XI, A, 1 Heise and Aguilar: Oxygen Consumption of Waters 39 



EFFECT OF TIME OF DIGESTION 



A sample of river water (from the reservoir of the city of 

 Manila) containing less than 5 parts per million of chlorides 

 was used for these determinations. The oxygen-consuming 

 capacity was determined practically as outlined by the American 

 Public Health Association ^^ by adding 10 cubic centimeters of 

 dilute (1:3) sulphuric acid and 10 cubic centimeters of potas- 

 sium permanganate (1 cubic centimeter =0.1 milligram oxygen) 

 to 200 cubic centimeter samples of water in the cold and digest- 

 ing for varying periods in a boiling-water bath. After digestion, 

 10 cubic centimeters of oxalic acid were added, and the sample 

 was titrated at boiling temperature with permanganate. 



The analytical results are recorded in Table I. 



Table I. — Effect of time of digestion on the determination of the oxygen- 

 consuming capacity. 

 [Temperature of digestion, 100° C] 



Time. 



Perman- 

 ganate 

 required. 



Oxygen con- 

 sumption 

 (parts per 

 million). 



Min. 

 

 15 

 30 

 45 

 60 

 120 

 240 



cc. 

 0.82 

 1.10 

 1.27 

 1.29 

 1.34 

 1.75 

 2.46 



0.35 

 0.49 

 0.57 

 0.58 

 0.61 

 0.81 

 1.16 



The decomposition showed no signs of reaching an end point, 

 and proceeded regularly, although at the end of a four-hour 

 digestion the volume of the sample was greatly reduced. The 

 time of digestion is, therefore, a very important factor, and 

 strict adherence to a fixed digestion period is necessary to en- 

 sure concordant results. 



With digestion at room temperature, the effect, although less 

 marked, is still appreciable, as shown by Table II. 



Table II.- 



-Effect 



of time 



of digestion 



on 



oxygen consumption. 





[Temperature of digestion. 



28 



' C] 





Hours. 







Oxygen consumption. 

 (Parts per million.) 





2 











0.60 





4 











0.64 





6 











0.68 





8 











0.69 





24 



18 



Op. 



cit., 27. 





0.74 



