388  The  U.S.P.  Inorganic  Chemicals.      { Am^li^m.m' 
very  little,  hence  we  must  adapt  ourselves  to  the  commercial  con- 
ditions. 
Hydrochloric  Acid. — A  31-9  per  cent,  or  20  Be.  is  the  usual  acid 
employed  in  the  arts.  Another  acid  used  much  industrially  is  the 
22  Be.,  which  is  about  36- 1 8  per  cent,  strength. 
Nitric  Acid. — The  strongest  nitric  acid  used  in  the  arts  and  manu- 
factures is  the  42  Be.,  which  represents  a  strength  of  68  per  cent. 
Hence,  for  like  reasons  cited  under  sulfuric  acid,  the  strength  of 
this  is  made  to  conform  to  commercial  standards. 
Not  the  least  difficulty  should  be  experienced  in  procuring  the 
above  acids  fully  up  to  the  pharmacopceial  standards. 
Boric  Acid. — The  strength  of  99  8  per  cent,  allows  for  the  presence 
of  about  0*2  per  cent,  of  moisture,  such  possible  impurities  as  calcium 
and  sodium  sulfate  are  not  found  in  more  than  traces  in  boric  acid 
of  reliable  origin. 
Hydrobromic  Acid. — In  order  to  provide  against  the  presence  of 
other  acids,  two  methods  of  titration  have  been  introduced,  namely, 
by  neutralization  and  by  precipitation. 
Sulfuric  Acid,  Aromatic. — In  order  to  secure  uniformity  in 
strength,  based  on  the  percentage  of  absolute  sulfuric  acid  present, 
as  is  the  case  with  our  dilute  acids,  the  quantity  of  sulfuric  acid 
lias  been  placed  at  1 11  ex.,  or  203-24  grammes. 
Phosphoric  Acid. — It  will  be  noted  that  a  saturated  solution  of 
sodium  chlorid  is  added  to  the  solution  of  phosphoric  acid  to  be 
titrated,  which  is  done  for  the  following  reasons :  When  titrating 
phosphoric  acid  under  ordinary  conditions  the  end-reaction  is  not 
sharp,  phenolphthalein  giving  a  rose  tint  when  two-thirds  of  the 
acid  has  been  neutralized,  thus: 
H3P04  +  2NaOH  =  Na2HP04  +  2H20. 
According  to  the  phenolphthalein  reaction,  di-sodium  phosphate 
is  slightly  alkaline.  If,  however,  this  solution  is  diluted,  the  colora- 
tion disappears,  for  dissociation  takes  place  as  follows : 
Na2HP04  +  HOH  =  NaOH  +  NaH2P04. 
In  order  to  suppress  this  hydrolysis,  a  slight  excess  of  sodium 
chlorid  is  added,  the  end-point  of  titration  is  then  very  sharp. 
A  sample  of  phosphoric  acid  which  assayed  82-8  per  cent,  gravi- 
metrically  (as  magnesium  pyrophosphate),  titrated  82-35  per  cent. 
