HYDROGEN ION CONCENTRATION U 



Blood, which contains 4-7 x 10"^ grams per litre of 

 hydrogen ions, will have 



pK = - log (4-7 X 10-8) = 8 - log 4-7 = 8 - 0-672 = 7-328 



Since, as we have seen, even strongly alkaline solutions 

 contain some hydrogen ions, and the amount of hydrogen 

 ions is inversely proportional to the amount of hydroxyl 

 ions (on which the alkalinity depends), the alkalinity of 

 a solution can also be expressed in terms of its hydrogen 

 ion concentration and the pH. scale. Thus an alkaline 

 solution containing 3-7x10-^ grams of hydrogen ions 

 per litre has 



pK = - log (3-7 X 10-9) = 9 - log 3-7 ^ 9 - 0-568 = 8-432 



As the product of hydrogen ion concentration and 

 hydroxyl ion concentration is constant at 10"^*, it is 

 obvious that the more hydrogen ions there are 

 present, that is, the more acid the solution, the loAver 

 will be the pH, tending to the value p}l = 0, which is the 

 theoretical limit when there are no hydroxyl ions present. 

 On the other hand, the more hydroxyl ions there are 

 present, that is, the more alkaline the solution, the higher 

 will be the pH value, tending to the hypothetical limit 

 ;:>H =14 when there are no hydrogen ions present. Since 

 neutrality occurs at pH 7, all values lower than this refer 

 to acid solutions, whilst higher values than 7 indicate 

 alkaline solutions. 



It must be remembered that the pH scale is logarithmic, 

 and that accordingly a change in ^^H value of 1 unit 

 means a tenfold change in acidity or alkalinity. Thus a 

 solution of pH 5 will be ten times as acid (will contain 

 ten times as many hydrogen ions) as one at ^^H 6, and a 

 hundred times as acid as one at pH 7. Similarly a solution 

 at ^^H 10 will be ten times as alkaline (will contain ten 

 times less hydrogen ions) than one at pK 9, and be one 

 hundred times as alkaline as a solution at pH 8. 



The titratable acidity of a solution must not be con- 

 fused with its pH value. The titratable acidity de]-)ends 



