PAPERS ON WATER SUPPLY 91 



the sample stands for a long time before titration. If the po- 

 tassium acetate is added from 15 to 30 minutes before the titra- 

 tion is carried out, the amount reduced is negligible. 



As to organic matter, Hale and Melia claim that the iodine 

 is not reduced to an appreciable extent by the organic matter 

 present in mixtures of sewage and water. 



The importance of comparing these methods has been real- 

 ized by the Committee of the Laboratory Section of the 

 American Public Health Association and a subcommittee was 

 appointed in 1914 to undertake a comparative study to de- 

 termine the accuracy, ease of manipulation and adaptability for 

 laboratory and field work of the four methods. The chairman 

 of the subcommittee, Professor E. B. Phelps, outlined the 

 work for the committee. 



In this paper the results and conclusions from my experi- 

 mental work on these four methods are presented. 



EXPERIMENTAL 



Distilled water was aerated, and after 24 hours, its dis- 

 solved oxygen content was determined by both the Levy and 

 Winkler methods. 



Then .2 parts per million as nitrite was added in the form of 

 sodium nitrite (NaN0 2 ) and the Levy Winkler, Hale Melia 

 and Rideal-Stewart determinations made. Three samples were 

 taken for each determination. Enough sodium nitrite was 

 then added to bring the N as nitrite content up to .5 parts per 

 million and the dissolved oxygen was again determined by the 

 four methods. The averages of three determinations are re- 

 ported in Table I. All except the Levy have been corrected 

 for the volume of reagents introduced, as Winkler 1 claims 

 that the concentrated solutions do not contain any dissolved 

 oxygen. The standard permanganate for use with the Levy 

 method and the standard thiosulfate for the other methods 

 were checked against each other a number of times, and both 

 solutions were found to be correct. 



TABLE I. 



Aerated, 

 distilled, HX> 



Levy 8.23 



Winkler 7.64 



Rideal-Stewart 



Hale-Melia 



1. Zeit. Anal. Chem., 11, pp. 665. 



