MEASUREMENTS OF OXYGEN EVOLUTION 849 



(If the liquid volume is sufficiently large compared with the gas volume, 

 KcOi will be much smaller than Kq,.) 



The knowledge of these constants and assumption of a photosynthetic 

 quotient, Qp = AO./— ACO2 (compare chapter 3, Vol. I), permit one to 

 calculate the rate of photosynthesis from manometric readings in a single 

 vessel. If doubts arise as to the value of the photosynthetic quotient, the 

 latter can be treated as a second unknown, and an additional equation for 

 its determmation can be obtained by using a second Warburg vessel of dif- 

 ferent dimensions (e. g., the same vessel filled to a different level, or a vessel 

 \Adth equal liquid volume but an enlarged gas volume; cf. fig. 25. 3B). 

 The pressure increase in the first vessel is : 



(25.3a) Ap' = K^A02 + Kco.ACO, = AO, (^K^, - ^^ 



and that in the second vessel : 



(25.3b) Ap" = AO2 (a'o, - ^^ 



If the K values are known, the two equations permit the calculation of the 

 two unkno\^Tis, AO2 and Qp (Warburg) . 



Of course, this purely manometric determination of the rate of photo- 

 synthesis is only reliable if it is definitely known that no other gas except 

 oxygen and carbon dioxide is involved in the gas exchange. After Gaffron 

 (1942) became suspicious that this is not the case with certain anaerobically 

 incubated algae, he introduced chemical absorbers for oxygen, carbon di- 

 oxide and hydrogen into the side arms of the manometer, and proved in 

 this way the occurrence of hydrogen fermentation and photochemical 

 hydrogen consumption by these algae {cf. Vol. I, chapter 6). 



Two conditions must be strictly fulfilled if manometric measurements 

 are to be reliable. In the first place, when pressure changes of the order 

 of 0.1 mm. Hg (1 mm. Brodie solution) are to be measured, while the total 

 pressure is of the order of 100-1000 mm. Hg, maintenance of constant 

 temperature becomes very important. The reaction vessels must be 

 placed in a precision thermostat, and shaken vigorously to ensure continu- 

 ous thermal equilibrium. Vigorous shaking is needed also to satisfy a 

 second condition — rapid establishment of equilil)riTnn between free and 

 dissolved gases. This is essential both to prevent a lag in the manometric 

 readings and to make sure that no depletion of carbon dioxide occurs in 

 the lowest layer of the reaction vessel, where illumination (through the 

 flat bottom!) is strongest, and carbon dioxide consumption bj^ photosyn- 

 thesis is fastest. It is thus necessary both to mix the liquid rapidly 

 (to equalize carbon dioxide concentration) and to splash it vigorously (to 



