346 WATER TIYGIENE 
bottle with a two-hole stopper having one glass tube extending to the 
bottom and another glass tube entering, but not projecting, into the 
bottle. Connect the short tube of the sample bottle with the long tube 
of the liter bottle. Immerse the sample bottle in the water and apply 
suction to the outlet of the liter bottle. To collect a sample at any 
depth arrange the two bottles so that the outlet tube of the liter bottle 
is at a higher elevation than the outlet of the sample bottle. Lower 
the two bottles in any convenient form of cage properly weighted, to the 
desired depth. Water entering during the descent will be flushed 
through into the liter bottle with a glass stopper in such a manner as 
to avoid entraining bubbles of air. 
Procedure. Remove the stopper from the bottle and add, first, 0.7 
c.c. of the concentrated sulphuric acid, and then 1 c.c. of the potassium 
permanganate solution. These and all other reagents should be intro- 
duced by pipette under the surface of the liquid. Insert the stopper 
and mix by inverting the bottle several times. After twenty minutes 
have elapsed, destroy the excess of permanganate by adding 1 c.c. of 
the potassium oxalate solution, the bottle being at once restoppered 
and its contents mixed. If a noticeable excess of potassium perman- 
ganate is not present at the end of twenty minutes again add 1 c.c. of 
potassium permanganate solution. If this is still insufficient use a 
stronger potassium permanganate solution. After the liquid has been 
decolorized by the addition of potassium oxalate add 1 c.c. of man- 
ganous sulphate solution and 3 e.c. of the alkaline potassium iodide 
solution. Allow the precipitate to settle. Add 2 ¢.c. of the hydro- 
chloric acid and mix by shaking. 
The procedure to this point must be carried out in the field, but 
after the acid has been added and the stopper replaced there is no 
further change, and the rest of the test may be performed within a 
few hours as convenient. Transfer 200 c.c. of the contents of the 
bottle to a flask and titrate with N/40 sodium thiosulphate, using a 
few cubic centimeters of the starch solution as an indicator toward 
the end of the titration. Do not add the starch solution until the 
color has become a faint yellow and titrate until the blue color 
disappears. 
The use of potassium permanganate is made necessary by high 
nitrite or organic matter. The procedure outlined must be followed on 
all work on sewage and partly purified effluents or seriously polluted 
streams or samples whose nitrite nitrogen exceeds 0.1 part per million. 
In testing other samples the procedure may be shortened by beginning 
with the addition of the manganous sulphate solution and proceeding 
