Complexation studies of 5,5'-dinitro BAPTA and Mag-quin-1, with the divalent cations of cadmium, lead, zinc, copper, nickel, and calciu

The protonation constants of 1,2-bis[2-[bis[carboxymethyl]amino]-5-nitrophenoxy]ethane - (5,5'-dinitro-BAPTA, DNB) and 6-Methoxy-2-methyl-8-aminoquinoline- N,N-diacetic acid - (Mag-quin-1, MQ1) and their complex formation constants with the divalent cations of lead, cadmium, copper, zinc, nickel, and calcium, have been determined in aqueous solution at 25.0°C. Values of these constants were determined using potentiometric and spectrophotometric titration techniques.;For DNB the log values for the first three stepwise, mixed-mode protonation constants were: log KH1 = 5.36 +/- 0.01, log KH2 = 4.51 +/- 0.01, and KH3 = 3.30 +/- 0.01. Titrations of DNB with Cd2+, Pb2+, Cu2+, Zn2+, Co, Ni2+, and Ca2+ at fixed values of pH produced changes in the UV-vis spectra which were analyzed to give conditional complex formation constants, K' 'MDNB. Data from potentiometric pH titrations of solutions containing DNB and metal salt were analyzed to determine complex formation constants as well. Log values for the 1:1 metal-DNB complex formation constants with these ions are: Cd2+ = 8.68; Pb2+ = 7.35; Cu2+ = 7.24; Zn2+ = 6.54; CO 2+ = 4.60; Ni2+, = 3.53; and Ca2+ = 2.75. Evidence for the formation of a 2:1 copper to DNB complex was apparent in titration data from both methods. All DNB experiments were performed at I = 0.1 M (KNO3). The stability constants with these metals indicate a retention of selectivity pattern with a lowered complexation strength in comparison to the parent compound BAPTAA(5,5' = H) of about 104.;Log values for the first three stepwise, mixed-mode protonation constants of MQ1 are: log KH1 = 7.57 +/- 0.01, log KH2 = 3.73 +/- 0.01, and KH3 = 2.41 +/- 0.02 at I = 0.05 (TEAP). Log values for the 1:1 metal to MQ1 complex formation constants of MQ1 with the indicated metal ions at I = 0.05 (TEAP) are: Cd = 7.98; Pb = 9.38; Cu = 13.8; Zn = 9.61; Ni = 11.5; Ca = 3.46. The formation of a 1:2 copper to MQ1 complex was indicated. These values indicate a retention of affinity for smaller metal ions with decreased affinity for the larger ions Ca2+ , Cd2+, and Pb2+ in comparison to quin-2, the parent compound.