Am.  Tour.  Pharm. 
Oct.,  1918. 
Dissolving  Zinc  Chloride. 
711 
Looked  at  from  the  view  point  which  attaches  to  precipitated 
oxychloride  another  significance  to  be  gathered  from  this  phenom- 
enon is  that  there  is  a  limit  to  the  ability  of  the  acidity  of  the  zinc 
chloride  solution  to  hold  the  oxychloride  in  solution,  and  as  through 
gradual  dilution  this  power  suffers  attenuation  the  separation  of 
oxychloride  progresses  until  exhaustion  is  attained  when  down 
comes  such  balance  of  zinc  in  the  form  of  oxychloride  as  the  weak- 
ened acidity  of  the  solution  can  no  longer  hold  dissolved: 
The  present  acidity  of  the  solution,  must,  therefore,  be  looked 
upon  as  the  only  reason  that  all  of  the  zinc  is  not  thrown  out  in 
preparing  clear  zinc  chloride  solutions  by  dissolving  or  diluting  with 
water. 
That  this  acidity  must  be  sufficient,  though  not  necessarily  of  a 
related  form,  was  proven  through  a  fortunate  thought  which  oc- 
curred just  before  finishing  the  work  upon  these  tests ;  namely,  zinc 
chloride  can  be  dissolved  in  a  well-charged  carbonic  acid  water, 
without  residue  of  oxychloride ;  and  further  evidence  that  acidity  is 
essential  was  shown  by  the  fact  that  if  this  same  solution  is  boiled 
or  agitated  to  expel  the  carbon  dioxide,  the  insoluble  oxychloride 
appears.  Boiling  caused  its  immediate  separation,  agitation  revealed 
it  but  slowly,  while  quiet  maintained  a  clear  solution  for^  over  four 
days ;  when  afterwards  the  application  of  heat  caused  the  escape  of 
a  few  bubbles  of  gas,  and  the  looked-for  precipitate  appeared.  But 
for  its  fleeting  nature,  what  could  be  better  for  our  need?  That 
there  may  be  a  trace  of  oxychloride  yet  held  in  solution  by  the  com- 
bined acidities  of  the  zinc  chloride  and  free  acid  which  have  ever' 
enveloped  it,  would  be  difficult  indeed  to  prove,  but  it  was  proven 
that  a  clear  solution  of  zinc  chloride  of  greatest  strength  possible 
will  not  dissolve  oxychloride  which  has  been  previously  separated 
from  a  solution.  A  portion  of  the  cold,  clear  concentrated  solution 
of  zinc  chloride  was  mixed  with  a  small  amount  of  zinc  carbonate 
without  change  in  appearance,  but  upon  boiling  the  mixture  the  zinc 
carbonate  dissolved.  Even  after  no  more  carbonate  would  be  taken 
up  the  solution  retained  its  power  to  redden  litmus.  On  the  other 
hand,  a  clear  solution  of  zinc  chloride  from  which  oxychloride  has 
been  completely  separated  by  sufficient  dilution,  did  not  dissolve  zinc 
oxide  or  carbonate,  even  upon  prolonged  digestion  with  heat,  thus 
indicating  that  in  this  dilution  the  affinity  between  the  zinc  oxy- 
chloride and  hydrochloric  acid  as  a  balanced  condition  within  the 
solution  is  greater  than  between  the  zinc  oxide  and  hydrochloric 
acid. 
