i6 PROPERTIES OF SOLUTIONS 



concentration. Corresponding to the concentration of hydrogen ions is a 

 certain definite concentration of hydroxyl ions. This concentration of 

 hydroxyl ions is indicated in Table 3 as pOH. It can be shown by the prin- 

 ciple of mass action that the mathematical product of the concentration of 

 hydroxyl ions and the concentration of hydrogen ions in a solution is a con- 

 stant. This may be expressed as follows : 



(H + ) X (OH-)=K. K=io-i4 at 22° C. 



Hence as the pH of a solution is increased the pOH decreases and vice versa. 

 For example, if the pH increases from 5 to 6 the pOH decreases from 9 to 8. 



Furthermore, only at pH 7 can the concentration of hydrogen ions equal 

 the concentration of hydroxyl ions. This is therefore the neutral point on 

 the pH scale and corresponds to the dissociation of pure water. This pH 

 value represents a dissociation of only one water molecule in approximately 

 every 555,000,000. 



Values below 7 on the pH scale represent the acid range, those above 

 7 the alkaline range of the scale. An "acid" solution is one with a larger 

 concentration of hydrogen ions than hydroxyl ions, while in an "alkaline" 

 solution the reverse is true. The lower the pH value the greater the hydro- 

 gen ion concentration of a solution. A pH value of 5 represents ten times 

 the hydrogen ion concentration of a solution with a pH of 6 and one hundred 

 times the hydrogen ion concentration of one with a pH value of 7, etc. This 

 is due to the fact, previously emphasized, that the numbers on the pH scale 

 are related to each other as logarithms and not as ordinary arithmetic numbers. 



The hydroxyl ion concentration of a solution could be expressed in pOH 

 units instead of in pH units. For alkaline solutions especially this would seem 

 to be a logical practice. But because of the definite mathematical relation- 

 ship between the pH and pOH the pH value alone also defines the pOH value. 

 Hence both the acidity of a solution in terms of H+ ions, and its alkalinity 

 in terms of OH~ ions may be, and customarily are, expressed in terms of 

 pH units. 



Table 3 shows only the pH values corresponding to the range between 

 a normal solution in terms of hydrogen ions and a normal solution in terms 

 of hydroxyl ions. It is possible for aqueous solutions to exist which have a 

 pH value of less than o {i.e. a minus pH value) or more than 14. A 5 N solu- 

 tion of hydrochloric acid, for example, would have a pH value of a little 

 less than o; correspondingly a 5 N solution of sodium hydroxide would have 

 a pH value a little greater than 14. In actual experience, however, solutions 

 of such extreme concentrations are seldom encountered. 



