230 
Hydrogen  Dioxide. 
( Am.  Jour.  Pharm. 
t       May,  1898. 
The  sulphuric  acid  should  be  free  from  sulphurous,  arsenous  and 
nitric  acids,  and  the  potassium  iodide  from  iodate.  Sodium  thiosul- 
phate  (commercially  called  "hyposulphite  of  soda")  is  very  fre- 
quently found  in  the  market  of  a  sufficiently  high  degree  of  purity 
N 
to  make  it  permissible  to  prepare  the  —  solution,  for  the  assay  of 
10 
hydrogen  dioxide,  by  dissolving  exactly  24-8  grammes  of  it  in  enough 
distilled  water  to  make  I  liter,  instead  of  standardizing  it  with 
iodine.  It  should  be  in  colorless,  transparent,  uneffioresced  crystals, 
and  contain  no  chloride  or  sulphate,  or  at  most  only  traces  of  sul- 
phate. The  crystals  should  be  coarsely  powdered  and  any  interstitial 
motherliquor  removed  by  pressing  between  folds  of  bibulous  paper 
before  weighing.  The  writer,  however,  prefers  to  recrystallize  the 
commercial  salt  and  finds  the  solution  prepared  from  this,  in  the 
manner  indicated  above,  fully  as  accurate  as  when  standardized  with 
pure  iodine. 
This  method  has  been  treated  of  more  at  length  than  the  others, 
because  it  proved  to  be  the  best  of  those  examined,  as  will  be 
explained  more  fully  below. 
Comparison  of  Methods  on  Commercial  Samples. — A  few  commer- 
cial solutions  of  hydrogen  dioxide  of  varying  strength  and  purity 
were  examined  with  the  object  of  gaining  some  knowledge  of  the 
comparative  accuracy  of  the  four  methods  described.  The  pharma- 
copceial  tests  were  also  applied  and  the  results  embodied  in  the  fol- 
lowing table: 
Sample  I. 
Sample  II. 
Sample  III. 
Sample  IV. 
St'ng'h  by  KMn04 
(titr.)  method  . 
10-3;  10-35  vol. 
i5'45;  I5'45 -vol. 
29-45;  29-35  vol. 
26-65;  26*65  p  c. 
St'ng'h  by  KMn04 
(gasom.)  method 
io'6;  10.3;  io'i;  1C4  vol. 
15*5;  15*35  vol. 
30-15;  31-6  vol. 
27-85;  26*75  P-C. 
St'ng'h  by  Ca(ClO)2 
method  
10*45;  I0"4  vol. 
I5'5;  15*55  vol. 
29-7;  29  9  vol. 
26-95;  27-2;  27-1 
p.c. 
26-2;  26*4  p.c. 
St'ng'h  by  Na2S203 
method  
10-3;  10-35  vol. 
15*5;  15*55  vol. 
29-4;  29-4  vol. 
Acidity  in  cub.cms. 
of  ^  soda  requ'd 
1-85  c.c. 
o'339  gm 
HC1,  NaCl,  H3P04 
0-95  c.c. 
0-029  gra. 
HC1,  H3PQ4 
2"I  C.C. 
Neutral 
Residue    from  50 
c.c.  at  ioo°  C.  .  . 
Impurities  found  . 
0-124  gm. 
HC1,  H2S04,  H3PO4 
0-015  gm.  from 
1  tube  (3-025  gm.) 
Not  identified 
