A DEFINITE H-ION CONCENTRATION 161 



STANDARDIZATION TO A DEFINITE H-ION 

 CONCENTRATION. 



It is well established that the controlling factor in the 

 growth of bacteria, in so far as "reaction" is concerned, is 

 not the titratable substances present but only the "free acid," 

 i. e., the nmnber of free H ions, consequently it is better to 

 determine the concentration of H ions and to standardize 

 to a definite H-ion concentration. A discussion of this subject 

 seems therefore appropriate. 



It is presumed that the student has had chemistry and 

 is familiar with modern chemical theories. How^ever, the 

 following discussion may not be out of place and may aid 

 in a better understanding of titration and standardization. 



By reaction is understood the relative concentration of 

 H ions and of OH ions in a solution. Both ions are always 

 present in watery solutions. If H ions are in excess the 

 reaction is called "acid"; if OH ions are in excess the reac- 

 tion is "alkaline." If the relative concentration of each is 

 the same the reaction is "neutral." 



Reaction may be indicated in yer cent of acid present, or 

 what is more common and much better, the relative amount 

 of H ions present as compared with a solution of known 

 strength. Solutions most commonly used for comparison 

 are normal solutions. A chemical normal solution is a 

 solution that contains that number of grams per liter of 

 water which corresponds to the hydrogen equivalent of the 

 substance. For substances in which there is one replaceable 

 H atom, or its equivalent, the number of grams of the 

 substance dissolved in a liter of water equals the molecular 

 iveight of the substance. For example, a normal solution 

 of HCl would contain 36.5 (H = 1, CI = 35.5) grams per 

 liter; of NaCl (Na = 23, and CI = 35.5) 58.5 grams per hter; 

 of H ions 1 (H = 1) gram per liter. A normal solution of 

 HCI is evidently a normal solution of H ions, since it con- 

 tains 1 gram H ions per liter. For substances containing 

 two replaceable H atoms or equivalent the normal solution 

 will contain the number of grams per liter equal to one-half 

 the molecular weight. Normal H2SO4 is 49 grams per liter 

 (H2 = 2, S = 32, O4 = 64, 2 + 32 + 64 = 98, J of 98 = 49). 

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