486  Analysis  of  Coal  Tar  Preparations.  {AmkeX;STm' 
Heavy  Oil,  boiling  between  2io°and  3000  C,  having  the  specific 
gravity  of  1  04,  and  containing  naphtalene,  anthracene,  phenan- 
threne,  and  other  hydrocarbons. 
The  tar  oils  from  bones  and  wood  are  respectively  richer  in  basic 
constituents  and  phenoloid  compounds,  and  therefore  afford  a  better 
source  than  the  coal-tar  oils  for  the  isolation  and  preparation  of 
definite  compounds,  so  that  they  are  less  employed  in  the  crude 
state. 
Estimation  of  Tar  Oils. — The  physical  characteristics  of  boiling 
point,  specific  gravity,  viscosity,  and  solidifying  point,  although  still 
frequently  solely  relied  upon,  afford  very  little  reliable  evidence  as 
to  the  composition  of  the  product.  As  the  hydrocarbons,  tar  acids, 
and  bases  have  very  different  and  distinct  values  with  regard  to  their 
disinfectant  properties,  the  first  step  in  arriving  at  a  true  knowledge 
of  the  chemical  composition  of  the  oil  is  the  separation  of  these 
three  classes  of  compounds.  This  is  effected  by  shaking  the  tar  oil 
with  concentrated  caustic  soda  solution,  containing  about  ten  per 
cent,  hydrate,  whereby  the  phenols  and  other  acid  constituents  are 
removed,  and  either  go  into  solution  in  the  alkaline  liquid  or  form 
a  separate  layer  of  sodium  salt. 
In  technical  analysis  this  process  is  carried  out  in  a  graduated 
tube,  and  the  difference  in  volume  of  tar  oil  before  and  after  the 
soda  treatment  gives  the  approximate  quantity  of  tar  acids  present. 
The  carrying  out  of  the  process  is*,  however,  accompanied  by  certain 
difficulties  of  manipulation  arising  from  the  viscous  nature  of  the 
liquids  and  the  difficulty  with  which  they  separate  into  layers.  If 
much  naphtalene  be  present,  it  is  also  liable  to  crystallize  out  when 
the  acids  are  removed  from  the  hydrocarbons  and  still  further  increase 
the  difficulties.  To  avoid  this  it  is  customary  to  add  ordinary  ether 
or  petroleum  ether  to  the  oil  before  shaking  with  soda,  as  these  sub- 
stances not  only  act  as  solvents  of  the  hydrocarbons  and  prevent 
any  tendency  to  crystallize,  but  also  by  mixing  with  them  lowers 
their  specific  gravity  and  allows  them  to  separate  more  easily. 
When  the  separation  is  complete  the  upper  layer  containing  the 
hydrocarbons  is  removed,  and  the  ether  allowed  to  evaporate  and 
the  volume  measured.  The  bases  which  are  retained  in  solution  by 
the  hydrocarbons  can  be  removed  from  the  latter  by  treatment  with 
dilute  mineral  acids  and  the  bases  obtained  from  the  acid  liquors  in 
the  form  of  salts. 
