394  Caustic  Soda  Sold  as  Caustic  Potash.  {Am^J5t,M£m" 
quantity  of  water,  is  accurrately  neutralized  with  a  concentrated  so- 
lution of  tartaric  acid,  with  phenolphthalein  as  indicator.  Litmus 
does  not  answer,  as  normal  alkali  tartrates  are  alkaline  towards  it. 
Exactly  the  same  volume  as  was  required  for  neutralization  is  then 
added  to  the  solution  to  convert  the  tartrates  into  bitartrate  of  potas- 
sium, which  is  precipitated  almost  completely,  owing  to  its  sparing 
solubility,  and  bitartrate  of  sodium,  which  remains  in  solution.  By  the 
addition  of  alcohol  the  precipitation  of  potassium  bitartrate  is  made 
complete.  The  precipitate  is  collected  on  a  filter  and  washed  with 
alcohol  until  a  drop  of  the  washings  no  longer  reddens  blue,  or  pre- 
ferably neutral,  litmus  paper  previously  moistened  with  distilled 
water.  The  filtrate  is  titrated  with  decinormal  potassium  or  sodium 
hydrate,  each  cubic  centimeter  corresponding  to  0-004  gramme  of 
sodium  hydrate.  The  precipitate  of  potassium  bitartrate  on  the 
filter  is  transferred  to  a  beaker,  water  added,  and  then  volumetric 
alkali  until  neutralized  with  phenolphthalein  as  indicator.  Heating 
the  mixture  hastens  solution  of  the  bitartrate.  Each  cubic  centi- 
meter of  normal  alkali  required  indicates  0-056  gramme  of  potassium 
hydrate.  If  preferred,  the  precipitate  may  be  dried  and  weighed, 
and  the  weight  obtained  multiplied  by  0-3,  which  gives  the  potas- 
sium present  in  the  sample  as  potassium  hydrate. 
The  actual  amount  of  caustic  and  carbonated  alkali  must  be  deter- 
mined by  titration  with  sulphuric  or  hydrochloric  acid.  The  fact  is 
frequently  overlooked  that  in  titrating  caustic  alkalies  in  presence  of 
phenolphthalein  in  the  cold,  one-half  of  the  carbonate  present  is  also 
included  and  calculated  as  hydrate.  This  is  due  to  the  fact  that  the 
bicarbonates  of  potassium  and  sodium  are  neutral  to  phenolphtha- 
lein. When  the  solution  is  sufficiently  dilute,  practically  no  carbon 
dioxide  will  have  escaped  when  the  neutral  point  is  reached  and  all 
carbonate  will  have  been  converted  'into  bicarbonate,  as  shown  by 
the  following  equations  : 
KOH  +  HC1  ==  KC1  -f  H20  (neutral  to  phenolphthalein). 
K2C03  +  HC1  =  KC1  +  KHCO3  «  " 
If  methyl  orange  be  now  added  and  the  titration  continued  until 
the  color  changes,  the  bicarbonate  will  have  been  converted  to 
chloride,  thus : 
ECHCO3  +  HC1  =  KC1  -f  C02  +  H20  (neutral  to  methyl  orange). 
