224  Apparatus  for  Estimating  Urea.  {^VaT'is^ys"""' 
ting  india-rubber  stopper,  perforated  with  a  hole  in  which  is  inserted  a 
short  piece  of  glass  tubing,  open  at  both  ends,  and  is  then  connected 
with  the  measuring  tube  by  means  of  a  piece  of  elastic  tubing.  It  is 
now  inclined  so  as  to  allow  the  urine  to  mix  with  the  hypobromite. 
Effervescence  at  once  commences,  and  as  it  proceeds  the  measuring 
tube  is  gradually  raised  so  as  to  relieve  the  disengaged  nitrogen  from 
the  hydrostatic  pressure.  The  flask  is  shaken  a  few  times,  and  when 
the  reaction  is  completely  over,  the  apparatus  is  left  for  a  few  minutes 
until  it  has  acquired  the  temperature  of  the  room  in  which  the  experi- 
ment is  performed.  Another  exact  levelling  of  the  measuring  tube  is 
made,  and  the  number  of  the  division  corresponding  to  the  volume  of 
the  developed  nitrogen  is  read  off.  Since  55  c.c.  correspond  to  0*15 
grm.  of  urea,  a  single  division  corresponds  to — 
0-15 
=  0*005  gi*"^'  urea. 
Consequently,  if  n  is  the  number  of  measures  of  nitrogen  obtained  in 
an  experiment,  0*005   X  ^  will  represent  the  amount  of  urea  present. 
But  as  the  quantity  of  urine  generally  experimented  on  is  5  c.c,  if  x 
be  the  percentage  of  urea  in  the  urine, —  will  be  the  urea  in  5  c.c. 
20 
X 
Hence  we  have  =  0*005  X  n.  and  x  =  0'i  x  n. 
20 
It  therefore  follows  that  if  we  operate  on  5  c.c.  of  urine  each  meas- 
ure of  nitrogen  evolved  will  correspond  to  0*1  per  cent,  of  urea. 
The  accompanying  rough  sketch  represents  the  apparatus  just  before 
the  flask  is  inclined  so  as  to  bring  the  urine  and  the  hypobromite  solu- 
tion into  contact. 
The  following  results,  obtained  from  known  quantities  of  pure  urea, 
will  give  an  idea  of  the  accuracy  which  is  attainable  by  this  process  : 
c.c.  of  a  2  p.c. 
Measures  of 
Weight  of  urea 
Weight  of  urea 
urea  solution. 
nitrogen  evolved. 
taken. 
found. 
7*4 
30*0 
0*148 
0*150 
7*2 
28*0 
0*144 
0*140 
6*0 
23*8 
0*120 
0*1 19 
5*0 
19*5 
0*100 
0*097 
4'4 
17*0 
0*088 
0-085 
4*0 
16*0 
0*080 
o-o8o 
3-0 
12*0 
o*o6o 
o*o6o 
2*0 
8*0 
0*040 
0*040 
1*0 
4.0 
0*020 
0-020 
In  working  with  a  specimen  of  urine,  three  experiments  gave  on 
