PCBs do not have a natural source in the environment, they are manufactured commercially, as explained in the Production section of this site. PCBs are either oily liquids or solids and range from colourless to light yellow, and are odourless and tasteless. Some PCBs are volatile and can exist as a vapour in air. PCBs exist in the environment as a combination of individual chlorinated biphenyl components, known as congeners, as well as impurities. There are 209 possible cogeners of Polychlorinated Biphenyls but only 130 of them readily exist in commercial products.

The chemical formula of PCBs can be presented as C(12)H(10-n)Cl(n) , where n is a number of chlorine atoms within the range of 1-10. PCBs consist of a biphenyl (two benzene rings with a carbon to carbon bond between carbon 1 on one ring and carbon 1' on the second ring) with a varying number of chlorines.

Examples of PCB Structures

Less toxic PCB structure

More toxic PCB structure

What makes one PCB cogener more toxic than another?

PCBs with chemical structures that will decrease its biodegradation are more toxic. The following is a list of characteristics that affect biodegradation:

• Increasing the number of chlorine substituents decreases biodegradation
• Two chlorines in ortho-position at the same or on different aromatic rings of the biphenyl molecule significantly decreases biodegradation
• Cogeners with chlorine substituents only on one ring generally degrade faster than a chlorobiphenyl with the same number of chlorine atoms but on both rings because an unsubstituted ring will open up more favourably
• PCB metabolism speeds up when a carbon atom with a chlorine substituent is between two unsubstituted carbon atoms;
• Higher chlorinated congeners having a 2,3,4-trichlorophenyl group, are resistant towards biological degradation

 

Certain chemical properties make PCBs a good choice for commercial applications. Their ability to resist acids and bases and their thermal stablility make them generally unreactive and are therefore practical to use in dielectric fluids in transformers and capacitors, lubricants, and heat transfer fluids. They make good insulators because of their low electric conductivity, and are generally a beneficial commercial compound since they are non-explosive.

 

Some General PCB characteristics::

• Liquid at room temperature; density: 1.182-1.566 kg/L
• Low water solubility; readily soluble in organic solvents
• Non-explosive
• Low electric conductivity
• Very high thermal conductivity
• Extremely high thermal and chemical resistance (very high stability).
• PCB mixtures are light yellow or dark yellow in colour
• Melting point increases with increasing degree of chlorination; vapour pressure and water solubility decrease
• High chemical stability

 

Sources:

http://www.nsc.org/library/chemical/polychlo.htm

http://www.inchem.org/documents/ehc/ehc/ehc140.htm#1.1.2

http://www.chem.unep.ch/pops/POPs_Inc/proceedings/stpetbrg/fiedler1.htm

http://environmentalchemistry.com/yogi/chemistry/pcb.html

 

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