Metallochromism

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 partial list of metallochromic indicators, and the metal ions and pH conditions for which they are useful, is given in Table 9.16. Even when a suitable indicator does not exist, it is often possible to conduct an EDTA titration by introducing a small amount of a secondary metal-EDTA complex, provided that the secondary metal ion forms a stronger complex with the indicator and a weaker complex with EDTA than the analyte. For example, calmagite can be used in the determination of... 

These results imply that at the specified pH the magnesium complex is appreciably dissociated, whereas the lead complex is stable, and clearly titration of an Mg( II) solution with EDTA at this pH will be unsatisfactory, but titration of the lead solution under the same conditions will be quite feasible. In practice, for a metal ion to be titrated with EDTA at a stipulated pH the value of log K H should be greater than 8 when a metallochromic indicator is used. 

The visual metallochromic indicators discussed above form by far the most important group of indicators for EDTA titrations and the operations subsequently described will be confined to the use of indicators of this type nevertheless there are certain other substances which can be used as indicators.11... 

Some examples of metal ion indicators. Numerous compounds have been proposed for use as pM indicators a selected few of these will be described. Where applicable, Colour Index (C.I.) references are given.12 It has been pointed out by West,11 that apart from a few miscellaneous compounds, the important visual metallochromic indicators fall into three main groups (a) hydroxyazo compounds (b) phenolic compounds and hydroxy-substituted triphenylmethane compounds (c) compounds containing an aminomethyldicarboxymethyl group many of these are also triphenylmethane compounds. 

Erbium, tris(2,2,6,6-tetrametbyl-3,5-hep tanedione) structure, 1, 65, 66 Erbium complexes acetylacetone, 2, 374 Erhium(ill) complexes glycolic acid, 2, 472 Eriochalcite, 6, 855 Eriochrome black T metallochromic indicators, 1, 555 Eriochrome blue black R metallochromic indicators, I, 556 Eriochrome cyanine R metallochromic indicator, 1, 556 Erythrocruorin, 6, 689 dioxygen transport, 6,683 stability... 

Naphthol, l-(2-pyridylazo)-hexaamminecobalt(III) complex electron recording system, 6, 127 metallochromic indicators, 1, 546, 556 Naphthols metal complexes color photography, 6, 107... 

Pyrocatechol dioxygenases. 6, 325 Pyrocatechol violet metallochromic indicator, 1,556 Pyrogallol... 

Solar energy, 6, 488 surface modified electrodes, 6, 30 Sol-Gel process fast reactor fuel, 6, 924 Solid state reactions, 1, 463-471 fraction of reaction, 1, 464 geometric, 1, 464 growth, 1, 464 nucleation, 1, 464 rate laws, 1,464 Solochrome black T metallochromic indicators, 1,555 Solubility... 

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

Shaw, C.F. Ill, Laib, J.E., Savas, M.M. and Petering, D.H. (1990) Biphasickinetics of aurothionein formation from gold sodium thiomalate a novel metallochromic technique to probe zinc (2 + ) and cadmium(2 +) displacement from metallothionein. Inorganic Chemistry, 29, 403-408. 

---