42 



The Philippine Journal of Science 



1916 



When the concentration of salt increased beyond 250 ±, it 

 became difficult to obtain concordant results, as the reaction 

 then appeared to be far from equilibrium at the end of the 

 half-hour digestion period. 



It is clear from the data presented that the analysis will give 

 concordant, comparable results only when the chlorine content 

 is extremely low. As little as 5 parts per million introduce an 

 appreciable error, while 50 parts per million, according to our 

 results, will cause a decomposition of permanganate correspond- 

 ing, in a 100 cubic centimeter sample, to an increase of 0.2 

 part per million in oxygen-consuming capacity. 



It is interesting to note that the method of digesting at room 

 temperature does not overcome the errors noted. Indeed 

 Dupre ^* had already pointed this out and recommended that 

 the digestion be carried on at a temperature as near 0° C. as 

 possible. We conducted a series of digestions at room tem- 

 perature (30° C.) over four hours and obtained the results 

 shown in Table VI. 



Table VI. — Effect of dissolved chlorides on the determinations of the 

 oxygen-consuming capacity. 



[Sample, 100 cubic centimeters ; digested for four hours at 80° C] 

 SERIES 1. 



Chlorine con- 

 tent (parts 

 per million). 



Perman- 

 ganate 

 required. 



Difference in 

 perman- 

 ganate 

 required. 



Error intro- 

 duced in 

 terms of 

 oxygen con- 

 sumed (parts 

 per million). 





 5 

 10 

 20 



ec. 

 1.31 

 1.38 

 1.46 

 1.48 



cc. 





0.07 

 0.15 

 0.17 



0.07 

 0.15 

 0.17 



SERIES 2. 

 [Twenty-four hours digestion period.] 







0.73 







253 



0.80 



0.07 



0.06 



355 



0.88 



0.15 



0.14 



442.5 



1.00 



0.27 



0.26 



591 



1.18 



0.45 



0.44 



887.5 



1.31 



0.58 



0.56 



1,183 



1.62 



0.89 



0.87 



1,775 



2.50 



1.77 



1.73 



3,550 



3.93 



. 3.20 



3.13 



'Analyst (1886), 10, 188. 



