Metallochromic dyes

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. 

Metallochromic dyes have been used to study the kinetics of aurothionein due to the lack of intrinsic and distinctive chromophoric changes during reaction [105]. 

The interaction of metallochromic indicators (Section 10.6.4) with metals has received a lot of attention since the resulting complexes are often solvent extractable. C. Woodward and H. Freiser73 have examined complexes of metals with sulfonated azo dyes with a view to their extraction by Aliquat-336. Vanadium in sea water can be determined at the 2 p.p.b. level by complexation with PAR (4-(2-pyridylazo)resorcinol) in the presence of CDTA and extracting the anion so formed into Zephiramine (tetradecyldimethylbenzylammonium chloride).74 The field has been reviewed up to 1971 by Ueno.2... 

Tennakone et al.%S) used triphenylmethane type (metallochromic) organic dye (Dye 21, 22), both of which show a large bathochromic shift on complexing with metal ions. The molecular orbital calculation of these dyes in chelating condition with the Tilv ion revealed that the LUMOs of these dyes are localized on the TiIV ion, but the HOMOs are delocalized in the whole dyes. Such MO distribution similar to the LMCT transition in transition metal complexes should contribute to the vectorial electron transfer (high rj ) from the excited dye to Ti02. 

A review of CE and CZE by Haddad covers metallochromic ligands, dye ligands detectable by UY-visible spectroscopy or laser fluorescence for low detection limits (PAR, BrPADAP, XO, as shown in Figure 2) in CE and CZE separations.69... 

There are now many metallochromic indicators which may be used in complexometric titrations. For successful end-point indication the pK value of the metal-dye complex should be at least 4 units less than that of the metal-EDTA complex which is formed during the titration if the difference is less than 4 units the titrant is unable to compete satisfactorily with the indicator dye and a sluggish end-point results. The indicators which are most widely used in the methods described in this book are solochrome black (for titrations carried out in ammoniacal solution), xylenol orange (for titrations carried out between pH 4 and 7) and catechol violet (for titration in mineral acid solution at pH 1 to 2). Table 63 lists the various indicators called for in methods in the monographs. Discussion of the merits of different indicators for specific purposes will be found in various parts of the book. 

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