72 Composition of the Atmosphere 



be fully compensated or readily corrected; fourth, barometric changes in 

 pressure taking place during an analysis should be fully compensated; 

 fifth, there should be an equal tension in the gas before and after absorbing 

 oxygen in the final measurement; and sixth, the contraction in volume as 

 measured should be due only to the absorption of oxygen. 



Absorbents for oxygen. Of the numerous absorbents for oxygen, in- 

 cluding phosphorus, potassium pyrogallate, and, more recently, sodium 

 hydrosulphite, there seems to be but little choice with regard to the com- 

 pleteness of absorption. Although both phosphorus and potassium 

 pyrogallate are affected somewhat by low temperature, when properly 

 handled they absorb oxygen completely. While the same is true of 

 sodium hydrosulphite, certain difficulties in the way of handling this re- 

 agent have precluded its general adoption by chemists. 1 



Formation of by-products. The absorption of oxygen is invariably 

 an oxidative process. Usually the products of oxidation are non-gaseous, 

 particularly when phosphorus and metallic absorbents are used. On the 

 other hand, it has been claimed that in the interaction between oxygen 

 and potassium pyrogallate a small amount of carbon monoxide is formed. 

 This militated greatly against the use of potassium pyrogallate in the 

 earlier stages of its introduction, but in more recent years a study of its 

 composition has led to changes in the general method of using this re- 

 agent so that the formation of any measurable amount of carbon monoxide 

 has been practically precluded; hence as satisfactory results can be obtained 

 with potassium pyrogallate as with phosphorus. 



Correction for temperature changes. From the time when the sample of 

 gas is first measured until after the absorption of either carbon dioxide 

 or oxygen and its subsequent measurement, there should be no material 

 alteration in the volume of the gas due to temperature. Modern ap- 

 paratus corrects for these temperature changes by means of a compensa- 

 ting vessel or pipette of the same size and in the same temperature en- 

 vironment as the vessel used for measuring the sample. Frequently 

 both vessels are immersed in a water-bath which is constantly stirred to 

 secure temperature equilibrium. 



Barometric fluctuations. While usually of slight moment, inasmuch 

 as the analyses can be readily carried out in a few minutes, barometric 

 changes should also be taken into consideration. Particularly is this the 

 case in exact gas-analysis when the period of contact between the air 

 and the various reagents must be longer than in the ordinary technical 

 analysis. In some analyses it may require 30 minutes or more for com- 



1 The use of sodium hydrosulphite, employed by Tobiesen (Skandinavisches Archiv 

 fur Physiologie, 1895, 6, p. 278), has been more recently brought to the attention of physi- 

 ologists by Durig, of Vienna. (See Biochemische Zeitschrift, 1907, 4, p. 65.) The 

 necessity for preventing the corrosive action of this reagent on glass has called for a 

 certain technique that is not readily acquired; for example, Durig coats the inside of his 

 pipette thinly with gutta percha. The absorption coefficient of the solution of this 

 reagent is extremely high, but as yet the chemical has not come into general use, al- 

 though it can now be readily purchased in a pure form. 



