Complex forming agents

Fi vre 17-15. Glyoxal. Low molecular aldehydes may form hydrates that act as complex forming agents. 

For polynuclear clusters similar reactions were not obtained These reactions proceed in the forward direction only at high temperatures ( 120 °C) and in the presence of an excess of the bidentate complex-forming agent, whereas the reverse reactions (20-22) also take place at room temperature in the presence of excess HC1 [11,46,49],... 

Chelating agents complex-forming agents having the ability to solubilize heavy metals. 

Fractional crystallization and precipitation are classical methods of separation of rare earth metal ions. Complex forming agents may be used to give better separations than simple or double salts. Some of the complexing agents used in fractionation separation are given below. 

K6nya, J., and N. M. Nagy. 1998. The effect of complex-forming agent (EDTA) on the exchange of manganese ions on calcium-montmorillonite. I. Reaction scheme and Ca-montmorillonite/Na2EDTA system. Coll. Surf. 136 297-308. 

Nagy, N. M., J. Konya, and T. Budai. 1998. Mn2+/54Mn2+ Heterogeneous isotope exchange reaction on montmorillonite in the presence of complex-forming agents. Coll. Surf. 138 81-89. 

The two different species of sorbed lead ions show different desorption properties. The desorption behavior in solutions of complex-forming agents will be discussed in Section 2.8.4. 

The effect of a complex-forming agent on the cation-exchange processes of montmorillonite is well demonstrated in calcium-montmorillonite, manganese(II) ion, and the sodium salt of the ethylene diamine tetraacetic acid (EDTA) system (K6nya and Nagy 1998 Konya et al. 1998). The reactions are illustrated in Figure 2.9. 

Iron(III) ion also appears in Figure 2.9. Its source is the montmorillonite (Box 4), the crystal lattice of which is destroyed under the influence of acidic media and the complex-forming agent (Section 2.7.2). It will be discussed later in detail (Section 2.8.1). 

FIGURE 2.11 The ratio of cCa aCa can be plotted as a function of the concentration of Ca2+ on montmorillonite (the exchange isotherm of a calcium-hydrogen-sodium ion exchange) with and without the EDTA complex-forming agent. (Reprinted from Konya and Nagy 1998, with permission from Elsevier.)... 

By means of Equation 2.54, knowing the pH, the total concentration of the complex-forming agent, its pH-dependent a parameter, and the stability constants of its complexes, the ratio of the free cation and the total metal ion concentration... 

FIGURE 2.13 The equivalent fractions of manganese(II) (XMn), calcium (XCa), hydrogen (XH), sodium ions (XNa), and their sum (X total) in the interlayer space of montmorillonite as a function of pH of the solution. The ratio of the total concentrations of EDTA, calcium, and manganese ions is 1 1 1. The equivalent fractions of calcium and hydrogen ions as a function of pH without the complex-forming agent is also shown. 

The sorption of calcium ions is as expected at low pH values, the calcium ions, present as Ca2+, sorb, as in the absence of a complex-forming agent the quantity of the sorbed calcium increases when the pH increases. At pH values where negative Ca-EDTA complexes are already formed and the concentration of hydrated Ca2+ ions decreases, the sorption of calcium on the surface of montmorillonite also decreases. 

As seen in Table 2.10, the stability constants of the tartarate and citrate complexes of lead and calcium ions are much smaller than those of EDTA and DTPA complexes. The calculations show that the dominant species of cations are Pb2+ and Ca2+ at pH < 4, and so the same ion exchange can be expected as would happen without complex-forming agents. At pH > 4, the effect of citric acid is significantly higher than expected from stability constants. The structure of citrate... 

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