34 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES 



Briefly stated the method for measuring the H-ion concentration con- 

 sists in preparing a series of solutions containing known concentrations 

 of H-ion that is to say, of known P H and adding to each solution an 

 equal amount of an indicator which exhibits easily distinguishable 

 changes in tint at H-ion concentrations approximating those believed 

 to be present in the unknown solution. The same indicator is added to 

 the unknown solution, which is then placed side by side with the stand- 

 ards to find with which of them it most closely matches. The series 

 of solutions of known H-ion concentration is prepared by mixing fif- 

 teenth normal solutions of Na 2 HP0 4 and KH 2 P0 4 in varying propor- 

 tions as given in the following table: 



PREPARATION OF STANDARD SOLUTIONS 

 The solutions are mixed in the proportions indicated below to obtain the desired PH:* 



PH 6.4 6.6 6.8 7.0 7.1 7.2 7.3 7.4 7.S 7.6 7.7 7.8 8.0 8.2 8.4 



Primary Potas. 73 63 51 37 32 27 23 19 15.8 13.2 11 8.8 5.6 3.2 2.0 



Phos., c.c. 

 Secondary Sodium 27 37 49 63 68 73 77 81 84.2 86.8 89 91.2 94.4 96.8 98.0 



Phos., c.c. 



(From Levy, Rowntree and Marriott.) 



*Standard phosphate mixtures are prepared according to Sorensen's directions as follows: 

 1/15 mol. acid or primary potassium phosphate. 9.078 grams of the pure recrystallized salt 

 (KH2PO4) are dissolved in freshly distilled water and made up to 1 liter. 



1/15 mol. alkaline or secondary sodium phosphate. The pure recrystallized salt (Na 2 HPC>4. 12H 2 O) 

 is exposed to the air for from ten days to two weeks, protected from dust. Ten molecules of water 

 of crystallization are given off and a salt of the formula! NaoHPO 4 .2H 2 O is obtained; 11.876 grams 

 of this are dissolved in freshly distilled water and made up to 1 liter. The solution should give a 

 deep rose red color with phenolphthalein. If only a faint pink color is obtained, the salt is not 

 sufficiently pure. 



The indicator method is extremely accurate when used with pure 

 solutions of acids, but, as mentioned above, it is apt to be inaccurate, at 

 least with most indicators, when protein or inorganic salts are pres- 

 ent in the solution, and of course it is quite unusable with colored 

 fluids such as blood. In order to overcome these difficulties, the 

 dialysis method has recently been evolved. It consists in placing the 

 fl u id blood, for example in a dialyser sac composed of celloidin and 

 about as large as a small test tube. The sac is placed in a wider test 

 tube of hard glass containing an isotonic solution of sodium chloride 

 that has been carefully tested to ascertain that it is strictly neutral. 

 The amount of blood or serum required for this method is only 2 or 

 3 c.c., and the amount of salt solution placed outside the sac should be 

 about the same. It takes only from five to ten minutes for dialysis to 

 occur. The celloidin sac is then removed, a few drops of the indicator 

 are thoroughly mixed with the dialysate, and the tube compared with 

 the series of standards until the corresponding tint is matched. This 

 indicates the H-ion concentration in the dialysate. The tints produced 

 by using sulphonephenolphthalein are reproduced as nearly as possible 



