Complex formation titrations

The main idea of research is application of accessible, simple and express methods that don t need expensive reagent techniques for analysis of phanuaceutical products based on bischofite. The determination of metal ions such as Mg, Zn, Cu, Fe by complex-formation titrations using a widely applicable chelating agent, EDTA, have been studied as a function of pH, complexing agents and indicators. The analysis consists of four parts ... 

Ethylenediaminetetra-acetic acid, largely as the disodium salt of EDTA, is a very important reagent for complex formation titrations and has become one of the most important reagents used in titrimetric analysis. Equivalence point detection by the use of metal-ion indicators has greatly enhanced its value in titrimetry. 

In principle, any type of titration can be carried out conductometrically provided that during the titration a substantial change in conductance takes place before and/or after the equivalence point. This condition can be easily fulfilled in acid-base, precipitation and complex-formation titrations and also the corresponding displacement titrations, e.g., a salt of a weak acid reacting with a strong acid or a metal in a fairly stable complex reacting with an anion to yield a very stable complex. However, for redox titrations such a condition is rarely met. 

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

Gran plots for other types of titrations. Gran64 gave the equations for dibasic acid titration and for precipication, complex-formation and redox titrations especially for the precipitation and complex-formation titrations the equations are complicated. 

Of much greater importance, however, is the amperometric method for precipitation and complex-formation titrations. Here the advantages are as follows ... 

Figure 12 [115] shows a series of complex formation titration curves, each of which represents a metal ion-ligand reaction that has an overall equilibrium constant of 1020. Curve A is associated with a reaction in which Mz+ with a coordination number of 4 reacts with a tetradentate ligand to form an ML type complex. Curve B relates to a reaction in which Mz+ reacts with bidentate ligands in two steps, first to give ML complexes, and finally close to 100% ML2 complexes in the final stages of the titration. The formation constant for the first step is 1012, and for the second 108. Curve C refers to a unidentate ligand that forms a series of complexes, ML, ML2. .. as the titration proceeds, until ultimately virtually 100% of Mz+ is in the ML4 complex form. The successive formation constants are 108 for ML, 106 for ML2, 104 for ML3, and 102 for ML4 complexes. 

Freese, F., and G. den Boef Amperometric Complex-Formation Titration of Traces of Cations. Talanta 13, 865 (1966). 

Complex-formation titration — A method based on the -> titration between a metal ion and electron-pair donor species to form coordination compounds named -> complexes. The donor species is usually called complexing agent or ligand and it can have one or several pairs of unshared electrons available to bond a metal species [i]. 

Fig. 9 Curves for complex formation titrations. Titration of the tetradentate ligand D (curve A), bidentate ligand (curve B), and unidentate ligand (curve C).

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. 

Inorganic complexing reagents such as Hg(N03)2, AgNOs, NiS04 and KCN can be used for complex-formation titrations. Mercury(II) ion forms neutral complexes with most of the anions that precipitate with silver nitrate such as Br, CD, SCN, CN and thiourea. 

Fig. 12 illustrates the titration of sodium chloride with silver nitrate. After all chloride is precipitated, the addition of excess silver nitrate causes a rapid increase in conductivity. The slope of the initial portion of the curve may be either downward or upward, depending on the relative conductance of the ion being determined and the ion of like charge in the reagent that replaces it. Slow reactions and coprecipitation are sources of difficulty with precipitation and complex-formation titrations. 

Potentiometric Titration Potentiometry may be used to follow a titration and to determine its end point. The principles have already been discussed in connection with acid-base or complex formation titrations where pH or pMe is used as a variable. Any potentiometric electrode may serve as an indicator electrode, which indicates either a reactant or a reaction product. Usually the measured potential will vary during the course of the reaction and the end point will be characterized by a jump in the curve of voltage versus amount of reactant added. 

The titration curve of iron(II) with cerium(IV) appears as A in Figure 19-3. This plot resembles closely the curves encountered in neutralization, precipitation, and complex-formation titrations, with the equivalence point being signaled by a rapid change in the ordinate function. A titration involving 0.00500 M iron(II) and 0.01000 M cerium(IV) yields a curve that for all practical purposes is identical to the one we have derived, since the electrode potential of the system is independent of dilution. A spreadsheet to calculate iisysiem a function of the volume of Ce(lV) added is shown in Figure 19-4. 

EDTA An abbreviation of ethylenediaminetetraacetic acid, a chelating agent widely used for complex formation titrations. Its formula is (HOOCCH2)2NCH2CH2N(CH2COOH)2. 

Essential water Water in a solid that exists in a fixed amount, either within the molecular structure (water of constitution) or within the crystalline structure (water of crystalli-atioii). Ethylenediaminetetraacetic acid Probably the most versatile reagent for complex formation titrations forms chelates with most cations. See EDTA. 

Precipitation and Complex-Formation Titrations A variety of coulometric titrations involving anodi-cally generated silver ions have been developed (sec Table 24-1). A cell, such as that shown in Figure 24-9, can be used with a generator electrode constructed from a length of heavy silver wire. End points are detected potentiometrically or with chemical indicators. Similar analyses, based on the generation of mer-cury(l) ion at a mercury anode, have been described. 

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