168 



ACIDITY 



stream of an equivalent amount of other common acids, even such acids 

 as citric and acetic, which the body normally oxidizes for energy, would 

 doubtless be fatal. In these two instances it is not the concentration 

 of total acid that determines whether or not injury results; it is the 

 concentration of hydrogen ions. Figure 7-2 shows the effect of various 

 hydrogen-ion concentrations on plants. Growth is poor when the active 

 acidity is too high (pH too low). 



Courtesy of Illinois Agricultural Experiment Station. Reproduced from Hiinper 

 Signs in Crops, a publication of the American Society of Agronomy and the 

 National Fertilizer Association, Washington, D. C. 



Fig. 7-2. The effect of increasing acidity (left to right) on growth of red 

 clover. The poor growth results from calcium starvation, because high 

 acidity interferes with the absorption and retention of calcium by the plant 

 roots. 



For these reasons measurement of the hydrogen-ion concentration fre- 

 quently is of more significance than determination of titratable acidity 

 or alkalinity of a given biological fluid or extract. But one must not 

 conclude that active acidity is always of prime importance. A familiar 

 illustration involves the use of soda and sour milk as a leavening agent 

 in the making of corn bread. If the housewife should add only enough 

 soda to react with the hydrogen ions initially present in the sour milk, 

 most of the lactic acid, i.e., the nonionized part, would not be neutralized, 

 and sour bread would be the inevitable result. Sufficient soda to react 

 with all of the acid must be added. 



Water is a neutral substance because it yields an equal, though rela- 

 tively small, number of hydrogen and hydroxyl ions. The concentra- 

 tion is known to be 0.0000001 (also expressed 10"'^) gram-ion per liter, 

 which is equivalent to 0.0000001 g. of hydrogen ions and 0.0000017 g. of 

 hydroxyl ions. It must be borne in mind that hydrogen ions exist even 

 in basic solutions. Their concentration is reduced as basicity increases, 

 but, theoretically, all are never entirely removed from a solution. 



