Colored cations

Although cations such as Zn, Cd, and Ag are transition metal cations and contain d-electrons, in these cations the d-orbitals are completely filled. The only electronic transitions that could occur would be from a d-orbital to a much higher energy orbital such as an s- or p-orbital. These transitions require light in the ultraviolet region. These ions therefore do not absorb visible light, and their salts are colorless (unless the anion is colored). 

The true colors of some minerals can only be seen when the sample is finely powdered. Rather than grinding up a specimen, a small area is rubbed against a hard porcelain surface until a streak of the mineral is obtained. The color obtained in this streak test can be quite different from the appearance of the sample itself. For example, samples of specular hematite (the word specular refers to the mirror-like surface) have a very shiny black appearance due to the reflective nature of the naturally polished surface. The streak test reveals the true color of hematite—a red-brown. 

Some anions, particularly ones that are more polarizable, may impart a covalent character to the compound and produce color. The sulfide ion, colorless in simple Group I salts such as Na2S, produces considerable covalent character in combination with the heavier and more polarizable transition metals, such as in yellow grenockite (CdS) and reddish acanthite (Ag2S), as well as in the cuprous and cupric sulfides mentioned above. 

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The resonating positive charge in Dobner s violet is formed not by internal ionization but rather by external ionization. Other colored cations of this general type are numerous the cation of acridine yellow, for example, can be ascribed the structure... 

Numerous colorless organic compounds with extended jt-electron systems can be converted to colored cations or anions with polymethyne chromophore by protonation or deprotonation. The intense coloration of the corresponding salts is usually attributable to the fact that the lone pairs of electrons of the heteroatoms participate in the mesomerism of the conjugated n-electron systems [4]. 

In general, most alkali borates are soluble, whereas most other borates are insoluble. Table 7.1 presents the names, formulas, states, solubilities, and standard free energies of a number of the more important simple inorganic species of B. All borate salts are white unless they contain a colored cation. 

Careful control of concentrations, temperature, and catalyst must be employed since this reaction can lead to methyl formate under different conditions. Neutralization of the acid with bases produces the salts. These compounds are usually soluble with the Ag+ and the Hg2 salts being only slightly soluble. Some basic acetates, for example those of Al+ and Fe+, are insoluble. Most of the acetates are white except those with a colored cation. The acetate ion is... 

The acid is a colorless moderately strong diprotic acid which gives anions 204 and HC204 with log Kp values of 4.3 and 1.2. Neutralization of the acid with bases produces oxalate and hydrogen oxalate salts which are white unless a colored cation is involved. Most of these salts are insoluble except those of the alkali metal cations. The acid and oxalates can be readily oxidized to CO2 andHOHbyMn04 and Cr04, but lesser oxidants maybe... 

CrflII) complexes of the type [(H20)5CrR]2+ have been widely investigated in solution (57). These orange-colored cations, absorption maxima —400 (—300), —280 (—300), —555 (-25), have a solution stability that is quite dependent on R and undergo a variety of reactions includ-... 

The term mixed metal oxide pigment denotes a pigment that crystallizes in a stable oxide lattice, and in which the color is due to the incorporation of colored cations in this lattice. Such compounds are regarded as solid solutions. The American Dry Color Manufacturers Association has abandoned the term mixed metal oxide inorganic colored pigments , it denotes these colorants as complex inorganic color pigments [3.73],... 

The more popular method to control leveling is to use cationic products that act as colorless dyes competing with the colored cationic dyes for the fiber sites. If amounts of colored modified basic dye and colorless modified basic dye equal to the saturation value of (he fiber are uniformly dissolved in the dyebalh. then level dyeing behavior is promoted. 

Cationic (Basic) Dyes. These water-soluble cationic dyes are applied to paper, polyacrylonitrile (e g. Dralon), modified nylons, and modified polyesters. Their original use was for silk, wool, and tannin-mordanted cotton when brightness of shade was more important than fastness to light and washing. Basic dyes are water-soluble and yield colored cations in solution. For this reason they are frequently referred to as cationic dyes. The principal chemical classes are diazahemi-cyanine, triarylmethane, cyanine, hemicyanine, thiazine, oxazine, and acridine. Some basic dyes show biological activity and are used in medicine as antiseptics. 

