﻿6 METHODS FOR THE DETERMINATION OF 



little of the standard permanganate, and when not in use some of the 

 solution is always kept in them so as to assure complete cleanliness 

 from any reducing substance. Before using the jars are drained. The 

 sample of air is collected by pumping out the air contained in the jars 

 with a small hand bellows and allowing the air to be examined to flow 

 in. Fifty cubic centimetres of the standard solution of permanganate 

 are run into each jar, when it is tightly stopped and well shaken for at 

 least five minutes. Twenty -five cubic centimetres of the permanganate 

 solution are then withdrawn by means of a pipette and placed in a 

 glass cylinder holding about 200 c. c. Twenty -five cubic centimetres 

 of the fresh standard solution are placed into a similar glass cylinder 

 for comparison. Both of these solutions are then diluted up to 150 

 c. c. with distilled water and allowed to stand for ten minutes, after 

 which the tints of the two solutions are compared. Standard solution 

 of permanganate is now run into the first cylinder from a burette, until 

 the tints of both solutions are of the same intensity ; usually from ^ 

 to 6 c. c. of the standard solution are required. The amount of stand- 

 ard solution added to the solution in the first cylinder is a measure of 

 the bleaching effected by the organic matter in the known volume 

 of air on one-haK of the permanganate employed. The results may be 

 expressed either in terms of the number of cubic centimetres of the 

 xcroQ- solution bleached by 1 litre of air, or, as they prefer, by the 

 number of volumes of oxygen required to oxidize the organic matter 

 in, say, 1,000,000 volumes of air, i e., the 25 c. c. of solution from a 

 8.5 litres flask, in which 50 c. c. of standard solution had been used, 

 required 3 c. c. to bring its tint up to that of the cylinder containing 

 the fresh standard, or the entire 50 c. c. would have required 6 c. c. 

 This represents the number of cubic centimetres of standard perman- 

 ganate bleached by 3,500 — 50 = 3,450 c. c. of air ; consequently -g-.f^- 

 = 1.74 c. c. is the bleaching effected by 1 litre of air, or 0.0000097 

 litre of oxygen, or 9.7 volumes of oxygen are required to oxidize the 

 organic matter in 1,000,000 volumes of air. Correction for tempera- 

 ture is considered unnecessary, as its effect falls within the limits of 

 experimental error. 



Nekam '^ repeated Uffelmann's experiments, giving particular 

 attention to the question whether all of the organic matter is absorbed 

 by the permanganate; and also whether the permanganate suffers 

 reduction from other reducing agents in the air. He found the 

 permanganate to undergo spontaneous reduction while it oxidizes the 

 organic matter very slowly. 



Archarow ^^ also employed Uffelmann's method, but so modified 

 the apparatus as to conduct the air through the permanganate in a fine 

 stream. The permanganate was also kept at a somewhat higher tem- 



