Colored Inorganic Compounds

It is often convenient, as it is now, to divide the fundamental ways in which color is created in chemical substances into two classes corresponding to the traditional division into organic and inorganic compounds. However, it wiU become apparent later in the discussion that this division is not a strict one since some compounds in both classifications exhibit color by the same type of mechanism. In aU cases, electronic transitions between energy level differences in the visible region of the spectrum must take place. 

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Oxidation. Hydrogen peroxide is a strong oxidant. Most of its uses and those of its derivatives depend on this property. Hydrogen peroxide oxidizes a wide variety of organic and inorganic compounds, ranging from iodide ions to the various color bodies of unknown stmcture in ceUulosic fibers. The rate of these reactions may be quite slow or so fast that the reaction occurs on a reactive shock wave. The mechanisms of these reactions are varied and dependent on the reductive substrate, the reaction environment, and catalysis. Specific reactions are discussed in a number of general and other references (4,5,32—35). 

Exempt colorants are made up of a wide variety of organic and inorganic compounds representing the animal, vegetable, and mineral kingdoms. Some, like -carotene and 2inc oxide, are essentially pure factory-produced chemicals of definite and known composition. Others, including annatto extract, cochineal extract, caramel, and beet powder are mixtures obtained from natural sources and have somewhat indefinite compositions. 

Some inorganic compounds are used as food additives and food colorants. They include titanium dioxide, carbon black, iron oxides, ultramarin, and calcium carbonate. Some of them are important for properties other than the ability to impart color. Titanium is the most commonly used inorganic pigment in food and will be briefly discussed below. ° "°... 

In this paper we will describe and discuss the metal-to-metal charge-transfer transitions as observed in optical spectroscopy. Their spectroscopic properties are of large importance with regard to photoredox processes [1-4], However, these transitions are also responsible for the color of many inorganic compounds and minerals [5, 6], for different types of processes in semiconductors [7], and for the presence or absence of certain luminescence processes [8]. 

Inorganic compound adsorptions, particularly those that are colored, involve the promotion of electrons in the d orbitals. Absorption can occur in both isolated compounds and inorganic moieties with organic ligands. 

A mineral is a naturally occurring, crystalline inorganic compound with a specific chemical composition and crystal structure. Minerals are commonly named to honor a person, to indicate the geographic area where the mineral was discovered, or to highlight some distinctive chemical, crystallographic, or physical characteristic of the substance. Each mineral sample has some obvious properties color, shape, texture, and perhaps odor or taste. However, to determine the precise composition and crystal structure necessary to accurately identify the species, one or several of the following techniques must be employed optical, x-ray diffraction, transmission electron microscopy and diffraction, and chemical and spectral analyses. 

By far the greatest number of photochromatic compounds, however, are organic. As in the inorganic compound mercury dithizon-ate, photochromism in organic compounds occurs when one form of a compound is exposed to light, which converts the compound to an isomer of a different color. In most such instances, the two isomers differ in that one is a closed form of the isomer and the other is an open form. In some cases, however, the two isomers differ in that one occurs in a trans form and the other in a cis form. Trans and cis isomers are forms of a compound in which two constituent atoms or groups are located on opposite or the same side of a molecule, respectively. 

At times, failures in processing may introduce such materials as carbon adsorbent particles, filter aid, or inorganic compounds. In the last category, iron oxides and sulfides and sodium and ammonium salts have frequently been reported to be factors in the color problem,60 not only because of their own color but also because of their catalytic effect on the formation of coloring materials from sugars. Turbidity may also result from presence of traces of such salts as calcium phosphate, silicate, or sulfate. 

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