40 



The Philippine Journal of Science 



1916 



Even at 10° C. the effect is still marked, although in this case 

 the difference between four- and eight-hour digestion is negli- 

 gible for practical purposes. An end point is apparently reached 

 after about seven hours, as is shown by the data in Table III. 



Table III.^ — Effect of time of digestion on oxygen consumption. 

 [Temperature of digestion, 10° C] 



ars. 



Oxygen consumption. 

 ( Parts per million. ) 



2 



0.63 



4 



0.66 



6 



0.67 



8 



0.71 



24 



0.71 



In digestion at or below room temperature with the method 

 described above, the oxygen consumed is the summation of that 

 used up during the digestion period and that required during 

 the time the sample is being heated to boiling temperature. 

 The latter amount is far from negligible; hence better results 

 are obtained by titrating the excess of potassium permanganate 

 in the cold with sodium thiosulphate, using potassium iodide 

 and starch indicator, than are obtained by adding an excess 

 of oxalic acid and titrating back with permanganate at boiling 

 temperature. The difference in results is shown in Table IV. 



The fact that no definite end point is attained, except per- 

 haps at low temperature, seems to show that great concordance 

 need not be expected when the digestion is carried on at different 

 temperatures. The discrepancy in results is indicated by the 

 data in Table IV. 



Table IV. — Effect of tem,perature and method of titration on oxygen- 

 consuming capacity. 



Tempera- 

 ture of 

 digestion. 



Time. 



Oxygen consumed (parts 

 per million). 



Cold titration 

 with thiosul- 

 phate. 



Hot titration 

 with oxalic 

 acid and per- 

 manganate. 



°C. 



10 



28 



100 



100 



Hours. 



24 



4 



0.5 

 W 



0.34 

 0.56 



0.62 

 0.64 

 0.79 

 0.5 





* Just heated to hoiling. 



