248 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



plus oxygen) groups, symbolized by (OH)-. Acids yield hydrogen- 

 ion and another ion consisting of the balance of their constituents; 

 thus from nitric acid, HN0 3 , arise H + and (N0 3 ) _ , the latter being 

 termed nitrate-ion. Alkalies yield hydroxyl-ion and another con- 

 sisting of the remainder of the compounds ; in potassium hydroxide, 

 KOH, the ions are (OH)- and potassium-ion, K + . 



The amount of hydrogen-ion present in a solution, expressed as 

 gram equivalents per liter, is often referred to as the hydrogen-ion 

 concentration. 



The water existing in soils, as a film around the solid soil particles, 

 does not differ essentially from free water in the above respects. 

 Substances can dissolve in it, undergo ionization, and give rise to a 

 definite hydrogen-ion concentration of the soil. This is what is 

 meant by the term " soil acidity." 



Observation has shown that the properties of substances which 

 lead to their classification as acids are directly connected with the 

 presence of hydrogen-ion in their solutions. The characteristic sour 

 taste — from which the term acid was of course originally derived — 

 is in considerable part the taste of hydrogen-ion. 3 Reddening of 

 blue litmus, the most widely known test for acidity, is produced by 

 hydrogen-ion. And it is hydrogen-ion which takes part in most of 

 the chemical changes into which acids enter. 



The inhibiting effect of acids on the growth of many bacteria, and 

 the favorable effect they show toward the growth of some molds and 

 fungi, are hydrogen-ion phenomena. There is every reason to sup- 

 pose that the action of acids on higher organisms, such as the flower- 

 ing plants, is identical in origin. It is evident from these considera- 

 tions that the hydrogen-ion concentration is a highly important fea- 

 ture of the solution of any acid substance. 



The difference between hydrogen-ion concentration and quantity 

 of acid in the case of two typical acids is here tabulated. The 

 numerical values given are only approximate. 



Table 1. — Difference beticeen hydrogen-ion concentration and quantity of acid 

 in normal solutions of tivo acids. 



Formula :•-.-•- 



Molecular weight= equivalent weight=grams per liter in normal (molar) solution 



Quantity of acidic hydrogen, grams per liter 



Physicochemical class 



Per cent to which ionized 



Corresponding hydrogen -ion concentration, grams per liter 



Specific acidity a ; 



Acid. 



Hydrochlo- 

 ric. 



HC1. 



36.5 



1 



Strong. 

 75 

 0.75 

 7, 500, 000 



Formic. 



H(CHO,). 



46.0 



1 



Weak. 



1 



0.01 



100,000 



a This method of stating acidity has been explained in: The statement of acidity and alkalinity, with 

 special reference to soils, Journ. Wash. Acad. Sci., 9, 305-309, 1919. 

 3 Harvey, R. B., Journ. Amer. Chem. Soc, 42, 712, 1920. 



