Bubble chlorine gas through red tomato juice and it will turn white. Treat old yellowed newspaper clippings with sodium borohydride and they likewise whiten.
Bleaching is the act of removing color. Chemical bleaching requires an agent that acts mostly by oxidation and has atoms with a strong attraction for electrons that will steal them from other atoms.
Colored substances contain groups of atoms called chromophores that absorb only certain wavelengths of light and reflect the remainder. For example, a chromophore that absorbs blue light will reflect the complementary yellow color, and the chromophore substance will appear yellow.
Bleaching agents disassemble and destroy chromophores by oxidation, removing the color.
Oxidation involves stealing electrons from atoms, while reduction adds electrons. Since electrons cannot be destroyed, oxidation and reduction are complementary and together are abbreviated as ‘redox’ reactions.
For example, take a hydrogen atom with its single electron in a quantum cloud around a single proton. Put an oxygen atom near the hydrogen atom and the oxygen atom will steal the electron from the hydrogen atom. Hydrogen is oxidized as it loses the electron, but oxygen is reduced as it gains the electron.
Chlorine has the same hunger for electrons as does oxygen, only stronger. Put them together in a single molecule and there is a lot of oxidation potential in a small volume.
Bleaching for pulp, paper and grain industries uses pure chlorine and chlorine dioxide.
Household bleach products use sodium hypochlorite. Calcium hypochlorite is a solid used in sanitizing. Sodium dichloroisocyanurate keeps the outdoor pool sanitized and at the ideal pH of human tears.
Tooth whiteners use peroxides such as hydrogen peroxide or carbamide peroxide. Sodium perborate is another bleach that has application as a phosphate replacement in detergents. Peroxide bleaches are available in powdered form for use in laundry and household cleaning.
Chlorine is deadly to all bacteria and viruses, hence its use for sanitization. Oxygen bleaches are less harsh, yet high concentration of hydrogen peroxide can strip coral and seafloor artifacts by oxidizing all cytoplasm effectively and completely.
If a colony of living coral were to be showered with concentrated hydrogen peroxide, the symbiotic algal zooxanthellae would die, leaving the coral skeleton as a bleached bare, white piece of limestone rock.
Coral are sensitive to change, so as changes in light, temperature or nutrients cause stress, the coral pushes out the symbiotic zooxanthellae living in their tissues, bleaching them just as effectively as a bath of peroxide.
Bleaching coral typically does not kill them, but it takes time to re-establish a healthy colony, and afterward they are more stressed and more vulnerable to dying.
Because of warmer-than-usual ocean temperatures this summer, the islands of Hawaii will likely see the worst coral bleaching ever experienced.
Water temperatures around Hawaii are 4 to 6 degrees above average for September, which is a large anomaly for the subtropical sea surface, which typically varies only a few degrees annually.
Richard Brill is a professor of science at Honolulu Community College. His column runs of the first and third Friday of the month. Email questions and comments to brill@hawaii.edu.
