44 The Philippine Journal of Science i9i6 



It is evident that the error due to the presence of chlorides 

 persists even when digestion is carried on at moderate temper- 

 atures. In order to eliminate this error, digestion should be 

 carried on in alkaline solution according to the method of Shulze 

 or Winkler.i^ 



Even in alkaline solution the presence of chlorides may intro- 

 duce discrepancies. After digestion, when the solution is acidi- 

 fied before the addition of potassium iodide and titration with 

 thiosulphate, sufficient chlorine may be liberated seriously 

 to affect the results. With a chlorine content as high as 250 

 parts per million no serious error could be detected ; with a chlo- 

 rine content of 1,000 parts per million the discrepancy was 

 appreciable. 



A further error is caused by the actual reduction of potassium 

 permanganate to potassium manganate in the presence of chlo- 

 rides in alkaline solution, even when digestion is carried on at 

 room temperature (30° C). This was clearly indicated in a 

 series of 24-hour digestions carried on at room temperature in 

 alkaline solution by the change in color from red to green in 

 the samples containing much salt. 



A large number of experiments have been performed on diges- 

 tions with alkaline solutions, but their results were not suffi- 

 ciently conclusive to warrant their being included in this paper. 

 As might be expected, the results obtained in alkaline solution 

 are highly dependent on the concentration of alkali and on 

 the temperature and time of digestion, so that comparable results 

 can be obtained only by adhering to fixed procedure. 



EFFECT OF HYPOCHLORITES 



In studying the daily variation in oxygen consumption of a 

 municipal supply, certain discrepancies were noted which in- 

 duced us further to study the effect of hypochlorites or "free 

 chlorine" on the amount of oxygen consumed. Glaser^^ states 

 that through the addition of chloride of lime the oxygen con- 

 sumption is first increased, because more organic matter is 

 brought into solution — that is, into a more rapidly oxidizable 

 form. After that a reduction in the required quantity of per- 

 manganate occurs, owing to the oxidation of organic material 

 by the chloride of lime itself. He believes that the initial in- 

 crease in oxygen consumption could not be ascribed to inter- 



" Dingier' s polytech. Journ. (1868), 188, 197; Zeitschr. f. analyt. Chem. 

 (1914), 53, 561. 



"Arch. f. Hyg. (1913), 77, 268. 



