XI, A, 1 Heise and Aguilar: Oxygen Consumption of Waters 43 



In these series the odor of chlorine could be plainly detected 

 during digestion with permanganate and sulphuric acid in the 

 samples containing over 10 parts per million of chlorides. The 

 results, although lacking somewhat in uniformity, showed that 

 an appreciable error is introduced even at 30° if the final titra- 

 tion is made at boiling temperature. 



At 10° C. the error, although slighter, is still appreciable, as 

 is shown by the data in Table VII. 



Table VII. — Effect of chlorides on the oxygen-consuming capacity. 



[Temperature of digestion, 10° C] 



Chlorine con- 

 tent (parts 

 per million). 



Perman- 

 ganate 

 required. 



Error intro- 

 duced in 

 terms of oxy- 

 gen con- 

 sumed (parts 

 per million). 







10 



100 



250 



1,000 



cc. 



11.43 

 11.50 

 11.50 

 11.61 



12.98 



0.0 



0.03 



0.03 



0.09 



0.8 



Only part of the error can be ascribed to interaction of the 

 permanganate and chlorides during digestion, as this reaction 

 goes on at greatly accelerated speed while the sample is being 

 heated to boiling; in fact, the amount of permanganate reduced 

 during digestion at low temperature, although not negligible, is 

 small as compared with that reduced during the short heating 

 period. It is, therefore, possible to eliminate to some extent 

 the error due to chlorides by performing the digestion at a 

 moderate temperature (preferably below 15° C.) and titrating 

 the excess of permanganate with sodium thiosulphate in the 

 presence of potassium iodide and starch indicator. This is 

 shown by the data in Table VIII. 



Table VIII. — Effect of chlorides on titration methods. 



Temperature 

 of digestion. 



Time of di- 

 gestion. 



Chlorine 



content 



(parts per 



million). 



Oxygen consumed (parts 

 per milloji) . 



Cold ti- 

 tration with 

 thiosulphate. 



Hot titration 



with per- 

 manganate. 



10 

 10 

 28 

 28 



Hrs. 



24 



24 



4 



4 





 1.000 





 1,000 



0.34 

 0.78 

 0.55 

 0.72 



0.62 

 1.53 

 0.64 

 1.03 



