Chemical analysis of colorants

The general aim of this chapter is to focus on the major instrumental techniques that are used to analyse colorants associated with the textile coloration industries. The principal techniques that have been covered are spectroscopic techniques and chromatographic techniques. The capability and value of the techniques are highlighted by reference to relevant applications. Prior to focusing on the techniques, consideration is given to the nature of colorants and also general issues, such as sampling, that are important in an overall analytical procedure. Section 

2 on colorants includes some consideration of the health, safety and environmental considerations hnked to colorants, which is a major driver for analysis of colorants. Note, in this chapter there has been a greater focus on the chemical nature, identity and quantitation of colorants (especially by instrumental methods) than on the property and measurement of colour . 

A dye (or dyestuff, as they are sometimes referred to, especially in the USA) is a substance, nearly exclusively organic in nature, that is applied to a substrate in order to impart colour with some degree of permanence. At some point during apphcation dyes are unimolecular, that is, they exist as individual molecules. Dye molecules often contain at least one water solubihsing group, such as a sulphonic acid group, to aid their normal application route, which is usually from an aqueous medium. By far the main application area for dyes is in textile coloration. 

When it comes to defining lakes and toners, both of which are essentially specific types of organic pigment, there is a problem in that the accepted definition/ understanding varies in different parts of the world. In the UK the following are accepted  

The term lake is widely used in the USA, and elsewhere, to describe what are called toners in the UK. To further add to the confusion with respect to American terminology, they often use the name Toner for all organic pigments, as they tone duller inorganic pigments. 

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It is shown that both Sb(III) and Bi(III) can speed reduction of 12-molybdophosphate (12-MPC) to the corresponding heteropoly blue (12-MPB) by ascorbic acid (AA). It is found that mixed polyoxometalates can be formed in solution which reduce considerably more rapidly than 12-MPC. Complete formation of mixed POM is observed only if significant excess of Me(III) ions is used in the reaction. POM responsible for blue color was synthesized by selective extraction. Chemical analysis of tetrabutyl-ammonium salt is in accordance with formula of (TBAl PMeflllfMo O j (Me = Sb, Bi). IR spectmm of mixed POM is identical to 12-MPC. 

The analysis of dyes and pigments for textile applications is really a very broad subject because it usually involves many different types of analyses and the use of many sophisticated instruments. It is difficult to provide a detailed coverage in a short section here. Therefore, only some basic and simple analyses are introduced. Readers who want to know more can access the books published elsewhere. - Analysis of colorants can also mean many different things. Identification of colorants on textile materials is an important aspect of forensic analysis and also very useful for textile dyers. Characterisation of colorants is critical for colour chemists to know what chemical structures the colorant has. Determination of dye classes has its practical significance in making up dyeing and printing recipes as well as in the analysis of historical textiles. 

The question of the use of chemicals in and on foods is now moot. Many articles have recently been written concerning such additions. There is one book devoted to the broadest aspect of this field covering the actual use of colors, fl[avors, preservatives, stabilizers, vitamins, and other food additives by the food industries— Jacobs Synthetic Food Adjuncts. Some information is also given by Blumenthal in Food Manufacturing. There are other texts with more limited scope in this connection—Hirsch, Chemische Konservierung von Lebensmittlen (1952) in which the stress is on East German practice Jacobs, Chemical Analysis of Foods and Food Products (1951) and the much older text by Folin, Preservatives and Other Chemicals in Foods (1914). 

An English translation of Cronstedfs book (1788) contained the same list of species and varieties cited previously, but also reported the chemical analysis of Kirwan as well as an observation by Fabroni, deputy-director of the Museum of Physics and Natural History of Florence. Fabroni observed that colored mica was dissolved by acqua regia or muriatic acid to produce a liquid that is yellow on account of iron. 

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