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The acidity constants of MNPPAP1 were determined by potentiometric pH titration. The stability constants of the 1:1 complexes formed between M2+: Ca2+, Mg2+, Mn2+, Co2+, Ni2+, Cu2+ or Zn2+ and MNPPAP2-, were determined by potentiometric pH titration in aqueous solution (I =0.1 M, NaNO3, 25℃). The order of the stability constants was reported. It is shown that the stability of the binary M(MNPPAP) complexes is determined by the basicity of the carboxyl or amino group. All the stability constants reported in this work show the usual trend. The obtained order is Ca2+ < Mg2+ > Mn2+ < Co2+ < Ni2+ < Cu2+ > Zn2+. The observed stability order for MNPPAP follows approximately the Irving-Williams sequence. It is shown that regarding to M ion-binding properties vital differences on complex bilding were considered. It is demonstrated, that in M-MNPPAP complexes, M ion is coordinated to the carboxyl group, M ion is also able to bild macrochelate over pyrimidine group. The up mentioned results demonstrate that for M-MNPPAP complex the stability constants is also largely determined by the affinity of M2+ for carboxyl group. It is shown that MNPPAP can exert a direct influence on reaction mechanism through different kinds of metal ions and donor groups of MNPPAP.