Titration, 40, Also complexometric

Variamine blue (C.I. 37255). The end point in an EDTA titration may sometimes be detected by changes in redox potential, and hence by the use of appropriate redox indicators. An excellent example is variamine blue (4-methoxy-4 -aminodiphenylamine), which may be employed in the complexometric titration of iron(III). When a mixture of iron(II) and (III) is titrated with EDTA the latter disappears first. As soon as an amount of the complexing agent equivalent to the concentration of iron(III) has been added, pFe(III) increases abruptly and consequently there is a sudden decrease in the redox potential (compare Section 2.33) the end point can therefore be detected either potentiometrically or with a redox indicator (10.91). The stability constant of the iron(III) complex FeY- (EDTA = Na2H2Y) is about 1025 and that of the iron(II) complex FeY2 - is 1014 approximate calculations show that the change of redox potential is about 600 millivolts at pH = 2 and that this will be almost independent of the concentration of iron(II) present. The jump in redox potential will also be obtained if no iron(II) salt is actually added, since the extremely minute amount of iron(II) necessary is always present in any pure iron(III) salt. 

Complexometric titration general discussion, 258, 309, 311, 322 see also EDTA Complexones 55 Computers 133... 

For analysis in solutions, the most frequently used CL reaction is alkaline oxidation of luminol and lucigenin in the presence of hydrogen peroxide as oxidant, although sodium hypochlorite, sodium perborate, or potassium ferricyanide may also be used. CL reactions involving alkaline oxidation have been used to indicate acid-base, precipitation, redox, or complexometric titration endpoints either by the appearance or the quenching of CL when an excess of titrant is present [114, 134], An example of these mechanisms is shown in Figure 14. 

The end points of precipitation titrations can be variously detected. An indicator exhibiting a pronounced colour change with the first excess of the titrant may be used. The Mohr method, involving the formation of red silver chromate with the appearance of an excess of silver ions, is an important example of this procedure, whilst the Volhard method, which uses the ferric thiocyanate colour as an indication of the presence of excess thiocyanate ions, is another. A series of indicators known as adsorption indicators have also been utilized. These consist of organic dyes such as fluorescein which are used in silver nitrate titrations. When the equivalence point is passed the excess silver ions are adsorbed on the precipitate to give a positively charged surface which attracts and adsorbs fluoresceinate ions. This adsorption is accompanied by the appearance of a red colour on the precipitate surface. Finally, the electroanalytical methods described in Chapter 6 may be used to scan the solution for metal ions. Table 5.12 includes some examples of substances determined by silver titrations and Table 5.13 some miscellaneous precipitation methods. Other examples have already been mentioned under complexometric titrations. 

The course of precipitation and complexometric titrations can also be followed conductometrically, but changes of slope are generally less pronounced. Redox reactions are difficult to follow because of the high... 

Titrations are veiy powerful techniques that contain two very different kinds of information and thus serve two different purposes (a) titrations are used for quantitative analytical applications, e.g. the determination of the concentration of an acid by an acid-base titration or the determination of a metal ion by a complexometric titration (b) titrations serve also as a method for the determination of equilibrium constants, e.g. the determination of the strength of the interaction between a metal ion and a ligand. Naturally, both objectives can be combined and the analysis of one titration can deliver both types of information. 

The Cd ISE is used primarily in complexometric titrations with EDTA and Cd as an indicator [327,386] this approach is also the basis of stability constant determination [268,268a]. 

The most important applications of Cu ISEs are in the direct determination of Cu " in water [169, 372,410], complexometric titration of various metal ions using Cu " as an indicator [30, 143,269, 385] and complexometric titrations of Cu " [409]. This ISE has also been used in the determination of the equilibrium activity of Cu in various Cu complexes in order to determine the stability constants (see [46, 285, 317, 318,427, 445]), in the determination of the solubility of poorly soluble salts [122] and in the determination of the standard Gibbs transfer energies [58]. It can also be used in concentrated electrolytes [170]. 

