PAPERS ON WATER SUPPLY 



93 



containing free iodine, was divided into two portions. One 

 portion was kept exposed to the air in beakers, the other por- 

 tion was kept in stoppered bottles out of contact with air. The 

 results are given in Table IV. 



TABLE IV. 



Reduction of iodine by organic matter. 



Percent Sewage 25 50 75 100 



Dissolved Oxygen 8.08 5.16 2.41 .053 0.00 



Dissolved Oxygen, in closed 



bottles after standing 1 week 8.00 4.40 1.54 3.30 0.00 

 Dissolved Oxygen in beakers 0.00 in 0.00 in 0.00 in 0.00 in 



1 wk. 2 da. 1 da. ^ da. 



It would seem from this table that organic matter does not 

 reduce iodine to an appreciable extent under the conditions of 

 the Winkler method, in which the iodine solution is kept in 

 closed bottles until ready for titration, and is exposed to the 

 air but a few minutes during titration. 



In the next series of experiments .5 p. p.m. N as nitrite and 5 

 per cent sewage was added. Then, keeping the nitrite constant, 

 the sewage content was increased to 10 per cent. Results are 

 given in Table V. 



table v. 



Aerated, .5 p. p.m. N as nitrite 



distilled, H 2 5 pet. sewage 10 pct.sewage 



Levy 8.23 7.90 7.60 



Winkler 7.55 7.54 6.99 



Rideal-Stewart 7.07 6.54 



Hale-Melia 7.14 6.47 



Finally, 1.0 p.p.m. N as nitrite and 5 per cent sewage was 



added, then, keeping the nitrite constant, the sewage content 



was increased to 10 per cent. Results are given in Table VI. 



table VI. 



Aerated, 1.0 p.p.m. N as nitrite 



distilled, FLO 5 pet. sewage 10 pet. sewage 

 H 2 



Levy 8.95 8.50 7.72 



Winkler 7.99 8.36 7.28 



Rideal-Stewart 8.36 6.76 



Hale-Melia 8.41 6.73 



Levy values are invariably higher than any of the others; 

 Winkler values are high in the presence of high nitrites; 

 Rideal-Stewart and Hale-Melia agree very well. 



The experiments shown in Tables I, II, III, V, and VI have 

 been duplicated, with similar results, but the tables have 

 not been included in this paper. In all, 300 dissolved oxygen 

 determinations were made. 



