Complexation titrations

Examples of titration curves for (a) a complexation titration, (b) a redox titration, and (c) a precipitation titration. 

The utility of complexation titrations improved following the introduction by Schwarzenbach, in 1945, of aminocarboxylic acids as multidentate ligands capable of forming stable 1 1 complexes with metal ions. The most widely used of these new ligands was ethylenediaminetetraacetic acid, EDTA, which forms strong 1 1 complexes with many metal ions. The first use of EDTA as a titrant occurred in... 

A second ligand in a complexation titration that initially binds with the analyte but is displaced by the titrant. 

The equivalence point of a complexation titration occurs when stoichiometri-cally equivalent amounts of analyte and titrant have reacted. For titrations involving metal ions and EDTA, the equivalence point occurs when Cm and Cedxa are equal and may be located visually by looking for the titration curve s inflection point. 

Finding the End Point with a Visual Indicator Most indicators for complexation titrations are organic dyes that form stable complexes with metal ions. These dyes are known as metallochromic indicators. To function as an indicator for an EDTA titration, the metal-indicator complex must possess a color different from that of the uncomplexed indicator. Furthermore, the formation constant for the metal-indicator complex must be less favorable than that for the metal-EDTA complex. 

A -visual indicator used to signal the end point in a complexation titration. 

Selection and Standardization of Titrants EDTA is a versatile titrant that can be used for the analysis of virtually all metal ions. Although EDTA is the most commonly employed titrant for complexation titrations involving metal ions, it cannot be used for the direct analysis of anions or neutral ligands. In the latter case, standard solutions of Ag+ or Hg + are used as the titrant. 

An alloy of chromel containing Ni, Fe, and Cr was analyzed by a complexation titration using EDTA as the titrant. A 0.7176-g sample of the alloy was dissolved in ITNOa and diluted to 250 mb in a volumetric flask. A 50.00-mb aliquot of the sample, treated with pyrophosphate to mask the Fe and Cr, required 26.14 mb of 0.05831 M EDTA to reach the murexide end point. A second 50.00-mb aliquot was treated with hexamethylenetetramine to mask the Cr. Titrating with 0.05831 M EDTA required 35.43 mb to reach the murexide end point. Einally, a third 50.00-mb aliquot was treated with 50.00 mb of 0.05831 M EDTA, and back titrated to the murexide end point with 6.21 mb of 0.06316 M Cu +. Report the weight percents of Ni, fe, and Cr in the alloy. 

Spectrophotometric titration curve for the complexation titration of a mixture. 

To evaluate a redox titration we must know the shape of its titration curve. In an acid-base titration or a complexation titration, a titration curve shows the change in concentration of H3O+ (as pH) or M"+ (as pM) as a function of the volume of titrant. For a redox titration, it is convenient to monitor electrochemical potential. 

As with acid-base and complexation titrations, redox titrations are not frequently used in modern analytical laboratories. Nevertheless, several important applications continue to find favor in environmental, pharmaceutical, and industrial laboratories. In this section we review the general application of redox titrimetry. We begin, however, with a brief discussion of selecting and characterizing redox titrants, and methods for controlling the analyte s oxidation state. 

Titrimetric methods have been developed using acid-base, complexation, redox, and precipitation reactions. Acid-base titrations use a strong acid or strong base as a titrant. The most common titrant for a complexation titration is EDTA. Because of their... 

Pyrocatechol Violet (tetraphenolictriphenylmethanesulfonic acid Na salt) [115-41-3] M 386.4, e 1.4 x 1(H at 445nm in acetate buffer pH 5.2-5.4, pKesi(i)>0 (SO3H), pK ,t(2) 9.4, pKEst(3) 13. It was recrystd from glacial acetic acid. Very hygroscopic. Indicator standard for metal complex titrations. [Mustafin et al. Zh Anal Khim 22 1808 1967.]... 

The vast majority of complexation titrations are carried out using multidentate ligands such as EDTA or similar substances as the complexone. However, there are other more simple processes which also involve complexation using monodentate or bidentate ligands and which also serve to exemplify the nature of this type of titration. This is demonstrated in the determination outlined in Section 10.44. 

For complexation titrations involving the use of EDTA, an indicator electrode can be set up by using a mercury electrode in the presence of mercury (II) EDT A complex (see Section 15.24). 

While sharp changes in pM are desirable for complexation titrations, they can be undesirable for electroless solutions. Thus an electroless solution that involves a metal ion-ligand system with the titration characteristics of curve (a) in Fig. 12 would... 

Those equilibrium processes that can be resolved explicitly are straightforwardly modelled in Excel. While it is possible to solve equilibrium problems of essentially any complexity in Excel, it is virtually impossible to develop a reasonable spreadsheet for the modelling of a complex titration. Iterative methods are generally difficult to implement in Excel. 

Coulometric titrations have been employed for redox, acid-base, precipitation, and complexation titrations of organics and inorganics in both aqueous... 

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