104  Methods  of  Water  Analysis.  '  {AmMS,iS5ym^ 
amount  of  chlorides  will  vary  with  the  natural  source  and  remains 
fairly  constant.  However,  when  the  water  is  polluted  with  sewage 
or  household  refuse  the  chlorides  will  increase  in  proportion  to  the 
degree  and  nature  of  the  pollution,  and  this  increase  serves  as  a 
reliable  indication  of  past  or  present  pollution.  This  index,  how- 
ever, is  of  value  only  when  the  normal  chlorine  contents  of  the  water 
in  question  or  of  waters  in  the  immediate  neighborhood  is  known. 
On  the  foregoing  considerations  are  based  the  various  methods 
employed  in  the  chemical  analysis  of  water.  As  these  methods  are 
fully  described  in  books  on  the  subject,  I  shall  not  dwell  on  them 
here,  but  will  mention  the  modifications  which  I  found  useful  in  my 
work.  For  the  determination  of  turbidity,  free  and  albuminoid 
ammonia,  nitrates,  nitrites  and  iron  I  employ  Jackson's  standards, 
which  are  used  in  the  Mt.  Prospect  Laboratory,  Brooklyn,  and  are 
described  by  Mr.  Jackson  in  the  Technology  Quarterly,  Vol.  XIII,  No. 
4,  1900.  A  constant  use  of  the  standards  convinced  me  of  their 
accuracy  and  convenience.  They  offer  the  great  advantage  of  being 
always  on  hand  and  presenting  a  uniformity  of  composition  (color) 
not  attainable  when  the  standards  are  made  up  extemporaneously. 
However,  in  the  determination  of  turbidity  I  depart  somewhat  from 
Mr.  Jackson's  recommendations  and  make  use  of  100  c.c.  xx  tincture 
bottles,  glass  stoppered  (W.  T.  &  Co.),  instead  of  100  c.c.  Nessler 
tubes.  I  found  that  by  means  of  these  bottles  it  is  possible  to 
determine  the  turbidity  with  much  greater  accuracy.  In  determining 
nitrates  and  nitrites  I  treat  200  c.c.  of  the  water  with  an  excess  of 
precipitated  and  washed  aluminum  hydrate,  decanting  the  clear 
supernatant  fluid.  This  brings  about  complete  decolorization  of  the 
water,  a  condition  most  desirable  in  the  case  of  surface  waters  which 
are  frequently  colored,  the  color  interfering  with  the  proper  determi- 
nation of  nitrites  and  nitrates.  I  do  not  determine  the  loss  on 
ignition  for  the  reason  that  it  is  not  a  reliable  method  of  determining 
the  organic  matter  in  the  residue.  When  the  latter  is  subjected  to 
heat,  the  nitrates  are  decomposed  and  the  chlorides  volatilized  to  a 
considerable  extent,  while  some  salts  retain  the  water  of  crystalliza- 
tion despite  the  heating.  The  loss  on  ignition,  therefore,  does  not 
represent  the  amount  of  organic  matter  burned.  I  do,  however, 
heat  the  residue,  but  only  to  observe  the  charring  on  ignition.  The 
degree  of  charring  of  the  residue  does  indicate,  roughly,  of  course, 
the  amount  of  organic  matter. 
