How does the pH of an acid change when an alkali is added to it?

1 Answer
Jun 12, 2016

Answer:

#pH# increases; sometimes it rises substantially.

Explanation:

#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#