EDTA (ethylenediaminetetraacetic acid) (H02CCH2)2NCH2CH2N-(CH2C02H)2, the most widely used reagent for complexometric titrations. It forms 1 1 complexes with virtually all cations with a charge of 2 or more, effective formation constant Equilibrium constant for formation of a complex under a particular stated set of conditions, such as pH, ionic strength, and concentration of auxiliary complexing species. Also called conditional formation constant. 

Only slightly less accurate ( 0.3—0.5%) and more versatile in scale are other titration techniques. Plutonium may be oxidized in aqueous solution to Pu02+2 using AgO, and then reduced to Pu4+ by a known excess of Fe+2, which is back-titrated with Ce4+. Pu4+ may be titrated complexometrically with EOT A and a colorimetric indicator such as Arsenazo(I), even in the presence of a large excess of U02+2. Solution spectrophotometry (Figs. 4 and 5) can be utilized if the plutonium oxidation state is known or controlled. The spectrophotometric method is very sensitive if a colored complex such as Arsenazo(III) is used. Analytically useful absorption maxima and molar absorption coefficients ( s) are given in Table 10. Laser photoacoustic spectroscopy has been developed for both elemental analysis and speciation (oxidation state) at concentrations of 10-4 — 10-5 M (118). Chemical extraction can also be used to enhance this technique. 

The mono- or disodium salt of EDTA may also be used in complexometric titrations. The formula weights of these salts in their dehydrated form are 372.3 and 349.3, respectively. EDTA, or its sodium salt, is standardized against a standard calcium solution. The indicators that are commonly used for such titrations are organic dyes. The structures of two common indicators are as follows ... 

C.I. Mordant Black 11 is also used as an indicator in the complexometric titration of various bivalent metals with which it forms complexes [8],... 

Titration — A process for quantitative analysis in which measured increments of a - titrant are added to a solution of an - analyte until the reaction between the analyte and titrant is considered as complete at the - end point [i]. The aim of this process is to determine the amount of an analyte in a -> sample. In addition, the determination can involve the measurement of one or several physical and/or chemical properties from which a relationship between the measured parameter/s and the concentration of the analyte is established. It is also feasible to measure the amount of a - titrand that is added to react with a fixed volume of titrant. In both cases, the -> stoichiometry of the reaction must be known. Additionally, there has to be a means such as a -> titration curve or an - indicator to recognize that the -> end point has been reached. The nature of the reaction between the titrant and the analyte is commonly indicated by terms like acid-base, complexometric, redox, precipitation, etc. [ii]. Titrations can be performed by addition of measured volume/mass increments of a solution,... 

Hydrated lanthanide salts may also be obtained by the addition of an excess of lanthanide oxide to a concentrated acid solution, heating at 80°C until the pH is between 5 and 6. The residual oxide is removed by filtration, and the filtrate is subjected to rotary evaporation. This procedure may lead to the presence of oxo and hydroxy species in solution. At present hydrated lanthanide salts of 99.9% purity are available commercially. Salts of the highest purity are generally used in spectroscopic and magnetic studies. The purity of lanthanide salts can be determined by complexometric titration with ethylenediamine tetraacetic acid [1]. 

Simple inorganic ions and molecules like NH3, CN , Cl-, H20 form monodentate ligands, that is one ion or molecule occupies one of the spaces available around the central ion in the coordination sphere, but bidentate (like the dipyridyl ion), tridentate, and also tetradentate ligands are known. Complexes made of polydentate ligands are often called chelates, the name originating from the Greek word for the claw of the crab, which bites into an object like the polydentate ligand catches the central ion. The formation of chelate complexes is used extensively in quantitative chemical analysis (complexometric titrations). ... 

