38 SEWAGE AND ITS PURIFICATION 



the Klibel modification, in order to intensify and shorten the 

 reaction, the liquid is boiled, in Germany usually for ten, in 

 America for two to five, minutes, with the objection that boiling 

 is apt to cause a spontaneous and irregular loss of oxygen from 

 the permanganate and acid alone, due to the variation in the 

 chlorides. Many years ago I found it safe to work at a tempera- 

 ture of 80° C. on an ordinary water-bath, instead of the 

 customary heat of 80° P., thereby shortening the time to two 

 and a half hours, in place of the usual four hours in the 

 cold. 



4. The greatest disturbing influence is the interference of 

 nitrites, which are abundant in certain stages of purification, 

 of high chlorides, and of iron and occasionally various other 

 salts derived from a chemical treatment. This objection has 

 not been satisfactorily eliminated, even by the adoption of the 

 different time limits proposed, such as three minutes, fifteen 

 minutes, two and a half or four hours. 



The process, as common to all modifications, is as follows : 

 A measured volume of the water or sewage is placed in a care- 

 fully-cleaned stoppered bottle of about double the capacity, 

 and acidified strongly with a uniform amount of pure sulphuric 

 acid ; then an excess of a standard solution of potassium per- 

 manganate is measured in, and the whole mixed by rotation. 

 At the same time a blank is mounted with equal volumes of 

 pure water and the reagents. Both are exposed to the same 

 temperature for the same time. The effect on the perman- 

 ganate must be watched ; should it happen that the red colour 

 pales rapidly, a further measured quantity of permanganate and 

 of acid must be at once added, as it is necessary that the 

 oxidizing agent should be present in excess up to the end of 

 the time. At the end both bottles are cooled, and a few drops 

 of freshly-prepared potassium iodide solution is added to each, 

 or a small crystal of the pure solid. Iodine is immediately 

 liberated in proportion to the amount of permanganate that has 

 remained unreduced by the organic matter. A centinormal 

 solution of sodium thiosulphate (2*4827 grammes Na2S203,8HoO 

 per litre) is then run in from a burette till the brown colour of 

 the iodine has nearly disappeared. A few drops of fresh, thin 

 starch solution are then added, and the addition of thiosulphate 

 continued till the blue colour has just disappeared. This titra- 

 tion must be accomplished rapidly, as it will be noticed that the 

 blue colour will reappear after standing. 



