Examples metal/ligand titration

Spectrophotometric titrations are particularly useful for the analysis of mixtures if a suitable difference in absorbance exists between the analytes and products, or titrant. Eor example, the analysis of a two-component mixture can be accomplished if there is a difference between the absorbance of the two metal-ligand complexes (Eigure 9.33). 

Another area where titration calorimetry has found intensive application, and where the importance of heat flow versus isoperibol calorimetry has been growing, is the energetics of metal-ligand complexation. Morss, Nash, and Ensor [225], for example, used potenciometric titrations and heat flow isothermal titration calorimetry to study the complexation of UO "1" and trivalent lanthanide cations by tetrahydrofuran-2,3,4,5-tetracarboxylic acid (THFTCA), in aqueous solution. Their general goal was to investigate the potential application of THFTCA for actinide and lanthanide separation, and nuclear fuels processing. The obtained results (table 11.1) indicated that the 1 1 complexes formed in the reaction (M = La, Nd, Eu, Dy, andTm)... 

In some complex-formation titrations, the endpoint is noted by the formation or disappearance of a solid phase. For example, in the titration of cyanide with silver ion, the solution remains clear, but the first excess of silver causes formation of a white solid that marks the endpoint. The electron-donor groups of most common ligands tend to combine not only with metallic ions but also with protons thus, the equivalence point in a complex-formation titration is often accompanied by a marked change in pH, which can be detected with an acid-base indicator. 

We have seen how the pH and the presence of several complexing agents can be taken into account in equilibrium calculations. If, as happens often, we can identify one reaction in the array of reactions as the principal reaction, then all the others can be properly be called side reactions, and treated in a convenient manner. For example, in complexometric titrations of metal ions with EDTA or some other polydentate chelating titrant, the presence of auxiliary ligands like NHj, citrate anion, etc., can best be accounted for by the use of the conditional constant, first introduced by Schwarzenbach and widely applied by Ringbom. 

Pyridylazo-ligands (Scheme 2) have widely been used in colorimetric analyses of various metal ions. For example, l-(2-pyridylazo)-2-naphthol (Hpan) is one of the most well-known reagents for the colorimetric determination and complexometric titration of... 

Complex stability constants are often determined by pH-potentiometric titration of the ligand in the presence and absence of the metal ion (129). This method works well when equilibrium is reached rapidly (within few minutes), which is usually the case for linear ligands. For macrocyclic compounds, such as DOTA and its derivatives, complex formation is slow, especially at pH-s where the formation is not yet complete, therefore a batch method is used instead of direct titration (130,131). A few representative examples of stability constant data mainly collected from Ref. (132), on MRI relevant Gdm complexes are presented in Table IV. 

Example 2.4 Shift in the Alkali metric Titration Curve of an Oxide in the Presence of an Adsorbable Metal Ion or Ligand... 

Effect of ligands and metal ions on surface protonation of a hydrous oxide. Specific Adsorption of cations and anions is accompanied by a displacement of alkalimetric and acidimetric titration curve (see Figs. 2.10 and 3.5). This reflects a change in surface protonation as a consequence of adsorption. This is illustrated by two examples ... 

It is possible to resolve complex equilibria in Excel. One feasible way of setting up a spreadsheet is represented in the example of Figure 3-14. It computes the equilibrium concentrations in a titration of a solution of a metal M with a ligand L. Two complexes are formed ML and ML2. It is a 2-component 4-species system. 

The following example demonstrates the implementation of known spectra and non-absorbing species into the algorithms. It is an aqueous spectrophotometric titration, investigating the complexation of a metal M by a ligand L to form the complex ML. The ligand also acts as a diprotic base and, additionally, the autoprotolysis of the solvent water needs to be taken into account. The complete model is ... 

For example, EDTA can be used to determine the percentage of nickel present in a nickel salt by complexometric titration. EDTA is a hexadentate ligand that binds in a 1 1 ratio with most metal ions, such as Ni +, forming a stable octahedral complex as shown in the diagram. 

The assembly formation of heterobidentate ligated complexes was studied in detail using bidentate ligand 13"b as a typical example (Figure 8.8). UV/Vis titrations and NMR spectroscopy experiments showed that the pyridyl moiety of b selectively coordinates to thezinc(ii)porphyrinl3withabindingconstantofK(ib) = 3.8x lO m. An increase in the Kfor 13 b to 64.5 X lO was observed in the presence of metal... 

This is a special case where the replacement of groups already coordinated to a cation by a new ligand is a slow process. In the case of chromium(III) this can be attributed to losses of ligand-field stabilization energy whether a five- or a seven-coordinate reaction intermediate is involved and the classical example is the slow reaction with EDTA at room temperature. This makes it possible to titrate a mixture with other metals (e.g. Fem) which react quickly without interference from Crm. 

Complexometric titrations are mainly used to determine the concentration of cations in solution. The method is based on the competition between a metal ion (for example) and two ligands, one of which acts as an indicator and the other is a component of a standard solution. 

In simple models a combination of different ligands (typically two to five) is used each ligand is described in terms of a concentration, a metal-ion complexation constant, and an acidity constant. An example of this modeling approach is given in Table 6.6. This model fits best the titration data shown in Figure 6.17. 

Some metal hydride values have been determined (Table the values are markedly ligand dependent, as seen for example, by replacement of one CO of HCo(CO)4, a strong acid, by PPhj, which decreases the acidity of 7 pK units However, the values, which must be a measure of the polarity of the M—H bond under a certain set of conditions (those of the titration procedures), have proved of little value in predicting the chemistry of the metal hydrides, e.g., whether behavior is more typical of Co "( H), Co ( H), or Co (H). This is critical in catalysis, particularly in the direction of addition of the M—H bond across olefinic groups (i.e., in olefin insertion), which is important in hydrogenation, hydroformylation, and isomerization . This is a complex question and, as well as electronic factors, steric factors, solvent polarity, the presence of radical initiators, and even temperature changes, can be important. 

In this chapter, approaches to estimates of (1) the polyelectrolyte (electrostatic) effect, (2) the hydrophobicity/hydrophilicity effect, and (3) the multicoordination effect, specified for metal ion/polyelectrolyte systems are described. As weak acidic polyelectrolytes, polyacrylic acid, PAA, as well as polymethacrylic acid, PMA, are exemplified as an example of weak basic polyelectrolyte, poI y(/V-vinyI i m idazoIc), PVIm, is chosen all the chemical structures of the polymer ligands are illustrated in Figure 1. Precise poten-tiometric titration studies by the use of a glass electrode as well as respective metal ion selective electrodes have been performed for the complexation equilibrium analyses. All the equilibrium constants reported in this chapter were obtained at 298 K unless otherwise stated. 

A number of anions form slightly soluble precipitates with certain metal ions and can be titrated with the metal solutions for example, chloride can be titrated with silver ion and sulfate with barium ion. The precipitation equilibrium may be affected by pH or by the presence of complexing agents. The anion of the precipitate may be derived from a weak acid and therefore combine with protons in acid solution to cause the precipitate to dissolve. On the other hand, the metal ion may complex with a ligand (the complexing agent) to shift the equilibrium toward dissolution. Silver ion will complex with ammonia and cause silver chloride to dissolve. 

Some organic ligands, notably 1,10-phenanthroline and 2,2 -bipyridine (Table 6.7), stabilize the lower of two oxidation states of a metal. This is apparent from the values of FF for the appropriate half-reactions in Table 7.1. The observation is associated with the ability of the phen and bpy ligands to accept electrons. Iron(II) complexes of bpy and phen are used as indicators in redox reactions. For example, in a redox titration of Fe with powerful oxidizing... 

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