The Chemical Causes of Color

The structural and chemical causes of color in natural materials can be very complex. But basically, visible colors are caused and affected by the amount of a pigment or impurities present in a material, or the physical structure of its surface. Color is consistent for some materials, and not for others. This depends on the causes of the color. 

Room-temperature fluorescence (RTF) has been used to determine the emission characteristics of a wide variety of materials relative to the wavelengths of selected Fraunhofer lines in support of the Fraunhofer luminescence detector remote-sensing instrument. RTF techniques are now used in the compilation of excitation-emission-matrix (EEM) fluorescence "signatures" of materials. The spectral data are collected with a Perkin-Elraer MPF-44B Fluorescence Spectrometer interfaced to an Apple 11+ personal computer. EEM fluorescence data can be displayed as 3-D perspective plots, contour plots, or "color-contour" images. The integrated intensity for selected Fraunhofer lines can also be directly extracted from the EEM data rather than being collected with a separate procedure. Fluorescence, chemical, and mineralogical data will be statistically analyzed to determine the probable physical and/or chemical causes of the fluorescence. 

It is tempting to speculate on the nature of the pigment formed in the caries process and on its relationship with caries arrestment. Knowledge of the cause of caries arrestment may help provide ways to stop caries in a preliminary phase. However, many causes could account for the discoloration in caries, often explaining the different opinions dentists have on this matter. Therefore, a review of the chemical backgrounds of the color changes was felt necessary. 

One example is the known interference by reducing compounds that affect the chemical conversion of substrate to a colored indicator. This is especially true for the tetrazolium assays (Ulukaya, Colakogullari, and Wood 2004 Chakrabarti et al. 2000 Pagliacci et al. 1993 Collier and Pritsos 2003). The growing list of interfering compounds includes ascorbic acid and sulfhydryl reagents such as glutathione, coenzyme A, dithiothreitol, etc. Similar interferences by compounds that affect the oxidation and reduction chemistry of cells are likely to cause artifacts with the resazurin reduction assay. Assays that measure markers of metabolism also can be influenced by the pH of the culture medium and other factors that may stimulate or stress the metabolic rates of cells. 

After review of the control chart, the process was determined to be unstable. On June 8 and 17, points were above the upper control limit on the X-bar chart. Beginning on June 21, four points were below the lower control limit on the X-bar chart and there was a run of eight points in a row below the average. Since the process was unstable, action was taken to eliminate the special causes of variation. The special cause detected on June 8 was associated with poor color of the feedstock supplied by the Quality Chemical Company. Discussions with this supplier were initiated immediately and the problem was corrected. Material with better color was introduced into the process on June... 

Contamination of post-consumer PET (POSTC-PET) is the major cause of deterioration of its physical and chemical properties during reprocessing. POSTC-PET is contaminated with many substances (1) add producing contaminants, such as poly(vinyl acetate) and PVC (2) water (3) coloring contaminants (4) acetaldehyde (5) other contaminants such as detergents, fuel, pestiddes, etc., stored in PET bottles. 

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