Thermal Decomposition of EDTA, NTA, and Nitrilotrimethylenephosphonic Acid in Aqueous Solution

Abstract

The decomposition of ethylenediaminetetraacetic acid (EDTA) in water solution at 200 and 260 °C, nitrilotriacetic acid (NTA) at 260 and 293 °C, and of nitrilotrimethylenephosphonic acid (NTPO) at 260 °C was studied by n.m.r. as a function of time at pH 9.5 and decomposition rates and products were determined. The primary (fast) decomposition reaction of EDTA involves the hydrolytic cleavage of the ethylenic C—N bond to produce the relatively stable pair: N-(2-hydroxyethyl)iminodiacetic acid and iminodiacetic acid. NTA does not cleave below 260 °C but decomposes at about 290 °C and above through a stepwise decarboxylation reaction. NTPO cleaves hydrolytically at 260 °C at two C—N bonds to produce aminomethylenephosphonic acid and 2 mol of hydroxymethylenephosphonic acid. The further breakdown of the primary products of EDTA at higher temperatures occurs by the loss of carbon dioxide producing the corresponding methylamines, concomitantly with the hydrolytic cleavage of the remaining CH2CH2—N bond giving ethylene glycol. The pseudo first order rate constant kobs for NTA decarboxylation at 293 °C and pH 9.3 is 0.19 ± 0.01 h−1. The value of kobs for EDTA hydrolysis at 200 °C is 1.4 ± 0.2 h−1.