850 METHODS OF KINETIC MEASUREMENTS CHAP. 25 



increase and renew the liquid-air interface), thus accelerating the release of 

 oxygen into the gas space. 



Because of these complications, direct chemical methods for determina- 

 tion of carbon dioxide and oxygen appeal to some investigators as more 

 satisfactory than pressure measurements; the latter give only indirect 

 evidence as to the identity of the gases causing the pressure changes. In 

 spite of this, the manometric techniques have been resorted to again and 

 again because of certain advantages not available with chemical analysis. 

 The manometer measures change in pressure, regardless of total pressure of 

 the gas in question. Thus, 10 cu. mm. of carbon dioxide can be determined 

 with equal precision (assuming constancy of temperature), regardless of 

 whether the total amount of gas present is 50 or 500 cu. mm. This is not 

 true of direct chemical methods. The manometer has a fast response, so 

 that measurements can be made over short periods of light or darkness. 



The two-vessel method further requires exact identity of physiological 

 processes and identical time course of pressure equilibration in the two 

 vessels. 



For the most precise manometric work, a differential manometer may be 

 substituted for the usual open-type manometer (Warburg 1926). This 

 eliminates the disturbing influence of barometric pressure. Differential 

 manometers can be conveniently read to 1/100 of a millimeter with a cathe- 

 tometer. This technique has been especially developed for measurements 

 of the quantum requirement of photosynthesis (c/. chapter 29) . 



Several methods of magneiometric oxygen determination (based on 

 paramagnetism of the O2 molecule) have been developed. Pauling's 

 magnetic oxygen meter (Pauling, Wood and Sturdivant 1946) is fabricated 

 by Beckman, Inc. Its range (0-1 atm. O2) makes it not directly applicable 

 to precision measurements of photosynthesis. 



A new method for continuous determination of oxygen content in solu- 

 tion was introduced in 1938, based on the measurement of conductivity. 

 It is a form of the so-called polarographic analysis, which has found 

 numerous applications in modern analytical chemistry. The essential de- 

 vice is a small-surface cathode, in a solution such that the passage of the cur- 

 rent involves cathodic reduction of dissolved oxygen to H2O2. The maxi- 

 mum current that can pass through a cell with such an anode is determined 

 by the supply of oxygen to the anode by diffusion, and is therefore propor- 

 tional to the oxygen concentration. 



An apparatus for polarographic oxygen determination in biological 

 studies was developed by Petering and Daniels (1938) and applied by 

 Petering, Duggar, and Daniels (1939) to the determination of the quantum 

 yield of photosynthesis in Chlorella. The range of determinable concen- 

 trations is from about 5 X 10 ~^ m./l. upward to saturation. (Water satu- 

 rated with air at 25° contains 2.4 X 10"* m./l. O2.) A similar method was 

 used by Blinks and Skow (1938^) for the investigation of induction phe- 

 nomena in photosynthesis. They found it advisable to replace the usual 



