#pH#, #"potenz hydrogen"# is a measure of hydrogen ion concentration, i.e. #[H_3O^+]#.
By definition, #pH# #=# #-log_10[H_3O^+]#, so that acidic solution have low (or even negative) values of #pH#, and alkaline solutions have high values of #pH#.
In water, the following equilibrium occurs:
#"H_2O(l) rightleftharpoonsH_3O^+ + HO^-#
At #298*K# we can measure the value of this equilibrium very accurately:
#[H_3O^+][""^(-)OH]# #=# #10^-14#
And taking #log_10# of both sides:
#log_10[H_3O^+] + log_10[HO""^-]# #=# #log_10(10^-14)# #=# #-14#
On rearrangement, #14=-log_10[H_3O^+] -log_10[HO""^-]#
To give (finally):
#14=pH+pOH#
