MEASUREMENTS OF GAS EXCHANGE 137 



niques. It must be included in the equation because a certain amount 

 of the gas we wish to measure will remain dissolved in the liquid. 



In biological experiments, Po is usually 10,000 mm of manometer 

 fluid. This round number is no coincidence, because the manometer fluid 

 is a specially prepared solution whose density will be 760/10,000 of the 

 density of mercury. The fluid most commonly used is Brodie's solution, 

 23 g of sodium chloride in 500 ml of water. A small amount of detergent 

 is added to prevent the liquid's sticking to the glass tube. The solution 

 is colored with a dye for ease in reading. 



Suppose that we wish to measure oxygen exchange at 25° C in one of 

 our manometers. We have found that the volume of the internal gas 

 space is 18.36 ml (18,360 /"-l). For convenience we prefer to use 3 ml 

 (3000 /Ltl) of fluid. Thus Vf = 3000 /ttl, and Vg is the total volume minus 

 Vf, or 18,360 — 3000 = 15,360 fA. For oxygen in water at 25° C, « is 

 found to be 0.028. If we use Brodie's solution, Po is 10,000 mm of 

 manometer fluid. The arithmetic is not difficult. We find that the con- 

 stant for oxygen in this vessel at this temperature is 1.42 (A of 02/mm, 

 This means that each miflimeter on the manometer corresponds to 1.42 

 microliters of oxygen. 



If we keep our slide rule handy, it is an easy matter to find the actual 

 amount of oxygen, even while the experiment is progressing. If we 

 wished to measure CO2, we could find another constant by substituting 

 the proper solubility figure in the equation. The example given here is a 

 reasonably sized constant. Most of the vessels in common use today are 

 of such a size that the constants for oxygen are between 1.00 and 2.00 

 lA 02/mm. 



The actual measurement is accomplished by placing the necessary 

 materials in the vessels and placing the vessels on the manometers. The 

 manometers are placed on the water bath with the vessels hanging into 

 the water. With the manometer stopcocks open, a period of 15 to 20 min- 

 utes is allowed for temperature equilibration. Then, as rapidly as pos- 

 sible, the manometers are closed, and the initial pressure and starting 

 time are recorded. The number of millimeters of pressure change is 

 recorded at five or ten minute intervals. The final treatment of data from 

 manometric measurements is used as an example in Chapter 14. 



In principle, the production or use of any gas could be measured by 

 this technique. Actually carbon dioxide and oxygen are the gases most 

 frequendy involved in biological reactions. Unfortunately, in respiration 

 and in photosynthesis one of these gases is utilized while the other is 

 produced. There is no net change, and the manometer will show no pres- 



