Table 2. Chemical and physical properties of 2,3,7,8-TCDD, 

 also known as CAS Registry No. 1746-01-6 (NIOSH 1984). 



Criterion Property 



Empirical formula C^pH^CKO^ 



Percent by weight 



Carbon 44.70 



Oxygen 9.95 



Hydrogen . 1.25 



Chlorine 44.1 



Molecular weight 322 ^ 



Vapor pressure, mm Hg at 25 C 1.7x10" 



Melting point, C 305 



Decomposition temperature, C >700 

 Solubilities, g/liter 



- dichlorobenzene 1.4 



Chlorobenzene 0.72 



Benzene 0.57 



Chloroform 0.37 



n - octanol 0.05 



Methanol 0.01 



Acetone 0.11 , 



Water 2.0x10"^ 



rodents, birds, lizards, fishes, and insects 12 years after application. This 

 half-time in soil was estimated at 2.9 years (Westing 1978). Uptake of 

 2,3,7,8-TCDD from soils by vegetation is considered negligible (Blair 1973; 

 Ramel 1978). 



Other than 2,3,7,8-TCDD, identification difficulties with specific 

 isomers make quantification difficult. To date, there has been little effort 

 towards resolution of deficiencies in analytical methodology (NRCC 1981). A 

 detection level of 1 pg (10 --^^g), or lower, might be required to find 

 2,3,7,8-TCDD in a one gram sample. Analyses at such low levels are 

 complicated by interference from a multitude of other compounds, as well as by 

 the large number of PCDD isomers and their variations in chemical properties 

 (Rappe 1984). Although 2,3,7,8-TCDD is the most extensively studied PCDD 

 isomer, data on its fate and persistence are generally poor, interpretations 

 are frequently absent, and extrapolations from case to case usually 

 impossible. The general result is a qualitative concept of this compound's 

 behavior in environmental situations (NRCC 1981). The issue is further 

 confounded by the presence in biological and abiotic samples of chemicals of 

 similar structure and toxicological properties to that of 2,3,7,8-TCDD. These 

 isosteric compounds include: 2,3,6,7-tetrachlorobiphenylene; 2,3,7,8-chlorine 

 substituted dibenzofurans; and 3,3' ,4,4'-tetra-, 3,3' ,4,4' ,5-penta-, and 



