Dyestuffs titration

In acid-base titrations the end point is generally detected by a pH-sensitive indicator. In the EDTA titration a metal ion-sensitive indicator (abbreviated, to metal indicator or metal-ion indicator) is often employed to detect changes of pM. Such indicators (which contain types of chelate groupings and generally possess resonance systems typical of dyestuffs) form complexes with specific metal ions, which differ in colour from the free indicator and produce a sudden colour change at the equivalence point. The end point of the titration can also be evaluated by other methods including potentiometric, amperometric, and spectrophotometric techniques. 

This colour change can be observed with the ions of Mg, Mn, Zn, Cd, Hg, Pb, Cu, Al, Fe, Ti, Co, Ni, and the Pt metals. To maintain the pH constant (ca 10) a buffer mixture is added, and most of the above metals must be kept in solution with the aid of a weak complexing reagent such as ammonia or tartrate. The cations of Cu, Co, Ni, Al, Fe(III), Ti(IV), and certain of the Pt metals form such stable indicator complexes that the dyestuff can no longer be liberated by adding EDTA direct titration of these ions using solochrome black as indicator is therefore impracticable, and the metallic ions are said to block the indicator. However, with Cu, Co, Ni, and Al a back-titration can be carried out, for the rate of reaction of their EDTA complexes with the indicator is extremely slow and it is possible to titrate the excess of EDTA with standard zinc or magnesium ion solution. 

The dyestuff is thoroughly mixed with 100 times its weight of sodium sulphate, and 1 g of the mixture is used in each titration. The indicator is not very stable in alkaline solution. 

Solochrome dark blue or calcon ( C.1.15705). This is sometimes referred to as eriochrome blue black RC it is in fact sodium l-(2-hydroxy-l-naphthylazo)-2-naphthol-4-sulphonate. The dyestuff has two ionisable phenolic hydrogen atoms the protons ionise stepwise with pK values of 7.4 and 13.5 respectively. An important application of the indicator is in the complexometric titration of calcium in the presence of magnesium this must be carried out at a pH of about 12.3 (obtained, for example, with a diethylamine buffer 5 mL for every 100 mL of solution) in order to avoid the interference of magnesium. Under these conditions magnesium is precipitated quantitatively as the hydroxide. The colour change is from pink to pure blue. 

Fast sulphon black F ( C.I.26990). This dyestuff is the sodium salt of 1-hydroxy-8-( 2-hydroxynaphthylazo) -2- (sulphonaphthylazo) -3,6-disulph onic acid. The colour reaction seems virtually specific for copper ions. In ammoniacal solution it forms complexes with only copper and nickel the presence of ammonia or pyridine is required for colour formation. In the direct titration of copper in ammoniacal solution the colour change at the end point is from magenta or [depending upon the concentration of copper(II) ions] pale blue to bright green. The indicator action with nickel is poor. Metal ions, such as those of Cd, Pb, Ni, Zn, Ca, and Ba, may be titrated using this indicator by the prior addition of a reasonable excess of standard copper(II) solution. 

A disadvantage of adsorption indicators is that silver halides are sensitised to the action of light by a layer of adsorbed dyestuff. For this reason, titrations should be carried out with a minimum exposure to sunlight. When using adsorption indicators, only 2 x 10-4 to 3 x 10 3 mol of dye per mol of silver halide is added this small concentration is used so that an appreciable fraction of the added indicator is actually adsorbed on the precipitate. 

Other dyestuffs have been recommended as adsorption indicators for the titration of halides and other ions. Thus cyanide ion may be titrated with standard silver nitrate solution using diphenylcarbazide as adsorption indicator (see Section 10.44) the precipitate is pale violet at the end point. A selection of adsorption indicators, their properties and uses, is given in Table 10.8. 

The presence of free bromine, and consequently the end-point, can be detected by its yellow colour, but it is better to use indicators such as methyl orange, methyl red, naphthalene black 12B, xylidine ponceau, and fuchsine. These indicators have their usual colour in acid solution, but are destroyed by the first excess of bromine. With all irreversible oxidation indicators the destruction of the indicator is often premature to a slight extent a little additional indicator is usually required near the end point. The quantity of bromate solution consumed by the indicator is exceedingly small, and the blank can be neglected for 0.02M solutions. Direct titrations with bromate solution in the presence of irreversible dyestuff indicators are usually made in hydrochloric acid solution, the concentration of which should be at least 1.5-2M. At the end of the titration some chlorine may appear by virtue of the reaction ... 

