314 PHYSIOLOGICAL CHEMISTRY 



and separately funnel. The stopper of the funnel should be inserted just before 

 the expiration is finished, so that there is no opportunity for air to be drawn 

 back into the funnel. In order to saturate the plasma the separatory funnel 

 is turned end over end for 2 minutes, the plasma being distributed in a thin 

 layer as completely over the surface of the funnel's ulterior as is possible. After 

 saturation is completed the funnel is placed upright and allowed to stand for a 

 few minutes until the fluid has drained from the walls and gathered in the con- 

 tracted space at the bottom of the funnel. 



Determination of Carbon Dioxide. A sample of i c.c. (or 0.5 c.c. in case the 

 amount of plasma available is very small) accurately pipetted, is allowed to run 

 into the cup b in the apparatus represented in Fig. 100, the tip of the pipette 

 remaining below the suface of the plasma as it is added. The cup should have 

 been previously washed out with carbonate-free ammonia, and together with 

 the entire apparatus should have been filled with mercury to the top of the capil- 

 lary tube by placing the levelling bulb of mercury in position i. 



With the mercury bulb at position 2 and the cock / hi the position shown in 

 the figure the plasma is admitted from the cup into the 50 c.c. chamber, leaving 

 just enough above the cock e to fill the capillary so that no air is introduced 

 when the next solution is added. The cup is washed with two portions of about 

 0.5 c.c. of water, each portion being added to the pipette in turn. A small 

 drop 1 of caprylic alcohol is then added to the cup and is permitted to flow 

 entirly into the capillary above e. Finally 0.5 c.c. of 5 per cent sulphuric acid 

 is run in. 



It is not necessary that exactly i c.c. of wash water and 0.5 c.c. of acid shall 

 be taken, but the total volume of the water solution introduced must extend 

 exactly to the 2.5 c.c. mark on the apparatus, if the table on page 330 is to be 

 used. 



If the amount of plasma available is small a little more than 0.5 c.c. are 

 saturated in a 50 c.c. funnel, and exactly 0.5 c.c. used for the estimation of 

 carbon dioxide. In this case the volume of distilled water and acid used to 

 wash the plasma into the apparatus is halved, so that the total volume of water 

 solution introduced is only 1.25 c.c., and in the calculation the observed volume 

 of gas is multiplied by 2. 



After the acid has been added a drop of mercury is placed in b and allowed 

 to run down the capillary as far as the cock hi order to seal the latter. Whatever 

 excess of sulphuric acid remains in the cup is washed out with a little water. 



The mercury bulb is now lowered and hung at position 3 and the mercury 

 in the pipette is allowed to run down to the 50 c.c. mark, producing a Torricellian 

 vacuum in the apparatus. When the mercury (not the water) meniscus has 

 fallen to the 50 c.c. mark the lower cock is closed and the pipette is removed 

 from the clamp. Equilibrium of the carbon dioxide between the 2.5 c.c. of water 

 solution and the 47.5 c.c. of free space in the apparatus is obtained by turning 

 the pipette upside down fifteen or more times, thus thoroughly agitating the con- 

 tents. The pipette is then replaced in the clamp. 



By turning the cock / the water solution is now allowed to flow from the 

 pipette completely into d without, however, allowing any of the gas to follow 



1 It is desirable to keep the amount of caprylic alcohol small, as larger amounts may 

 appreciably increase results. With plasma 0.02 c.c. is sufficient to prevent foaming and 

 is measured most conveniently from a burette made by fusing a capillary stopcock to a 

 pipette graduated into o.oi c.c. divisions. 



