Colourings analytical methods

In 1904 the Russian scientist Tswett coined the term chromatography (= colour writing) with regard to his work in separating plant pigments. Today, some of the most potent analytical methods available are chromatographic. 

Field pH, Munsell soil colour (dry moist), EC 1 5 (soil water), pH 1 5, XRD, laser particle size analysis, XRF (multiple elements), ICP-MS (after HNO3/HCKV HF/HCI digestion for multiple elements), ICP-MS (after HF/HCI/HNO3 digestion for Se), ISE (for F), GF-AAS (for Au), and ICP-MS after MMI extraction were performed. Full details of the sampling and analytical methods are given in Caritat Lech (2007). 

Natural colours include annatto, anthocyanins, beetroot red (betalaines), caramel, carotenoids, cochineal and lac pigments, flavanoids, chlorophylls and tumeric. There is a trend towards encapsulating natural colours for food use, but this is not yet reflected in the extraction techniques described in the published analytical methods. Lancaster and Lawrence (1996) described the extraction and... 

Chlorophylls occur very frequently in the plant kingdom, they are responsible for the colour of vegetables and some fruits. They also occur in algae and several bacteria. Chlorophylls in plants are photoreceptors and in photosynthesis the presence of a closed circuit of conjugated double bonds allows them to absorb light. Because of their predominant importance as photoreceptors a considerable number of analytical methods have been developed for the separation and quantitative determination. The analytical methods applied for the measurement of chlorophylls and carotenoids in food products have been reviewed previously [273],... 

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-... 

In 1935, the Committee was renamed the Analytical Methods Committee (AMC) but the main analytical work was carried out by sub-committees composed of analysts with specialised knowledge of the particular application area. The earliest topics selected for study were milk products, essential oils, soap and the determination of metals in food colourants. Later applications included the determination of fluorine, crude fibre, total solids in tomato products, trade effluents and trace elements, and vitamins in animal feeding stuffs. These later topics led to the publication of standard methods in a separate booklet. All standard and recommended methods were collated and published in a volume entitled Bibliography of Standard, Tentative and Recommended or Recognised Methods of Analysis in 1951. This bibliography was expanded to include full details of the method under the title Official, Standardised and Recommended Methods of Analysis in 1976 with a second edition in 1983 and a third edition in 1994. 

The determination of the colouring matters constituting the mixtures used to obtain a given colour is usually a complex problem which varies from one case to another and can be solved only by one possessing an exact knowledge of the analytical method and of the various processes of dyeing. 

This volume provides comprehensive information about additives in the food industry. The book opens with an explanation of risk analysis and analytical methods in relation to the use of additives in food products. This is followed by full details of relevant EU and USA regulations. The second part of the book provides information about specific subjects including flavourings, sweeteners and colourings. 

Genuine chemical methods were preceded by techniques such as the touchstone,26 and also by the direct investigation of the material or object by the use of the senses. A material could be felt, it could be tasted, its colour could be observed, it could be smelt, and the noise it could be induced to make could be listened to. The use of such methods of organoleptic analysis in the work of the Georgian King, Vakhtang VI (1675-1737) have been described.27 Probably the first demand for a quantitative analytical method arose from a desire to estimate the purity of samples of gold. While... 

The widespread occurrence of iron ores, coupled with the relative ease of extraction of the metal, has led to its extensive use as a constructional material with the result that the analysis of steels by both classic wet and instrumental methods has been pursued with vigour over many years.3 Iron complexes are themselves widely used as the basis of convenient analytical methods for the detection and estimation of iron down to parts per million. Familiar tests for iron(III) in aqueous solution include the formation of Prussian blue as a result of reaction with [Fe(CN)6]4, and the formation of the intensely red-coloured [Fe(H20)5SCN]2+ on reaction with thiocyanate ion.4 Iron(II) forms particularly stable red tris chelates with a,a -diimines such as 1,10-phenanthroline or 2,2 -bipyridine that have been used extensively in spectrophotometric determinations of iron and in the estimation of various anions.5 In gravimetric estimations, iron(III) can be precipitated as the insoluble 8-hydroxyquinoline or a-nitroso-jS-naphthol complex which is then ignited to Fe203.6 In many situations the levels of free [Fe(H20)6]3+ may be controlled through complex formation by addition of edta. 

One of the characteristic features of aromatic nitro compounds, particularly those with two or more nitro groups is their ability to react with bases and in many instances the reaction is accompanied by an intense colour. The colour had caught the attention of chemists by the late nineteenth century and initiated a considerable amount of research on the nature of the formed substances. Thus considerable literature appeared on reactions known as Janovsky or Jackson-Meisenheimer or Meisenheimer reactions which are discussed later. They are nucleophilic reactions and are not only of theoretical but also of practical importance. Thus the Janovsky reaction became an important analytical method, and reactions of di- and trinitrobenzene and their homologues with bases, are of great importance in the problem of the safety of manufacture and of the stability of these nitro compounds. 

In addition to transfer techniques in quantitative analyses, such as polarography, titrimetry, spectroscopy and other analytical methods used after separation by TLC, the in situ optical measurement is the most widely employed technique for quantitative determinations. In most cases UV-absorption is used, while coloured substances can be determined by absorption measurement in the visible range of the spectrum. Fluorescent substances are preferably determined by fluorescence measurement. Infrared absorption techniques are not used in routine analysis up to this date. 

The AATCC system is a very detailed dye class identification system. The brief description of some basic tests in this chapter is only for the purpose of introduction. Readers are recommended to obtain thebook Analytical Methods for a Textile Laboratory by the AATCC in order to comprehend the complexity of the dye analysis. Of course the other books and journal articles listed in the references are also excellent information sources. Whenever possible, reading these materials would definitely help develop a better understanding of colour chemistry, which will ultimately ensure that the dye analysis is performed more effectively and efficiently. 

To conclude, the common physico-chemical characteristics of oligo-polyols for polyurethanes determined by standard analytical methods are hydroxyl number, hydroxyl percentage, primary hydroxyl content, molecular weight, equivalent weight, molecular weight distribution, viscosity, specific gravity, acidity and colour (See Chapters 3.1-3.11). 

Determination of red-free G-G has allowed observation, for the first time, of the sharp development in secondary metabolites in Shiraz fruit in the latter stages of soluble solids accumulation. The rate of formation of these important compoimds with degrees Brix, and possibly also the time of maturity at which the development accelerates, are influenced by irrigation as well as by the growing season (25). These characteristics are observable only with this analytical method and no other measure of fruit composition, including colour, leveab these trends. 

---