Contamination color compounding

During the synthesis of food dyes, as with any chemical reaction, the product that is formed contains impurities, since the reactants themselves are usually impure and side reactions can also occur. The impurities can be of three types Uncombined intermediates are compounds from which a color is directly synthesized and do not impart color to the product subsidiary colors are colored compounds that are structurally related to the product other impurities include substances that may arise from contamination of the intermediates or other reagents used during synthesis... 

Bleaching is usually practiced to remove unwanted colored compounds and other contaminants in vegetable oils. Adsorptive clays normally used for bleaching also serve as a catalyst to decompose or remove peroxides and metals that may stiU be present in the oil. The absorbents used in bleaching effectively remove these contaminants. The next step is usually deodorization, and if any metals are present in the oil during deodorization, it may in fact worsen the quality of the final product. 

Many of the types in Subgroup A represent colored compounds and the few individual members which are not colored when pure are often contaminated with colored impurities. The simple nitro and azoxy compounds are usually light yellow or cream colored, whereas the azo compounds are more highly colored. Additional substituents, for example, amine groups, will deepen the color of nitro compounds. Many simple nitroso compounds are green. 

Methanol can be converted to a dye after oxidation to formaldehyde and subsequent reaction with chromatropic acid [148-25-4]. The dye formed can be deterruined photometrically. However, gc methods are more convenient. Ammonium formate [540-69-2] is converted thermally to formic acid and ammonia. The latter is trapped by formaldehyde, which makes it possible to titrate the residual acid by conventional methods. The water content can be determined by standard Kad Eischer titration. In order to determine iron, it has to be reduced to the iron(II) form and converted to its bipyridyl complex. This compound is red and can be determined photometrically. Contamination with iron and impurities with polymeric hydrocyanic acid are mainly responsible for the color number of the merchandized formamide (<20 APHA). Hydrocyanic acid is detected by converting it to a blue dye that is analyzed and deterruined photometrically. 

The cmde phthaUc anhydride is subjected to a thermal pretreatment or heat soak at atmospheric pressure to complete dehydration of traces of phthahc acid and to convert color bodies to higher boiling compounds that can be removed by distillation. The addition of chemicals during the heat soak promotes condensation reactions and shortens the time required for them. Use of potassium hydroxide and sodium nitrate, carbonate, bicarbonate, sulfate, or borate has been patented (30). Purification is by continuous vacuum distillation, as shown by two columns in Figure 1. The most troublesome impurity is phthahde (l(3)-isobenzofuranone), which is stmcturaHy similar to phthahc anhydride. Reactor and recovery conditions must be carefully chosen to minimize phthahde contamination (31). Phthahde [87-41-2] is also reduced by adding potassium hydroxide during the heat soak (30). 

Color can be removed effectively and economically with either alum or ferric sulfate at pH values of 5—6 and 3—4, respectively. The reaction is stoichiometric and is a specific reaction of the coagulant with the color to form an insoluble compound (17). The dosage required may be as high as 100—150 mg/L (380—570 mg/gal). Raw-water colors may be as high as 450—500 units on the APHA color scale. The secondary MCL (maximum contaminant level) for color in the finished water is 15 units, although most municipal treatment plants produce water that seldom exceeds 5 units. 

Barium sulfide solutions undergo slow oxidation in air, forming elemental sulfur and a family of oxidized sulfur species including the sulfite, thiosulfate, polythionates, and sulfate. The elemental sulfur is retained in the dissolved bquor in the form of polysulfide ions, which are responsible for the yellow color of most BaS solutions. Some of the mote highly oxidized sulfur species also enter the solution. Sulfur compound formation should be minimized to prevent the compounds made from BaS, such as barium carbonate, from becoming contaminated with sulfur. 

Chemical contaminants are usually not reduced or removed by processing steps. Chemical risks must preferably be controlled as early as possible in the agri-food chain. Food color additives (Section 7.1.3) are chemical compounds and are considered potential risks. Therefore a safety evaluation is part of the approval of a food colorant before its use is acknowledged by legislation (see also Section 7.1.6). This section explains the principles of risk assessment and includes an example of such an assessment of a specific food colorant. 

While using the 0.1 % threshold as a determinant for when an impurity should be isolated to meet regulatory requirements is a good practice, there are other times when it becomes necessary to work with extraneous compounds at still lower levels. During the development of process chemistry for the synthesis of the protease inhibitor Tipranavir several synthetic lots of the drug were discolored, appearing pinkish rather than white as they should. It was determined that a low-level ( 0.1%) highly colored material was responsible for the problem with those lots and a request for the isolation and characterization of that contaminant was received [66]. 

A-Hydroxyphthalimide 178a (NHPI Chart 4), described for the first time in 1880, is a weak acid (plfa = 7), which forms highly colored salts with alkali metals, heavy metals, ammonia or amines. More precisely, the two crystalline forms of NHPI reported in the literature display different colors, colorless (white) and yellow, with the colors depending on the solvent in which the NHPI is prepared. The variations in physical properties, including color, have been attributed to synthetic problems , such as impurities, and contamination from a fluorescent compound that could not be isolated or characterized. 

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