280 WATER HYGIENE 
Determination. Place 100 c.c. of the sample in a tared platinum dish 
and evaporate to dryness over a water bath. Then heat the dish in an 
oven at 103° C. or 180° C. for one hour. The weight of the residue mul- 
tiplied by 10 gives the parts per million in the sample. 
CHLORINE 
Chlorine in a water is often a good index of pollution since it is usually 
high in sewage. A normal urine may contain in a day from 10-15 gms. 
of chloride calculated to sodium chloride. The interpretation of 
chlorine results demands a knowledge of the local conditions about the 
source of the sample. The amount of chlorine which is normal for the 
district from which the sample was taken must be known. 
Determination. Chlorine may be determined by titrating the water 
with silver nitrate in the presence of potassium chromate as an indicator. 
Add 1 «.c. of the indicator to 50 ¢.c. of the sample and titrate with the 
standard solution of silver nitrate. A standard of 1 c.c. of indicator 
in 50 c.c. of distilled water should be kept for comparison. The number 
of cubic centimeters of silver nitrate solution required multiplied by 10 
gives the titrated chloride (Cl) in parts per million. 
The solutions which are required in the determination of chlorine 
in water may be prepared as follows: 
1. Potassium Chromate. Dissolve 50 gms. of potassium chromate 
in a small amount of distilled water. Add sufficient silver nitrate 
to produce a slight red precipitate, filter and dilute to 1 liter with dis- 
tilled water. 
2. Standard Sodium Chloride Solution. Dissolve 16.48 gms. of pure 
fused sodium chloride in distilled water. One hundred c.c. of this 
stock solution should be diluted to a liter in order to secure a solution 
which will contain 0.001 gm. of chloride. 
3. Standard Silver Nitrate Solution. Dissolve 2.40 gms. of silver 
nitrate in a liter of distilled water. Standardize this with the above 
standard sodium chloride solution and adjust the volume so that each 
cubic centimeter is equivalent to .0005 gm. of chloride. 
NITROGEN 
Nitrogen furnishes the chemist a convenient index of bacterial 
activities in water. There were times in sanitary work when waters 
were condemned for, what was considered then, excessive amounts of 
the various nitrogen compounds. At one time nitrites were regarded 
as an infallible index of pollution. These different determinations of 
