SOIL ACIDITY — WHERRY. 



249 



Table 1 shows that although a normal (molar) solution of both 

 acids contains the same quantity of acidic hydrogen, the strong acid 

 yields seventy-five times as much hydrogen-ion as does the moderately 

 weak one, and may be expected to have seventy-five times as much 

 effect in the directions listed in the preceding paragraph. 



The situation is analogous to that of two men, both possessing $100, 

 but one having $25 in a savings bank and $75 in his pocket, the other 

 having $99 and $1 in these respective places. The first man can pur- 

 chase seventy-five times the amount of any commodity that the second 

 can, even though the total quantity of money they own is the same. 

 Purchasing power, in this illustration, corresponds exactly to hydro- 

 gen-ion concentration ; for the amount of li} T drogen which is ionized, 

 not the total amount, determines most of the things an acid can do. 



Several different methods of stating acidity are in use, and in 

 Table 2 the way two values selected at random appear under all of 

 them is brought out; and the reader may judge whether the one 

 chosen for use in the present paper (the last one tabulated, specific 

 acidity) is not the best from the point of view of clearness and con- 

 venience. The hyclrogen-ion concentrations of the two solutions, in 

 gram equivalents per liter, are 0.0004 and 0.0000002, respectively, the 

 former representing two thousand times as much hydrogen-ion as the 

 latter. 



Table 2. — Comparison of methods of stating acidity (hydrogen-ion) . 



Solution 1. 



Solution 2. 



Starting at normality: 



Concentration — 



Actual number 



Power of 10 with a coefficient 



Power of 10 



Potential due to H + (P H ) 



Starting at neutrality: 



Chemical potential, X H 



Concentration- 

 Power of 10 



Actualnumber (specific acidily) 



0. 0004 



0.4X10" 8 



10-j.« 



3.4 

 3.6 



103.6 



4,000 



0. 0000002 



0.2X10-« 



10-*' 



6.7 



0.3 



100.1 

 2 



The method illustrated in the last line of Table 2 obviously shows 

 with a minimum of calculation on the part of the reader that one of 

 the solutions is two thousand times as acid as the other ; and it gives 

 directly the information that the hydrogen-ion concentration is in 

 the one case four thousand times, in the other case twice, that of pure 

 water. 



As the P H method is rather widely used in the statement of hydro- 

 gen-ion concentration, however, it seems desirable that another table 

 be given showing the relations between P H values and specific acidi- 

 ties (and alkalinities) over the range covered by dilute aqueous 

 solutions. 



