208  Volumetric  Solutions  of  US.P.        { Am'Mayri9ioarm' 
But  even  if  it  should  be  desired  to  obtain  the  metallic  silver  in 
a  condition  which  may  be  regarded  as  absolutely  pure  for  all  analyt- 
ical purposes,  it  can  readily  be  obtained  by  electrolyzing  a  solution 
of  silver  nitrate,  as  shown  by  the  work  of  Gooch  and  Perkins ; 19 
while  if  it  also  be  desired  to  use  precautions  taken  by  Richards  in 
his  atomic  weight  determinations,  we  need  only  fuse  the  silver  in 
a  boat  of  pure  lime  placed  in  an  atmosphere  of  hydrogen.  In  other 
words,  if  we  adopt  pure  metallic  silver  as  the  standard  for  our 
volumetric  solutions,  we  enable  chemists  and  pharmacists  to  obtain 
such  standard,  even  commercially,20  in  a  condition  not  requiring 
further  purification,  or  to  prepare  same  in  the  laboratory  with 
comparatively  very  little  work.  It  would  probably  also  avoid  such 
dissimilarity  of  results  as  that  seen  in  the  case  of  potassium  bitar- 
trate,  for  example,  which,  as  already  mentioned,  Parsons  states  that 
although  following  Borntrager's  directions,  he  had  to  recrystallize 
three  times  in  addition  in  order  to  prepare  the  pure  salt ;  or  the 
different  results  obtained  by  Lunge  when  studying  Kiihling's  potas- 
sium tetroxalate,  to  which  reference  has  already  been  made. 
And  we  can  also  readily  see  why  there  should  be  such  a  striking 
difference  between  metallic  silver  as  the  standard  and  most  of  the 
other  substances  which  have  been  proposed  for  this  same  purpose. 
For  in  the  case  of  metallic  silver  we  are  dealing  with  an  element — a 
substance  which  is  undecomposable  by  any  means  at  present  known 
— whereas  in  the  case  of  most  of  the  other  substances  proposed  as 
standards,  we  are  dealing  with  compounds,  many  of  which  are  more 
or  less  easily  hydrated,  some  of  which  are  hygroscopic,  and  nearly 
all  of  which  are  more  or  less  soluble  in  water  and  hence  readily 
affected  by  moisture. 
The  advantages  of  metallic  silver  as  the  ultimate  standard  in 
volumetric  analysis  are,  however,  not  entirely  due  to  its  elementary 
nature  but  largely  to  its  own  peculiar  properties.    Thus,  when  com- 
19  Zeit.  anorg.  Chem.,  63,  pp.  322-323  (1909). 
20  It  is  interesting  to  note  in  this  connection  the  various  descriptions 
given  to  Merck's  "  Tested  Reagents."  Thus  we  note :  Potassium  tetroxalate, 
for  use  in  "  preparation  of  volumetric  solutions."  Sodium  carbonate,  an- 
hydrous— for  use  as  "  starting  material  for  preparation  of  volumetric  solu- 
tions." Oxalic  acid — for  use  in  "  preparation  of  volumetric  solutions." 
Potassium  bitartrate— for  use  as  "  starting  material  for  preparation  of 
volumetric  solutions."  Silver  sheets,  for  use  in  "standardizing  volumetric 
solutions."  (Prices  and  Uses  of  Tested  Reagents,  Merck  &  Co.,  New  York, 
1909). 
