Chelation formation curves

The titration curves of the dendrimers in the presence of calcium ions are also shown in Fig. 6. In the presence of calcium ions, the titration curve of the G1.5 dendrimer shifted downward, suggesting chelate formation. Both titration curves of the G3.5 dendrimers in the absence and presence of calcium ions are the same, especially in the high pH region. The protonation of the G4.5 dendrimer, however, was promoted in the presence of calcium ions. These results... 

There are available also several kits for the assay of calcium, in 10 or 20 microliter samples by chelate formation colorimetrically or fluorimetrically. (Pierce Chem. Co., Rockford, 111.). These are read either with the spectrophotometer or by spectrofluorometry. In our experience, while these systems can be used for approximate results, the plot of concentration versus reading curves are rather flat and only an approximation of the values can be obtained. This may be very important late at night or at times when the atomic absorption machine is down, but if the atomic absorption instrument is available it should be used in preference to these procedures. 

The Bjerrum complex formation curves of the metal chelates are obtained by plotting n, the number of monomer units per bound metal ion as calculated from the potentiometric titrations, against pA = -log[ j, [A] being the free ligand concentration (Figure 3). 

Whatever the actual mechanism in anion-exchange distributions of rare earths between an aminopolycarboxylate-charged resin and an aqueous solution of the same (or different) aminopolycarboxylate anion, the Ln distribution curve (log Ka VS. Z) peaks at (or near) the same Z as does the second step chelate formation constant. For example in the case of -HjfEDTAl /HzfEDTA) , Minczewski et al. (1962) found the Ka a maximum at about Z = 63 (Eu), corresponding rather well with the K2 value maximum observed by Brucher et al. (1975) at Z = 62 (Sm). 

In both cases n is the number of hydrogen ions displaced in the formation of the complex. In solutions where the ratio of free chelating agent to complex, ], is held constant, the slopes of the curves pM vs pH are equal. o n m. the region where H A is the principal form of the chelating... 

Equation (23-46), however, is less convenient than (23-32) because a varies with [H ]. This variation causes the extraction curves to be distorted from the symmetrical shapes shown in Figure 23-11, in accordance with the magnitudes of the formation constants of the lower chelates. 

Figure 17 shows zinc ions exchange-adsorbed and hydrogen ions released versus final pH for SA calcined at 750°. Inspection of the curves of Fig. 17 reveals that the ratio of hydrogen ion released to zinc ion adsorbed is about 2.5, indicating the formation of the two kinds of chelates shown in Fig. 16. SA calcined at 1000° behaved in a different way, as shown in Fig. 18.

We see that only complexes with formation constants of the order of 106 M-1 or more will lead to titration curves with a sufficiently steep change in pL near the equivalence point (at CM VM / CL VL = 1) to be useful for volumetric analysis. None of the common monodentate ligands, such as the halide anions (Cl-, Br , I-) or the pseudohalides (CN , SCN-, N3 ), form such strong complexes, nor do the carboxylic acid anions (such as acetate) or ammonia (NH3). However, in section 5.2 we will encounter special ligands, the chelates, that do form sufficiently strong 1 1 complexes. 

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