MEASURING PHOTOSYNTHETIC ACTIVITY 251 



A gas analysis apparatus for carbon dioxide which can be used in the 

 field has been devised by Lundegardh.^" The air sample is drawn into a 

 gasometer and the amount of carbon dioxide is determined from the de- 

 crease in volume after this gas has been absorbed in barium hydroxide. 



Brown and Escombe ^^ used a pure 4 per cent caustic soda (made from 

 sodium) solution for the absorption of the carbon dioxide. This solution 

 was then subjected to double titration with two indicators, one of them, 

 phenolphthalein, sensitive to free carbon dioxide and the other, methyl 

 orange, reacting only with the excess mineral acid used in the titration. 

 These titrations are carried out in flasks provided with a cap and tubulures 

 for the delivery tubes of the burettes to prevent access of the carbon 

 dioxide of the air. 



An electrometric method has been devised by Spoehr and McGee " 

 based upon the absorption of carbon dioxide in barium hydroxide solution 

 and the determination of the change in concentration of the latter through 

 electrical resistance measurements. The method permits the determination 

 of relatively large quantities of carbon dioxide in an air-stream with a 

 high degree of accuracy so that changes in a stream containing over twenty 

 times the amount of carbon dioxide normally in the atmosphere can be 

 determined to 0.002 per cent. The type of cell used depends very much 

 upon the amount of carbon dioxide which it is necessary to measure and 

 the accuracy demanded. 



The fact that hot barium hydroxide absorbs carbon dioxide more 

 rapidly than a cool solution has been used by Warburg ^^ for the deter- 

 mination of small quantities of this gas. Thus weaker solutions of barium 

 hydroxide and a more compact apparatus can be used. 



It has long been known that aquatic plants are capable of using bi- 

 carbonates as a source of carbon dioxide, and that with the abstraction 

 of carbon dioxide from the solution by the plant the former takes on an 

 alkaline reaction. Osterhout and Haas =^* have made use of this fact, and 

 have developed a method for estimating the rate of photosynthesis based 

 upon the change in color of an indicator added to the water. Phenol- 

 phthalein is used as the indicator, and a comparison is made of the time 

 required to produce the same change in color. Within a restricted range 

 of the pH., the amount of photosynthesis as indicated by the evolution of 

 oxygen is approximately a linear function. Under these conditions it is 

 permissible to measure the amount of photosynthesis by determining the 

 change in pH. It is, of course, essential to use glass vessels which do not 

 give an alkaline reaction, and in the case any nutrient substance is added 

 to the solution which may change the buffer value, this must be allowed 



"Lundegardh, Biochem. Zcit., 131, 109 (1922). 



"Brown and Escombe, Phil. Trans. Roy. Soc, 193, 289 (1900). 



"'Spoehr and McGee, hid. and Eng. Chcm., 16, 128 (1924); Carnegie Inst, of 

 Washington, Pub. No. 325, pp. 28, 89 (1923). 



■"Warburg, Zeit. physiol. Chem., 61, 261 (1909). 



"Osterhout and Haas, Jour. Gen. PhvsioL, 1, 1 (1918). Osterhout, Jour. Bio. 

 Chew.. 35, 237 (1918). 



