueous reaction systems showed a noticeable decrease in run time from freshly frozen to 2 week frozen, but minimal to no decrease with increasing preliminary freeze time. Perchlorate-based reaction systems showed minimal to no decrease in reaction time with increasing preliminary freeze time, and were found to be capable of tolerating up to three <5-minute excursions into room temperature before reaction time was significantly affected. It was also found that all reaction systems studied were capable of reaching their target run times within the intended ±10% limit of accuracy, as average run times for each system ranged from 0.4% to 6.9% off from the target run time. These systems were found to be reproducible with considerable precision as well, with the average coefficient of variation for each system ranging from 2.6 to 5.5.