Metallochromism chelates

The interest in colour indicators has recently increased as they are used for the direct determination of pH (acid-base indicators) and free calcium ions (fluorescent derivatives based on the calcium chelator EGTA as metallochromic indicators) in biological systems at cellular level. 

Silica- or polymer-based materials with chelating agents grafted onto the surface, available on the market or laboratory made, or metallochromic ligands used in HPCIC have been extensively reviewed [130,212,227], Recent developments in HPIC have been reported as well [91-93,237],... 

Metal ions, especially transition metal ions, form coloured complexes with chelating ligands that are the basis of the analytical methods and indicators for these ions that have been used for many decades. " The term metallochromism has been apphed to this phenomenon, and hence metallochromic indicators. Typical bidentate ligands are dimethylglyoxime, 1,2-dihydoxybenzenes and 1-hydroxyanthraquinones, 8-... 

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. 

This test is based upon formation of barium sulfate in the presence of excess barium, followed by chelation of the remaining barium with methylthymol blue, a metallochrome indicator recently developed for complexometric titrations (13, 14, 15, 16, 17). The reagent is a solution containing equimolar amounts of barium chloride and methylthymol blue at a concentration equivalent to the largest amount of sulfate ion anticipated in the test solution 18). 

The most important metallochromic indicators are probably those derived from familiar acid-base indicators of the phenolphthalein or sulfonphthalein series. Although themselves unable to form chelate complexes, this facility can be incorporated by introducing a grouping such as -OH, —COzH, -CH2NHCH2CO2H or —CH2N(CH2C02H) ortho to the phenol group. 

Methylthymol Blue (3,3 -bis-(N,N-dicarboxymethyl)aminomethylsulfo-naphthalene), a metallochromic indicator, shows a yellow color and forms a deep blue chelate with scandium in acid conditions. At pH 1.5, this color reaction is selective. ... 

A number of valuable metallochromic reagents are based on the triphenylmethane structure. A typical example is Pyrocatechol violet which is an acid-base indicator, as well as forming coloured (usually blue) chelates with a wide range of metal ions. Compounds such as Xylenol orange are also based on this structure, with the addition of two iminodiacetic acid groups. 

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