Thermal Stability of Ethylenedinitrilo Tetraacetic Acid and its Salts. Part 3. Oxygen Effect on the Thermal Decomposition of Tetrasodium Ethylenedinitrilo Tetraacetate in Aqueous Solutions

The thermal decomposition of aqueous Na4EDTA solutions containing varying amounts of dissolved oxygen was studied by a pmr technique. The presence of oxygen increases the rate of decomposition; at 200C, the estimated time for one-half of the original EDTA(4-) to decompose increases from 4 to 5 hours for air-saturated solutions to 13 to 15 hours for degassed solutions. The pmr data are consistent with a postulated loss of a - CH2CO2 - group from EDTA(4-) followed by the loss of a second group from the same nitrogen. The second-order kinetics of the first decomposition step show an increase in rate after an apparent induction period which is inversely proportional to the quantity of oxygen dissolved in solution. Apparently, dissolved oxygen causes the rapid formation of product(s) that, in turn, catalyze the decomposition of EDTA(4-). A chain-reaction mechanism is proposed for the thermal decomposition. The rapid rates of thermal decomposition observed for aqueous EDTA(4-) solutions suggest that stability studies are a necessary pre-requisite to any high-temperature use of systems containing this chelating agent. See also Part 2, AD676733.