136 METHODS FOR THE ANALYSIS OF BLOOD 



5, but it usually varies slightly from that because of difference 

 in the factors of acid and alkali and because of the calibration 

 error of the 5 c.c. pipette used for measuring the acid. Conse- 

 quently the maximum accuracy is obtained by performing a 

 preliminary titration on 5 c.c. of the acid plus 20 c.c. of the distilled 

 water, using the same pipette, indicator, and end-point as in the 

 plasma titration. 



The following is a typical calculation on a normal human 

 plasma: 



0.02 N NaOH = HCl added 5.09 c.c. 



. 02 N NaOH taken in titration 2 . 03 c.c. 



0.02MNaHCO 3 in 2 c.c. plasma or 



0.01 M NaHCOs in 1 c.c. plasma 3.06 c.c. 



3 . 06-f- 100 = . 0306 = molecular concentration of NaHCO 3 in plasma. 

 3.06X22.4 = 68.5 volume per cent CO 2 bound as bicarbonate in the 

 plasma. 



Since the titration result represents c.c. of 0.01 M NaHCOs 

 rer c.c. of plasma, it is transformed into terms of molecular con- 

 centration of NaHCOs in the plasma merely by dividing by 100. 



For the sake of comparison with results of bicarbonate deter- 

 mination by the CO2 method (Van Slyke andCullen), the molecular 

 concentration is multiplied by 2240 in order to give results in terms 

 of c.c. of CC>2 per 100 c.c. of plasma. According to the gas laws the 

 amount of C(>2 contained in a M carbonate solution is 22,400 c.c. 

 per liter (measured as CO2 gas at 0, 760 mm.) or 2240 c.c. of gas 

 per 100 c.c. of solution. Hence multiplying the bicarbonate 

 molecular concentration by 2240, or multiplying the c.c. of 0.02 

 N acid used in the titration by 22.4, gives the volume per cent of 

 bicarbonate C(>2 in the plasma. 



Inversely, of course, dividing the volume per cent of CC>2, as 

 determined by Van Slyke and Cullen, by 2240 transforms the CCb 

 figures into terms of molecular concentration. 



Solutions. The Standard 0.02 N Sodium Hydroxide. The 0.02 

 N NaOH as a basis for the determination must, in order to main- 

 tain its value, be protected from contact with atmospheric carbon 

 dioxide and from glass. Even standing over night in a burette of 

 soft glass is likely to result in the solution of enough alkali to 

 raise the titration value of the standard solution. The standard 

 alkali should be kept in paraffined bottles, and the burette filled 

 with fresh solution each day that it is used. 