Some evidence of the reaction mechanism was obtained from the UV spectra. Dissolving MA in dry trichlorogermane at low temperature leads to an intensively blue [MA]+" cation-radical. Generation of the intensively colored cation-radical MA+ was ascribed to a secondary reaction of the weakly colored a-complex [MAH]+ with [MA]94,95 (equations 58 and 59). 

Substitution of hydroxyl and methoxyl groups influences the color of the anthocyanins. This effect has been shown by Braver-man (1963) (Figure 6-25). Increase in the number of hydroxyl groups tends to deepen the color to a more bluish shade. Increase in the number of methoxyl groups increases redness. The anthocyanins can occur in different forms. In solution, there is an equilibrium between the colored cation R+ or oxonium salt and the colorless pseudobase ROH, which is dependent on pH. 

Protonation of the triaryl-1,2-diazepines (17 and 18) gave cations (45).21 In the case of 18 [R=p-(CH3)2N—C6H4] the compound is diprotonated and the effect of this on the ultraviolet spectrum implies planarity in the parent diazepine. It is also possible to monoprotonate the compound 18 [R=p-(CH3)2N—C6H4] and the deeply colored cation suggests aromatic resonance (46) for the diazepinium salt.21... 

One can see that the method would work best with a colored cation, e.g., a cation that... 

Basic (cationic) dyes. Basic dyes are water-soluble and produce colored cations in solution. They are mostly amino and substituted amino compounds soluble in acid and made insoluble by the solution being made basic. They become attached to the fibers by formation of salt linkages (ionic bonds) with anionic groups in the fiber. They are used to dye paper, polyacrylonitrile, modified nylons, and modified polyesters. In solvents other than water, they form writing and printing inks. The principal chemical classes are triaryl methane or xanthenes. Basic brown 1 is an example of a cationic dye that is readily protonated under the pH 2 to 5 conditions of dyeing [5]. 

Other examples where the dye cation was produced from the radical cation of a leuco dye include photooxidation of TAM dye leucolactones by iodonium salts that produce colored cations with high efficiency (Scheme 1.3) [51, 52]. Color formation was observed from the irradiation of TAM leucolactones alone in acetonitrile. It was suggested that the formation of S (7t7i ) was followed by the homolytic cleavage of the lactone P-bond and by subsequent electron transfer [53]. Because the quantum yield for the color formation was low ( 10 -10 ), it was suggested that krecomb > ket (Scheme 1.4). 

Methods using DPD DPD is reversibly oxidized to a magenta-colored cation radical known as a Wiister cation [XIII] ... 

Anionic dyes are those dyes that, in aqueous solution, yield colored anions. Cationic dyes are those dyes that, in aqueous solution, yield colored cations. T Apically anionic dyes contain carboxylic or sulfonic acid groups as the ionic moiety. Cationic dyes usually contain quaternary nitrogen groups. 

The Carr-Price reaction for the colorimetric determination of retinoids, which has long been in use, is based on colored cations formed via antimony (III) chloride (Hubbard et aL, 1971 Blatz and Estrada, 1972 Bridges and Alvarez, 1982). In the same way, the protonation of retinoids with trifluoroacetic acid has been used for colorimetric analysis (Dugan et aL, 1964). 

The addition of these color-on-demand systems may have an impact on the sensitivity if it interferes the initiation mechanism. Particular reactive initiator radicals (In) can easily abstract a hydrogen from leuco-dyes (XV). This results in the formation of a less reactive triaryl methane radical XVI. Oxidation of this intermediate yields the stable deeply colored cationic structure XVII. Thus, the reactivity/sensitivity of the initiator system reduces requiring to find an agreement between plate sensitivity and color formed. XVI can easily oxidize resulting in formation of the intensively blue-colored triaryl methane cation XVII. 

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