The increasing use of chelating reagent in analytical chemistry has created a demand for reliable values of equilibrium constants. Modern chelating ageants (TTHA: tetraethylentetraaminehexaacetic acid, TPHA : tetraethylenepentaamineheptaaceticacid) often contain a large number of coordination center, which means that in addition to normal chelates, they may form also mono- and diprotonated uni- nuclear and binuclear chelates. 1) pM and pH measurement (Stability constants) Ringbom's new method for the calculation of the stability constant of their chelate complexes by the measurement of pH and pM values has been reviewed. The method, which is based on a consistent use of side-reaction coefficient, is applicable to systems containing a large number of complexes. For example, the stability constants of mononuclear, binuclear and tinuclear silver complexes of TTHA are calculated. The constant K^{M, L}_MLL is given by eq. (1).
log K^{M, L}_ML=pM_0.5+logα_L(H)-logα_{ML(H, OH)}(1)
The last term of eq. (1), log ML(H, OH), is zero if no acid or basic complex is formed by ML. The values of log KK^{M, L}_(ML)L are given in the forth column of Table 1 and plotted as a function of pH in Fig. 1.The formation of mixed complexes by metal ion and two competing ligand has been atracted interest in recent years. It was found that the 2: 1 HgTTHA chelate formed reacted with the buffer to form complexes of the type Hg₂XA₂hich X denotes the anion of TTHA and A-the secondary ligand (ammonia, irnidazole, urotropin). A method for the determination of the stability constants of mixed ligand complexes of metals is described. It is also interesting that the ion selective electrodes working as indicator electrode and lead (IV) acetate have been used in the TTHA titration. 2) Determination of a composition and structures of the chelates with TTHA or TPHA. On the basis of nuclear magnetic resonance (n.m.r.) spectroscopy, plausible solution structures are proposed. Resolution of structure of metal-chelates is the most important thing for analytical chemist. The preferable protonation sites determined in TPHA, TTHA, and metal-TPHA chelates by means of n.m.r. spectroscopy are summarized (Fig. 5 and 6). The possible instantaneous configurations (at various pH values) of the metal-TPHA complexes are also proposed (Fig. 7). For TTHAthe chemical shifts of the various proton are calculated by eq. (l4)-(2l). By the development in the esr theory for the determination of the distance between the copper ion in polynuclear species, two major structural possibilities presented by the binding sites of TTHA chelates are shown by the structures Fig. 8.a and 8.b (Cu-Cu bond length-5.5 Å). An interesting series of synthetic chelating reagents that illustrated as the analogs of NTA and EDTA in the behaviour is listed in Table 7. Inspection of these stability data shows that the stabilities increase up to the point (DTPA for Ni (II), Cu (II), Zn (II)) and decrease slightly (TTHA and TPHA), according to the number of donor group and negative charges of the ligands increase. This behavior is characteristic of the increasing the number of coordinate bonds formed, which reaches a limit, when the number of donor groups matches the characteristic coordination number of the metal ion. Cu (II), has the lower coordination numbers (i.e., 6), while the coordination numbers of Th (IV) are believed to be much high (i.e., 8). The fact is indicated that the increasing number of donor groups of the ligand beyond 6 greatly increases the stabilities of the chelates formed. .The stability constant of the 1: 1 metal (II)-TPHA normal complex is lower than the corresponding constants of the EDTA, DTPA, TTHA chelates by 3-4 logarithmic units. The following reasons are proposed:

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The standard electrode potentials of R^-/R⁺ redox couples may be estimated from polarographic reversible half-wave potentials of R/R⁺ and R^-/R couples. When R⁺ and R^- are large ions, similar in size and structure, the standard potential may be assumed as a solvent-independent reference point of electrode potential in various solvents. A few possible candidates for the "reference redox couple" have been discussed.

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