28 STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



to a large carboy and from this to the electric pump. The rate at 

 which air is drawn through the apparatus is of minor importance 

 as long as it allows no accumulation of carbon dioxid in the respiration 

 chamber and as long as the carbon dioxid is completely absorbed 

 in the barium-hydroxide solution. 



The carbon dioxid absorbed by the barium-hydroxide solutions 

 was calculated from determinations of the changes in concentration 

 of this solution. At first this was done by titration with standard 

 acid; later this method was supplanted by the more rapid and more 

 accurate method of determining the electrolytic conductivity of the 

 solution. 



For the titration method a solution of about 0.1 normal strength 

 was used. This was made Up in 18-liter lots, and to each 1,000 c. c. 

 1 gram of barium chloride was added. The barium-hydroxide 

 solutions, through which carbon dioxid had passed, were transferred 

 to narrow bottles, the barium carbonate was allowed to settle, and 

 three aliquot portions of the clear supernatant solution were used for 

 titration with 0.1 normal hydrochloric acid. In order to attain very 

 accurate results with this method it is necessary to realize that 

 barium carbonate is slightly soluble in the barium-hydroxide solution. 

 From the work of Vesterburg^ and of Weissenberger^ it is evident 

 that the addition of barium chloride to suppress the hydrolysis of 

 the barium carbonate is essential in order to attain satisfactory 

 results with this method. This is a precaution frequently neglected 

 in determining rates of respiration by this procedure. 



On account of the labor and time involved in making the many ti- 

 trations required by the foregoing method, an electrolytic method 

 was devised which gave exceedingly satisfactory results. For this 

 purpose a pure barium-hydroxide solution was used, prepared with 

 C02-free water. Considerable preliminary experimentation was 

 necessary to work out a satisfactory and simple apparatus for the 

 electrolytic determinations. It was found quite impracticable to 

 incorporate the electrodes for the conductivity determinations within 

 the absorption tubes. The chief difficulty encountered here was, 

 in brief, that the electrodes became covered with barium carbonate 

 and thus greatly increased the resistance of the cell. Hence the 

 cell-constant at the beginning and end of the determination was not 

 the same; even a very slight deposit of barium carbonate, such as is 

 formed by wetting the electrodes with a barium-hydroxide solution 

 and allowing this to remain in the air for a few minutes, gave spurious 

 results. The procedure which was followed, therefore, was to transfer 

 the entire contents of the absorption tube into a glass vessel with a 

 wide mouth. This was carefully stoppered and the barium car- 



> Vesterburo, A. Zeit. physik. chem., 70, 550-568 (1910). 

 » Weissenberger, G. Ibid., 88, 257-270 (1914). 