The titrations should be carried out slowly so that the indicator change, which is a time reaction, may be readily detected. If the determinations are to be executed rapidly, the volume of the bromate solution to be used must be known approximately, since ordinarily with irreversible dyestuff indicators there is no simple way of ascertaining when the end point is close at hand. With the highly coloured indicators (xylidine ponceau, fuchsine, or naphthalene black 12B), the colour fades as the end point is approached (owing to local excess of bromate) and another drop of indicator can be added. At the end point the indicator is irreversibly destroyed and the solution becomes colourless or almost so. If the fading of the indicator is confused with the equivalence point, another drop of the indicator may be added. If the indicator has faded, the additional drop will colour the solution if the end point has been reached, the additional drop of indicator will be destroyed by the slight excess of bromate present in the solution. 

The measurement of absorbance of light by a dyestuff-anionic surfactant complex, which has been extracted into an organic solvent is a key feature of many methods, and Sodergren has successfully used segmented flow colorimetry for an automated version of this procedure (2 ). An alternative is the two phase titration technique, pioneered by Herring (3) which uses dimidium... 

More than 50 such dyestuffs have been proposed as indicators,8 but they have in common at least one o-hydroxy group from which the titratable metal displaces a hydrogen atom and forms a five-membered chelate ring with the azo nitrogens (cf 51) most have two o-hydroxy groups and can form two chelate rings. Of the few that are still used calcon (eriochrome blue black R ... 

Electrochemistry finds wide application. In addition to industrial electrolytic processes, electroplating, and the manufacture and use of batteries already mentioned, the principles of electrochemistry are used in chemical analysis, e.g.. polarography, and electrometric or conductometric titrations in chemical synthesis, e.g., dyestuffs, fertilizers, plastics, insecticides in biolugy and medicine, e g., electrophoretic separation of proteins, membrane potentials in metallurgy, e.g.. corrosion prevention, eleclrorefining and in electricity, e.g., electrolytic rectifiers, electrolytic capacitors. 

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 titration end-point is registered with a suitable indicator, which plays an important role in the titration process. A variety of indicators has been described in the literature [31,36], An example of an effective indicator used in two-phase titration is a mixture of anionic dyestuff, disulfine blue VN, and cationic dyestuf, dimidium bromide, the structures of which are shown in Fig. 11-32 (b,c). 

Barbituric acid, 5,5 -nitrilodi-, monoammonium salt EINECS 221-266-6 Murexide Naples red NSC 215208 2,4,6(1H,3H,5H)-Pyrimidinetrione, 5-((hexahydro-2,4,6-trioxo-5-pyrimid-inyl)imino)-, monoammonium salt. A red basic dyestuff, now obsolete, obtained by the action of nitric acid upon guano, and subsequently treating the product with ammonia. Used as an indicator in complexometric titrations. Crystals slightly soluble in H2O, insoluble in organic solvents km = 520 nm (H2O),... 

This association equilibrium is used in the so-called two-phase titration of anionic surfactants by a cationic dyestuff in which the association compound is displaced in an oil phase and results in a coloration [89,90]. 

Detection of the end-point depends on the fact that anionic surfactants react with some cationic dyestuffs, and cationic surfactants with some anionic dyestuffs, to form salts which, while not particularly insoluble in water, can be extracted with chloroform. Over the last 50 years, many methods have been proposed which are all variants on the general idea of titrating an anionic with a cationic, or vice versa, in a chloroform-water system in the presence of an ionic dyestuff, the end-point being disclosed by the migration of colour from water to chloroform or vice versa. One of these variants [5] was developed at the behest of the Commission Internationale d Analyses, and a critical review [6] was published two years later. These two papers repay close study. 

Metal indicators are derivatives, more precisely dyestuffs, that exhibit a color change when the concentration of the metal cation they indicate changes within a certain concentration range. They form complexes with specific metal cations. In principle, they exhibit two colors depending on whether they are free or engaged in the complex with the metallic ion. They are used to detect endpoints in complexometric titrations. 

A method of assay, applicable to most medicinal dyestuffs, depends upon their reduction with titanous chloride. The procedure has to be modified in detail for dissolving the dyestuff and for some dyes the endpoint is not indicated by a sharp decolorisation and an excess of titrant must be added, the excess being back titrated with ferric ammonium sulphate. 

There are two main types of indicator. One involves a complex of a metal ion, which changes colour when the oxidation state of the metal ion changes. The iron-1,10-phenanthroline system is a typical example, the iron(//) chelate (ferroin) being red, the iron(//7) chelate (ferrion) being essentially colourless (actually very pale blue). The transition potential is about 1,1 V, so the indicator is very suitable for use in titrations with cerium(Ty) in sulphuric acid. The second type of indicator comprises various types of organic compound (aromatic amines, triphenylmethane dyestuffs, for example) which can be oxidised and reduced reversibly, and change colour on doing so. 7V,iV -Diphenylbenzidine is a typical example. 

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