Thermal Stability of Ethylenedinitrilo Tetraacetic Acid and Its Salts. Part 2. Rate of Decomposition in Aqueous Solution Determined by NMR Techniques

摘要

To prevent pitting and scaling in marine boilers, tetralithium ethylenedinitrilo tetraacetate has been proposed as a boiler-water additive. However the thermal stability of the ethylenedinitrilo tetraacetate anion (EDTA(4-)) has not been thoroughly investigated. Consequently, the rate and mechanism of the thermal decomposition of EDTA(4-) salts in solution were studied using nuclear magnetic resonance (nmr) techniques. Aqueous and deuterium oxide solutions of ethylenedinitrilo tetraacetic acid (H4EDTA), disodium ethylenedinitrilo tetraacetate (Na2H2EDTA), and tetralithium ethylenedinitrilo tetraacetate (Li4EDTA) were heated at 200C in sealed nmr tubes for periods ranging from 1 hr to over 200 hr. Periodic examination of the contents using proton nmr showed that in all solutions the EDTA(4-) decomposed rapidly and in the solution order H4EDTA > Na2H2EDTA > Li4EDTA. Approximate rates for the degradation processes were determined. The longest half-life, that for 0.1M Li4EDTA solution, is approximately 13 hr at 200C. In view of these results, the identity of the species which prevent scale formation and which act as a corrosion inhibitor in boiler water at high temperatures is obviously open to question. (Author)