Oxyacids, like citric or tartaric acids, and polyols, like saccharose are also used, mainly as masking agents, in qualitative analysis. The action of some specific reagents, like oc-a -bipyridyl for iron(II) and dimethylglyoxime for nickel(II), is also based on the formation of chelate complexes. In quantitative analysis the formation of chelates is frequently utilized (complexometric titrations). ... 

A simple and rapid method for the iodometric determination of microgram amounts of chromium(ni) in organic chelates is based on the oxidation of chromium(III) with periodate at pH 3.2, removal of the umeacted periodate by masking with molybdate and subsequent iodometric determination of the liberated iodate . Iodometric titration was also used for determination of the effective isoascorbate (see 2) concentration in fermentation processes . The content of calcium ascorbate can be determined with high sensitivity by complexometric titration with edta, which is superior to iodometry. The purity of /3 -diketonate complexes of Al, Ga, In and Ni was determined by complexometric titration with edta at pH 5.5-3, with RSD < 0.01 for determining 5-30% metal ion. Good analytical results were obtained by a similar procedure for the metal content of 15 lanthanide organic complexes. ... 

Go to http //chemistry.brookscole.com/skoogfac/. From the Chapter Resources menu, choose Web Works, and locate the Chapter 17 section, where you will find links to several good educational sites that give additional help on complexation equilibria and complexometric titrations. Several Web sites describe experiments that can be done in the laboratory based on complexation methods. Find the abstract of the paper from the Journal of Chemical Education that deals with the determination of zinc by EDTA titration. Find the indicator and the buffer pH used in the titration. There is also a link to additional information on chemistry applied to aquatic systems. Compare some of the complexation equilibria described in these documents to those discussed in this chapter. 

The first group, which is developed in this chapter, use ion selective electrodes (ISE). The principle of these chemical sensors is to create an electric cell in which the analyte behaves in such a way that the potential difference obtained relates to its concentration. Measurement of pH, probably the most common and best known electroanalytical method, is part of this group. Most of the measurements concern the determination of ions in aqueous solution, though particular electrodes with selective membranes also allow the determination of molecules. The sensitivity of these methods is very great for certain ions but matrix is sometime responsible for lack of reliability in these measurements. In such cases, complexometric or titrimetric methods must replace direct potentiometry. It remains however for potentiometry multiple applications in which the instruments range from low-cost pH meters to automatic titrators. 

Ultraviolet-visible spectrophotometry has also been applied to titrimetry. In this case the variation in the absorbance of the analyte with addition of titrant is used to obtain a spectrophotometric profile from which titration end points and/or equilibrium constants, etc., can be determined. This has been applied to the whole range of titrations in which a chromophore is generated. These include acid-base, redox, and complexometric titrations. 

The complexometric titrations were performed at the natural pH values of the ground-water samples and also at pH 7.0 and 6.0. The pH was buffered using HEPES (N-2-Hydoxyethylpiperazine-N -2-hydroxyethanesulfonic acid), PIPES (piperazine-N-N -bis[2-ethanesulfonic acid], or MES (2-[N-Morpholino]ethanesulfonic acid), which have PK3 values of 7.5,... 

Another sensor, with calcein immobilized on cellulose, has been designed [79]. It is based on the observation that Co ", Cu " and Ni "> form nonfluorescent complexes at neutral pH (5-7), whereas calcein fluoresces strongly by itself. However, this sensor is not reversible in the pH zones analyzed. The addition of a metallic ion (non-quenching) such as Zn ", which displaces the metal from the fluorescent complex, has also been proposed, and also the use of immobilized calcein as a complexometric titration indicator [83]. 

Complexometric titrations require indicators that complex with metal ions and change color between the free state and the complex state. See also absorption indicator. 

The end point of acid-base titrations, complexometric titrations, or redox titrations can also be potentiometrically indexed by the use of suitable electrodes, for example, the glass electrode discussed in Sect. 22.7 in the case of acid-base reactions. The advantage of this method is that colored or cloudy solutions can also be titrated and it is simple to automate because an easily measured electric quantity participates. 

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