178 RESPIRATION 



remains the same, in accordance with the law of mass action, how- 

 ever acid or alkaline a solution may be. Hence the concentration 

 of hydroxyl ions diminishes in proportion as that of hydrogen 

 ions increases, and vice versa. 



All acids and' bases combine with one another in chemically 

 equivalent proportions, but different acids and alkalies vary very 

 greatly in the extent to which they are ionized. The "strengths" 

 of different acids and alkalies were found by the electrical con- 

 ductivity method to depend upon the extent of their ionization. 

 The "strong" acid HC1 is, for instance, very completely ionized 

 into hydrogen and chlorine ions, and the "strong" base NaHO is 

 similarly ionized into sodium and hydroxyl ions ; while "weak" 

 acids, such as carbonic acid, or weak bases, such as ammonia, are 

 very slightly ionized. 



Water itself is slightly ionized into hydrogen and hydroxyl 

 ions, and can thus act as either a very weak acid towards bases or 

 a weak base towards acids. In the case of strong or highly ionized 

 acids and bases this property of water is practically of no account, 

 as the ionization of water is so very small ; but in the case of weak 

 acids or bases the water competes appreciably with the acid or 

 base. For instance in the case of potassium cyanide, a compound of 

 an extremely weak acid with a very strong base, the following re- 

 action occurs : 



KCN + H 2 * KOH + HCN. 



Thus free KOH and free HCN are both present in a solution of 

 this salt. But the KOH is highly ionized into K and HO ions, 

 while the HCN is hardly ionized at all. Hence HO ions pre- 

 dominate, and the solution is alkaline. Carbonic acid is not such a 

 weak acid as hydrocyanic acid; but the same relations hold, so 

 that both carbonates and bicarbonates form solutions which are 

 distinctly alkaline ; and bicarbonate solutions are still slightly 

 alkaline, even though much free carbonic acid is present, as in the 

 case of blood in the living body. 



The ordinary indicators appear to be extremely weak acids 

 or bases which change color on combination. When the only 

 other acids or bases present are strong ones, the change of color 

 is of course very sharp ; but with other weak acids or bases present, 

 the change is gradual and the complete color change does not 

 occur until the solution is distinctly alkaline or acid. This is be- 

 cause the indicator competes with other weak acids for the base ; 

 and different indicators compete in varying degrees. Thus dif- 

 ferent indicators turn with different degrees of slight variation 



