The Thermal Stability of Nitrilotriacetic Acid and Its Salts in Aqueous Solutions

A proposed mechanism for the thermal decomposition of ethylenedinitrilotetraacetic acid (H4EDTA) and its salts suggests an intermediate having a hydrogen three-centered bridge structure. To elucidate the mechanism of EDTA(4-) decomposition and to establish structures which would impart inherent thermal stability to a chelating compound, nuclear magnetic resonance (nmr) techniques were used to study the thermal stability of nitrilotriacetic acid (H3NTA) and its salts. These compounds have a structure similar to EDTA(4-) but are not capable of forming a three-center intramolecular bridge under any circumstance. Degassed and undegassed aqueous solutions of Na3NTA, and Na2HNTA and a degassed solution of H3NTA were heated at 200C in sealed nmr tubes for periods ranging from 1 hr to over 670 hr. Periodic examination of the contents using proton magnetic resonance (pmr) showed that dissolved oxygen has little effect on the decomposition of Na3NTA and Na2HNTA solutions. Although the laboratory results show that NTA(3-) is far more stable than EDTA(4-) at 200C, it would be unsafe to predict that NTA(3-) would continue to show this excellent thermal stability up to 300 through 320C. A different mode of thermal decomposition, not dependent on the proposed protonation of NTA(3-), may occur readily at temperatures higher than 200C. In basic solutions at 200C and lower, the uncomplexed or weakly complexed NTA(3-) should be stable enough to be useful as a boiler-water additive for sludge control and feed-water conditioning.