Some Metal Ion Indicators

The principal metal ion indicators are derived from several chemical families. 

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Table 5.9 Some metal ion indicators for EDTA titrations...

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. 

Some ligands contain more than one atom that can function as an electron pair donor. For example, SCN is known to bond to some metal ions through the nitrogen atom but to others through the sulfur atom. In some instances, this situation is indicated in the name as thiocyanato-N- and... 

The now familiar alternatives of visual and potentiometric detection are available. A number of organic dyes form coloured chelates with many metal ions. These coloured chelates are often discernible to the eye at concentrations of 10 6-10 7 mol dm 3 and can function as visual indicators. Most metal ion indicators will also undergo parallel reactions with protons bringing about similar colour changes. Hence, a careful consideration of pH is prudent when selecting an indicator. Some typical indicators appear in Table 5.9. Of these, eriochrome black T, which forms red complexes with over twenty metal ions, is amongst the most widely used. Its behaviour will serve as a general example of indicator function. 

As can be seen in Figure 1, the shape of the titration curve of a solution of the acid H4L differs from that obtained in presence of an equal quantity of a metal ion, indicating that some reactions take place between edta and the cation. Let us consider the case of three different metal ions Li+, Mg2+ and Cu2+, for which the stability constant A, = [ML]/([M][L]) is equal to 1028, 108-7 and 10l8 > respectively (/ = 0.1 M (KC1) and 20 °C). 

The use of metal-ion indicators to indicate the end-point of complexometric titrations is based on a specific colour change. Some individuals may find it difficult to detect a particular colour change (e.g. those with colour blindness). Alternative approaches for end-point detection are available based on a colorimeter/spectrophotometer (devices for measuring colour, see Chapter 26) or electrochemical detection (see Chapter 34). 

An important factor in the application of EDTA titration methods has been the development of suitable metal ion indicators, which permit visual titrations to be carried out in dilute solutions. A metal ion indicator is usually a dyestuff that forms metal ion complexes of a color different from that of the uncomplexed indicator. The complex forms over some characteristic range of values of pM, exactly as an add-base indicator forms a hydrogen ion complex over a characteristic range of pH... 

The value of P is usually taken as constant with potential, a fair assumption for slow reactions over a wide range of potentials or overpotentials. With fast reactions, however, such as some metal ion discharge reactions, experimental evidence indicates that P is constant only at low overpotentials (66). 

Some of the metal ion indicators used for calcium and magnesium are discussed below Solochrome dark blue or calcon... 

Sonication of 68 or 69 in the presence of transition metal ions produced coordi-natively polymerized bilayer membranes (CPBMs) as schematically illustrated in 70 26.27.130 Upon complexation with the transition metal ions, stability of bilayer membranes was remarkably improved. Amphiphiles 68 and 69 form complexes with transition metal ions with 1 1 stoichiometry. The pronounced enhancement of the stability of bilayer membranes suggests that each metal ion of the resulting CPBM is bound to two nitrogen atoms of the two adjacent azobenzene moieties as indicated by 70. Some metal ions of the CPBMs may be attached to one azobenzene unit. Even if metal ions are singly attached to the azo ligand, a polymeric cluster of metal ions bound to the dihydroxyazobenzene ligand is obtained upon sonication of the amphiphile with transition metal ions. 

As a result, less barium ion is required to complete the precipitation of the sulfate ion, and the calculated results are low. Some metal ions will complex the indicator and interfere. Foreign anions may coprecipitate as the barium salts to cause high results. Errors from chloride, bromide, and perchlorate are small, but nitrate causes large errors and must be absent. 

USE OF OXIDATION OF PHENOLS WITH HYDROGEN PEROXIDE AS AN INDICATOR REACTION FOR DETERMINING SOME METAL IONS... 

For some metal ions, there is no satisfactory indicator, but a displacement titration is feasible. In this procedure, analyte is usually treated with excess Mg(EDTA) to displace Mg, which is later titrated with standard EDTA. 

Polymers are employed frequently as particle stabilizers in the chemical synthesis of colloidal particles (20). Interactions between the small metal particles and the macromolecules at the early stages of growth prevent precipitation processes. The appearance of the characteristic surface plasmon resonance of small Au crystallites in the spectra of Figure 1 indicates that Au(III) complexes are reduced to the elemental state by the PEG polymers. Thus, poly(ethylene glycol)s and poly(vinyl alcohol)s can reduce some metal ions to yield metal particles, and also act as stabilizing agents. 

Through this example, we see that the ay coefficients may considerably change with the pH value. Eor example, at pH = 2.3, log ay = 12, and at pH = 12, log ay = 0. Diagrams of this sort, but fitting some other basic ligands such as oxalate, acetate, fluoride, etc., have been drawn definitively. A particularly important case of such ligands for chemical analysis is that of metal ion indicators (see Sect. 26.7). 

The oxidation of alcohols in iron(n)-hydrogen peroxide systems has been re-examined, and the effects of copper(ii) ions described. A new kinetic analysis has been developed, and the nature of the interactions of the radicals produced on reaction of the peroxide with the metal ion indicates that, in the reaction with isopropyl alcohol, )S-hydroxyalkyl radicals are not oxidized by iron(m) but dimerize, whereas in the presence of copper(n) they are oxidized to glycols. It is suggested that oxidation of radicals by iron(m) involves an electron-transfer process, which may in some cases be reversible,... 

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