Potentiometric titrations complex-formation

Kimura and co-workers have synthesized a series of alkoxide complexes with the alcohol functionality as a pendent arm.447 674 737 A zinc complex of l-(4-bromophenacyl)-l, 4,7,10-tetraaza-cyclododecane was also synthesized by the same workers to mimic the active site of class II aldolases. The X-ray structure shows a six-coordinate zinc center with five donors from the ligand and a water molecule bound. The ketone is bound with a Zn—O distance of 2.159(3) A (Figure 12). Potentiometric titration indicated formation of a mixture of the hydroxide and the enolate. Enolate formation was also independently carried out by reaction with sodium methoxide, allowing full characterization.738... 

The experimental results may be represented both by the titration curves or property-composition dependences. The extremums or bends on the titration curves indicate the formation of complexes and their composition. Thus, investigating the-possi-bility of complex formation in polyelectrolyte - nonionic polymer systems, one can use the methods of conductometric and potentiometric titration. The formation of interpolymer complexes in these systems, as some authors suggest18,211, is caused by a co-operative formation of hydrogen bonds between carboxy groups of the polyacid and oxygen atoms of nonionic polyvinylpyrrolidone or poly(ethylene glycol) and is therefore accompanied by an increase of pH of the solution. The typical titration curves for the system polyvinylpyrrolidone - copolymer maleic anhydride and acrylic add are shown in Fig. 1. The inflection points of the titration curves indicate the ratio at which the macromolecular components react with each other, i.e. the composition of the formed complexes. 

Interaction between osmium(IV) and osmium(VI) and derivatives of dimerkaptotiopiron (DT) has been studied by amperemetric, potentiometric and spectral photometric methods in different mediums. It has been found out that in reactions of methyldimerkaptotiopiron (R) with Os(IV) and Os(VI) complex formation prevails. It has a step-like nature, being revealed by such change of ratio of Os R in the course of titration as Os(VI) R - from 1 1 to 1 4 in acid medium and from 2 1 to 1 4 in weak alkaline medium Os(IV) R - from 1 1 to 1 5 in medium of H SO and from 1 1 to 1 6 - in HCl medium. 

In fact, any type of titration can be carried out potentiometrically provided that an indicator electrode is applied whose potential changes markedly at the equivalence point. As the potential is a selective property of both reactants (titrand and titrant), notwithstanding an appreciable influence by the titration medium [aqueous or non-aqueous, with or without an ISA (ionic strength adjuster) or pH buffer, etc.] on that property, potentiometric titration is far more important than conductometric titration. Moreover, the potentiometric method has greater applicability because it is used not only for acid-base, precipitation, complex-formation and displacement titrations, but also for redox titrations. 

In the practice of potentiometric titration there are two aspects to be dealt with first the shape of the titration curve, i.e., its qualitative aspect, and second the titration end-point, i.e., its quantitative aspect. In relation to these aspects, an answer should also be given to the questions of analogy and/or mutual differences between the potentiometric curves of the acid-base, precipitation, complex-formation and redox reactions during titration. Excellent guidance is given by the Nernst equation, while the acid-base titration may serve as a basic model. Further, for convenience we start from the following fairly approximate assumptions (1) as titrations usually take place in dilute (0.1 M) solutions we use ion concentrations in the Nernst equation, etc., instead of ion activities and (2) during titration the volume of the reaction solution is considered to remain constant. 

Fig. 2.21. Potentiometric complex-formation titration (under conditions favouring the overall reaction Cu2+ + Y4- -> CuY2-). 

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... 

Potentiometric titrations can also be used to determine stability constants for complexes in some cases, which can be useful as a substitute or complementary approach to spectrophotometric titrations (56). By starting at a low pH, where the relative amount of complex formation is low, and then measuring the pH upon addition of standardized base, one can determine the relative amounts of the species in solution or how many equivalents of H+... 

As to Eq. (7), it is to be remembered that AG, in a general case is a function of p. Therefore, the experimental dependencies of p on concentration, chain length of oligomer and temperature may be employed to find thermodynamic parameters only for a fixed value of p, e.g., for p = 0.5 using Eqs. (8 a- b). These equations have been taken by various authors to calculate the enthalpy and entropy of complex formation between simple synthetic oligomers and polymers 28). In a number of cases the correspondence between the values of complex formation enthalpy thus obtained and determined, either by calorimetry or by potentiometric titration 26), has been found satisfactory although it is obvious that in a general case these values do not necessarily coincide. 

Potentiometric titration can determine the end point more accurately than the color indicators. Thus, the quantitative consumption of a titrant in an acid-base neutralization, oxidation-reduction reaction, or complex formation reaction can be determined precisely and very accurately by potentiometric titration. The titration involves the addition of large increments of the titrant to a measured volume of the sample at the initial phase and, thereafter, adding smaller and smaller increments as the end point approaches. The cell potential is recorded... 

Binding of the components in a neutral aqueous solution was confirmed by potentiometric titrations. The feasibility of electron transfer between the components was predicted by cyclic voltammetry and an efficient outer-sphere fast electron transfer was foreseen. Fluorescence spectroscopy measurements showed that the formation of a defined donor-acceptor complex worked even in water at neutral pH. Electron transfer as the quenching mechanism was proved by laser flash photolysis. 

The kinetics of the hydrolysis of di(2,4-dinitrophenyl) phosphate (DDNPP) were studied in basic solutions buffered with Bis-Tris propane (BTP) in the presence of La3+, Sm3+, Tb3+, and Er3+. Two equivalents of the 2,4-dinitrophenolate ion were liberated for each equivalent of DDNPP and the reaction showed first-order kinetics. Potentiometric titrations showed the formation of dinuclear complexes such as [Ln2(BTP)2(OH) ](6 " i, with values of n varying as a function of pH for all studied metals. Hence the catalytic effect depends on the formation of dinuclear lanthanide ion complexes with several hydroxo ligands.97... 

Oxygen Donors. The formation and stability constants oi complexes between Pt (Pd, Rh, Ir, Os, Ru) and o-coumaric acid have been determined by pH titration.31 The results indicate that a 1 2 complex is formed with Pt. Acid-base properties of aquo-complexes formed from [Pt(X)2(OH)2(NH3)(MeNH2)] (X = Cl, Br or N02) in aqueous solutions have been examined using potentiometric titration experiments.186 The Ka of co-ordinated water was lower for (X)2 = (H20)2 than for (X)2 = (H20) (OH-). 

---