Am.  Tour.  Pharm. 
Oct.,  1918. 
Dissolving  Zinc  Chloride. 
707 
Chloride-Dissociation  by  Water."  "  Perrot  has  observed  that  when 
zinc  chloride,  free  from  oxide  was  dissolved  in  water,  a  precipitate 
was  formed,  which  had  the  composition  ZnCl2,  5Zn06H20.  An 
inquiry  into  the  relations  of  the  quantity  of  water  to  the  amount  of 
oxychloride  formed  resulted  in  the  determination  that  by  employing 
100  mol.  of  water,  3.25  per  cent,  of  oxychloride  was  formed,  the 
same  quantity  was  formed  with  75  mol.,  3  per  cent,  was  formed  with 
50  mol.  and  2.6  per  cent,  with  25  mol. 
The  limit  of  the  reaction  is,  therefore,  with  75  mol.  of  water, 
the  addition  of  large  quantities  giving  no  increase  in  the  yield  of 
oxychloride.  Pharm.  Centralh.,  January  23,  1896,  46  from  Bull. 
Soc.  Chim.,  1895,  975." 
Commercial  zinc  chloride  may  contain  some  preexistent  oxy- 
chloride as  an  impurity  from  overheating  in.  manufacture,  and 
samples  of  medicinal  zinc  chloride  such  as  the  official  grade  are 
expected  to  show  a  separation  of  oxychloride  under  the  condition 
named ;  the  United  States  Pharmacopoeia  accordingly  provides  a  test 
for  limit  of  oxychloride. 
The  equation  commonly  used  to  explain  the  change  of  zinc 
chloride  through  the  effect  of  water  into  this  basic  salt  is 
ZnCl2  +  H20  —  Zn(OH)  CI  +  HC1. 
As  already  mentioned  the  precipitate  is  strikingly  bulky,  but  is 
by  no  means  a  great  part  of  the  zinc  salt  taken.  The  production  of 
free  hydrochloric  acid  as  shown  by  the  equation,  is  perhaps  the 
reason  that  all  of  the  zinc  is  not  precipitated  by  the  water.  A  proper 
conception  of  this  reaction  demonstrates  the  fact  that  every  clear 
solution  of  zinc  chloride  that  is  obtained  by  dissolving  zinc  chloride 
in  a  large  quantity  of  water  is  a  solution  of  zinc  chloride  plus  hydro- 
chloric acid.  This  is  a  matter  to  be  noted  toward  a  better  under- 
standing of  the  behavior  of  this  peculiar  substance,  for  although  at 
this  point  the  statement  is  largely  inference,  it  will,  in  due  course, 
become  a  proven  fact. 
Even  though  the  undissolved  matter  is  no  great  part  of  the  zinc 
chloride,  it  does  of  course,  represent  activity  which  it  is  the  duty 
of  the  pharmacist  to  vouchsafe  to  the  patient.  Therefore,  to  remove 
it  would  be  wrong,  to  dispense  it  in  eye-drops,  or  similar  prepara- 
tions, would  be  useless,  if  not  productive  of  more  serious  trouble 
than  an  explanation. 
When  first  we  encountered  this  behavior  we  could  not  take  time 
