Chelating agents in high temperature aqueous chemistry. 1. The kinetics of the thermal decomposition of aqueous nitrilotriacetate (NTA), iminodiacetate (IDA), and N -methyliminodiacetate (MIDA)

Abstract

Aqueous H 3 NTA, H 2 MIDA, H 2 IDA, and their anions decompose under hydrothermal conditions (400–580 K) according to first order kinetics by successive decarboxylations, oxidation by O 2 being unimportant except at the highest temperatures. In the presence of added H + , the species H 4 NTA + and, to a lesser extent, H 3 MIDA + (but not H 3 IDA + ), provide significant decomposition pathways through elimination of a —CH 2 COO— group (deacetylation). For H n NTA( 3−n ) − , first order rate coefficients k n for decomposition are k 0 = 4.5 × 10 −7 , k 1 ∼ 1 × 10 −6 , k 2 ∼ 7 × 10 −5 , k 3 = 2.1 × 10 −4 , and k 4 = 1.0 × 10 −2 s 1 , at 503 K and ionic strength 2.0 m, the spread in rates being due to differences in ΔS* rather than ΔH*. H 2 MIDA and H 2 IDA are comparable in reactivity to H 3 NTA, while their anions are much less reactive than the NTA species of the same charge. The good thermal stability of aqueous NTA commends it as a reagent for boiler servicing and for decontamination of water-cooled nuclear reactors. A potentiometric method for the estimation of mono-, di-, and tribasic aminoacids in aqueous mixtures of these is described.