Metal ligand stability constants potentiometric titrations

The ligand 6,13-dimethyl-l,4,8,ll-tetra-azacyclotetradecane-6,13-diamine coordinates as a hexadentate ligand to zinc in neutral aqueous solution. Potentiometric titrations were used to determine the stability constant for formation. The pXa values were determined for five of the six possible protonation steps of the hexamine (2.9, 5.5, 6.3, 9.9 and 11.0).697 Studies of the syn and anti isomers of 6,13-dimethyl-1,4,8, ll-tetraazacyclotetradecane-6,13-diamine reveal that they offer different shapes for metal binding, which is reflected in the stability constants for 1 1 zinc ligand ratio complexes. The selectivity of binding to the zinc ion compared to the cadmium(II) ion by both isomers is significant.698... 

Complex stability constants are most often determined by pH-potentiometric titration of the ligand in the presence and absence of the metal ion.100 This method works well when equilibrium is reached rapidly (within a few minutes), which is usually the case for linear ligands. For macrocyclic compounds, such as DOTA and its derivatives, complex formation is very slow, especially for low pH values where the formation is not complete, therefore a batch method is... 

In the use of potentiometry for the evaluation of stability constants for complex ions, the expressions can become extremely complicated if multiequilibria are present. For a simple one-to-one complex a direct potentiometric titration curve again provides die most satisfactory route to an accurate evaluation of the constant. The curve looks similar to that for an acid-base titration, and the appropriate point to pick is the half-equivalence point. If the complex is extremely stable, then die amount of free metal ion at this point on die dtration curve (ligand titrated with metal ion) is sufficiently low that it can be disregarded. If not, it must be handled in a way similar to the first point on the titration curve for phosphoric acid. Assuming that it is a stable complex, at the first half-equivalence point the concentration of complexed metal ion will be equivalent to that of the free ligand. The potential will give a direct measure of the free metal ion and allow the stability constant for the complex to be evaluated at the half-equivalence point ... 

Abstract. Crown ethers derived from tartaric acid present a number of interesting features as receptor frameworks and offer a possibility of enhanced metal cation binding due to favorable electrostatic interactions. The synthesis of polycarboxylate crown ethers from tartaric acid is achieved by simple Williamson ether synthesis using thallous ethoxide or sodium hydride as base. Stability constants for the complexation of alkali metal and alkaline earth cations were determined by potentiometric titration. Complexation is dominated by electrostatic interactions but cooperative coordination of the cation by both the crown ether and a carboxylate group is essential to complex stability. Complexes are stable to pH 3 and the ligands can be used as simultaneous proton and metal ion buffers. The low extractibility of the complexes was applied in a membrane transport system which is a formal model of primary active transport. 

Stability constants have been measured for a wide range of dithiocarbamate complexes from ultraviolet (UV)-visible spectroscopy (316,392,596,597), pH (598), and potentiometric (599,600) titrations, electron spin resonance (ESR) spectroscopy (601), and HPLC (602-605). They are determined typically from exchange studies between a free metal salt and its dithiocarbamate complex (Eq. 47). Sachinidis and Grant (597) identified two pathways for ligand transfer the first involves dissociation of a dithiocarbamate followed by substimtion at the metal ion, while the second results from direct electrophilic attack by the metal ion on the dithiocarbamate complex. 

There are two methods that are considered fundamental in the use of potentiometric titrations to determine the properties of a solution. The first was developed by Bodlander and Fittig (1902). The main feature of this method was to obtain a description of the stoichiometric constant in a reaction between a metal (M) and a ligand (L). However, it is usually desirable to also obtain a value for the stability constant itself since only how the expression looks for the reaction is not sufficient. A method for doing this using potentiometric titrations was introduced by Bjerrum (1941). In this method, Bjerrum usedthe average ligand number defined by... 

For many purposes the cumulative stability constant (P) is required this is the product of the individual constants. If there are two individual constants, the product is designated P if there are three constants, P3. Stability constants are usually determined by potentiometric (glass electrode) titration of the ligand, in the presence and absence of the metal, and the results are processed by a set of rather complex calculations for which a computer programme can conveniently be used. 

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