Blue colorants, examples

Some treatments are practiced so widely that untreated material is essentially unknown ia the jewelry trade. The heating of pale Fe-containing chalcedony to produce red-brown carnelian is one of these. Another example iavolves turquoise where the treated material is far superior ia color stabiUty. Such treatments have traditionally not been disclosed. Almost all blue sapphire on the market has been heat treated, but it is not possible to distinguish whether it was near-colorless comndum containing Fe and Ti before treatment, or whether it had already been blue and was only treated ia an attempt at marginal improvement. The irradiation of colorless topa2 to produce a blue color more iatense than any occurring naturally is, however, self-evident, and treatments used on diamond are always disclosed. 

Qualitative Analysis. Several quaUtative analyses can be employed. For example, in the oxamide method (59), oxaUc acid is first heated at approximately 200°C with concentrated aqueous ammonia in a sealed tube. When thiobarbituric acid is added and heated to 140°C, a condensed compound of red color forms. The analysis limit is 1.6 pg. In the diphenylamine blue method (59,60), oxaUc acid is heated with diphenylamine to form a blue color, aniline blue. The analysis limit is 5 pg. 

Sapphyrins were discovered serendipitously by Woodward33 during the course of synthetic studies directed towards the total synthesis of vitamin B, 2. The sapphyrins were the first example of expanded porphyrins in the literature. Due to sapphyrin exhibiting a deep-blue color in the crystalline state and intense green in solution. Woodward coined the expression sapphyrin with reference to the deep-blue color of sapphire stones. 

Many of the d-block elements form characteristically colored solutions in water. For example, although solid copper(II) chloride is brown and copper(II) bromide is black, their aqueous solutions are both light blue. The blue color is due to the hydrated copper(II) ions, [Cu(H20)fJ2+, that form when the solids dissolve. As the formula suggests, these hydrated ions have a specific composition they also have definite shapes and properties. They can be regarded as the outcome of a reaction in which the water molecules act as Lewis bases (electron pair donors, Section 10.2) and the Cu2+ ion acts as a Lewis acid (an electron pair acceptor). This type of Lewis acid-base reaction is characteristic of many cations of d-block elements. 

The blue color of 83 has been observed in numerous experiments. For example, a brilliant blue color occurs if a potassium thiocyanate melt is heated to temperatures above 300 °C [132] or if eutectic melts of LiCl-KCl (containing some sulfide) are in contact with elemental sulfur [132, 133], if aqueous sodium tetrasulfide is heated to temperatures above 100 °C [134], if alkali polysulfides are dissolved in boiling ethanol or in polar aprotic solvents (see above), or if borate glasses are doped with elemental sulfur [132]. In most of these cases mixtures of much 83 and little 82 will have been present demonstrating the ubiquitous nature of these radicals [12]. 

When a strip of zinc metal is added to a solution of copper(II) sulfate, the blue color slowly fades, and the zinc metal is replaced by copper metal (Figure 4-13). As copper ions in the solution are reduced to copper metal, zinc atoms are oxidized to Zn cations. This is an example of a metal displacement reaction, in which a metal ion in solution (Cu ) is displaced by another metal (Zn) by means of a redox reaction. Figure 4-13 also shows molecular views of this displacement reaction. 

Blue copper proteins. A typical blue copper redox protein contains a single copper atom in a distorted tetrahedral environment. Copper performs the redox function of the protein by cycling between Cu and Cu. Usually the metal binds to two N atoms and two S atoms through a methionine, a cysteine, and two histidines. An example is plastocyanin, shown in Figure 20-29Z>. As their name implies, these molecules have a beautiful deep blue color that is attributed to photon-induced charge transfer from the sulfur atom of cysteine to the copper cation center. 

If the arylamino group is substituted with an appropriate group to extend the conjugated double bond system, blue color can also be obtained. For example, 10 -(4-anilinoanilino)benzo[c]fluoran (32 R = C6H5NH)24 and 10 -(4-styrylanilino)benzo[c]fluoran (32 R = C6H5CH=CH)25 develop blue color. 

Fluoran compounds having two diarylamino groups at 3 - and 6 -positions generally develop blue tone colors. For example, 3, 6 -bis-(diphenylamino)fluoran (34 R1, R2 = H)29 develops reddish blue color, and 3 -diphenyl amino-6 -di-/ -tolylaminofluoran (34 R1 = H, R2 = CH3)29 and 3, 6 -bis(di-/i-tolylamino)fluoran (34 R1, R2 = CF13)29 blue color. 

An excellent example of this type of analysis involves the determination of phosphate in soil extracts. Soil is extracted with an appropriate extractant and added to a solution of acid molybdate, with which the phosphate reacts to produce a purple- or blue-colored solution of phosphomolybdate. Standard phosphate solutions are prepared, reacted with acid molybdate, and the intensity of the phosphomolybdate color produced is measured. A standard curve (also called a calibration curve) is prepared (see Section 14.10) from which the intensity of the color is directly related to the concentration of phosphate in the extract. 

Hydrochloric acid. Strong acids are used frequently for the purpose of sample dissolution when water will not do the job. One of these is hydrochloric acid, HC1. Concentrated HC1 is actually a saturated solution of hydrogen chloride gas, fumes of which are very pungent. Such a solution is 38.0% HC1 (about 12 M). Hydrochloric acid solutions are used especially for dissolving metals, metal oxides, and carbonates not ordinarily dissolved by water. Examples are iron and zinc metals, iron oxide ore, and the metal carbonates of which the scales in boilers and humidifiers are composed. Being a strong acid, it is very toxic and must be handled with care. It is stored in a blue color-coded container. 

Mavicyanin (Mj = 18,000) is obtained from green squash (Cucurbito pepo medullosa), where it occurs alongside ascorbate oxidase [64]. It has a peak at 600 nm (e 5000 M cm and reduction potential of 285 mV. Further studies on this and the mung bean and rice bran proteins [65, 66] would be of interest. All the above type 1 Cu proteins have an intense blue color and characteristic narrow hyperfine EPR spectrum for the Cu(II) state. Table 3 summarizes the properties of those most studied. There is some variation in reduction potential and position of the main visible absorbance peak. In the case of azurin, for example, the latter is shifted from 597 to 625 nm. Stellacyanin has no methionine and the identity of the fourth ligand is therefore different [75]. The possibility that this is the 0(amide) of Gln97 has been suggested [63b]. It now seems unlikely that the disulfide is involved in coordination. Stellacyanin has 107 amino acids, with carbohydrate attached at three points giving a 40% contribution to the M, of 20,000 [75]. 

Iodine is also used as a test for starch. When placed on starch (a potato for example), iodine turns the starch a dark blue color. Silver iodide is used in the manufacture of photographic film and paper. It is also used to seed clouds because of its ability to form a large number of crystals that act as nuclei upon which moisture in the clouds condenses, forming raindrops that may result in rain. 

Zinc exhibits a valence of +2 in all its compounds. It also is a highly electropositive metal. It replaces less electropositive metals from their aqueous salt solutions or melts. For example, a zinc metal bar put into Cu2+ solution acquires a brown-black crust of copper metal deposited on it. At the same time the blue color of the solution fades. Zinc reduces Cu2+ ions to copper metal. The overall reaction is ... 

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