Kinetic determination of Mo(VI)

Abstract

A catalytic reaction occurs when a certain species in the reaction medium (the catalyst) causes a decrease in the system's activation energy, without being consumed in the process. These reactions are particularly useful when it is desired to detect and/or determine the presence of very small amounts (traces) of substances contained in a sample. Some reactions occur (or are accelerated) only in the presence of catalysts and the kinetic study of these reactions provides valuable information about their presence or absence in the reaction medium. The first examples of analytical methods based on chemical kinetics date back to the end of the 19th century and are related to the catalytic activity of enzymes. Despite the variety of methods developed in the first half of the 20th century, kinetic methods were no longer in common use, as they were susceptible to errors caused by poorly controlled variables, such as temperature, pH and the presence of interferents, which could activate or inhibit the reactions. It was only after the 1980s that this technique began to be studied more closely again. This didactic article aims to demonstrate the use of the kinetic-catalytic method in determining traces of Mo(VI) ions in solution, based on the indicator reaction of hydrogen peroxide with iodide ions in an acidic medium. Iodine production can be monitored spectrophotometrically, monitoring the formation of the complex [I₂ – starch], with an intense blue color, which presents its maximum absorbance at l=590 nm. The reaction can also be monitored by monitoring the formation of I₂ in solution at its absorption maximum at l=390 nm, but the sensitivity of this procedure is much lower. The development of the indicator reaction was observed at regular time intervals in the presence of different concentrations of Mo(VI), allowing the construction of kinetic curves that relate the reaction speed with the catalyst concentration. The main objective of this article is to demonstrate the use of the kinetic-catalytic analysis technique, using molecular spectrophotometry as a detection method, applying the procedure to determine the concentration of Mo(VI) ions in a sample containing (only) ions molybdate. This method is attractive as a teaching experiment because, in addition to being relatively accurate, it uses simple instruments that are commonly used in teaching laboratories.