232        Pharmaceutical  Notes  from  Purdue  University {AmMay^Sf rm' 
2.  A  mixture  of  20  c.  c.  of  sulphide,  with  excess  of  ferric  sulphate 
and  dilute  sulphuric  acid  was  digested  half  an  hour,  and  then  titrated 
with  permanganate,  requiring  14" 73  c.  c.  to  give  a  rose  tint. 
3.  Ten  c.  c.  of  solution  of  arsenic  were  mixed  with  20  c.  c.  of  the 
sulphide,  and  acidified  by  the  addition  of  45  c.  c.  decinormal  solu- 
tion of  hydrochloric  acid.  The  whole  mixture  was  diluted  to  250 
c.  c.  and  filtered.  100  c.  c.  (or  two-fifths j  of  the  filtrate  were  made 
alkaline  by  the  requisite  quantity  of  solution  of  bicarbonate  of  soda, 
and  4*8  c.  c.  of  solution  of  iodine  were  then  required  to  oxidize  the 
arsenious  acid  still  present;  the  whole  filtrate  would  therefore  have 
required  f  X  4*8  =  12*0  c.  c.  of  iodine;  while  10  c.  c.  of  the  original 
arsenic  solution  required  21.5  c.  c.  iodine;  hence  the  arsenic  precipi- 
tated as  sulphide  would  have  required  21*5 — 12'0  —  9*5  c.  c.  of  deci- 
normal iodine.  In  thus  comparing  a  sulphide  with  iodine,  we  must 
refer  to  the  equations, 
4  14-2  H20+As203  =  4  H20+As205,  and 
6  HC1+3  Am2S+As203  =  6  AmCl—As2Sj— 3  H20; 
it  is  readily  seen  that  the  same  quantity  of  arsenic  that  requires  four 
times  the  atomic  weight  of  iodine,  would  require  three  times  the 
molecular  weight  of  a  sulphide;  and  the  atomic  weight  of  the  former 
represents  three-fourths  times  the  molecular  weight  of  the  latter.  But 
since  sulphur  has  a  combining  power  of  two,  we  take  half  the  mole- 
cular weight  for  a  normal  solution  of  a  sulphide,  and  the  atom  of 
iodine  represents  three-half  times  this  amount.  Hence,  9'5  c.  c.  of 
decinormal  solution  of  iodine  (which  represents  the  arsenic  precipi- 
tated by  the  sulphide)  would  represent  IX  9*5  =  14*25  c.  c.  of  a  deci- 
normal solution  of  sulphide. 
The  results  of  the  three  methods  of  titration  may  now  be  compared ; 
the  amount  of  sulphide  present  in  20  c.c.  of  the  solution  used,  when 
compared  with  decinormal  solution,  was  found  equivalent  to — 
14*85  c.  c.  by  titration  with  zinc 
1473  "  "         "  permanganate. 
14'25  "  "         "  iodine. 
This  agreement  is  considered  satisfactory,  and  the  first  method  was 
adopted  for  further  u«e.  1  c.c.  decinormal  solution  of  zinc  repre- 
sents 0'0036  gm.  CaS ;  but  this  salt  was  treated  with  ammonia  before 
making  the  titration. 
0*36  gm.  of  a  commercial  sample  (presented  by  the  Mallinckrodt 
