136 MEASUREMENTS OF GAS EXCHANGE 



(10-1) that the relationship will take this form if V and T are constant. 

 If we know the value of the constant k, we can easily find the amount of 

 gas corresponding to any change on the manometer. The value of k 

 will depend upon the conditions under which the experiment is per- 

 formed and upon the characteristics of the manometer-vessel combina- 

 tion. If the vessel is small, a small change in the amount of gas will make 

 a large change in the pressure and in the reading of the manometer. 

 In contrast, if the vessel is larger, the same amount of gas will make a 

 smaller change in the pressure. 



In order to compare the results of different experiments, we express 

 the amount of gas used or produced with reference to a set of standard 

 conditions, 0° C (273° K) and atmospheric pressure (760 mm Hg). 

 Chemists very often express amounts of gas in these terms because they 

 know that one mole of any gas, under these conditions, will occupy 22.4 

 liters, or any given volume of gas under these conditions will contain 

 a specific number of moles. Although it may seem difficult to transform 

 manometer readings to volume of gas under "standard conditions," we 

 can incorporate these corrections during the computation of the constant 

 k. An equation has been developed such that the constant k transforms 

 h millimeters of manometer fluid directly into microliters (at 0° C and 

 760 mm Hg) of the gas being measured. The complete equation follows: 



273° 



k = '—p (10-4) 



ro 



Vg = volume (in /Ltl) of the gas space in the particular vessel-manome- 

 ter combination. 



T = absolute temperature, or Celsius temperature +273°. 



Vf = volume (in fA^ of liquid in which the living cells are suspended. 

 oi m solubility of the gas in this hquid at this temperature. 



Po = the number of mm of this manometer fluid that would exert the 

 same pressure as 760 mm of Hg. In other words, this is the 

 "standard pressure" in terms of millimeters of manometer fluid. 



For any separate vessel-manometer combination we must measure the 

 internal volume. The easiest accurate means of finding this volume is to 

 find how much liquid the vessel will hold. Usually we fill the gas space 

 with mercury and then weigh the mercury. The density of mercury is 

 known quite precisely. Since it is great, a small difference in volume 

 brings about a large easily-measured change in weight. 



The solubility factor, a, may be found in physical tables, or more 

 easily, in the Umbreit, Burris, and Stauffer book, Manometric Tech- 



