What is the difference in concentration between a pH of 7 and 12?

The pH of a solution is simply a measure of the concentration of hydrogen ions,  

H

+

, which you'll often see referred to as hydronium cations,  

H

3

O

+

.

More specifically, the pH of the solution is calculated using the negative log base  

10

of the concentration of the hydronium cations.

∣

∣

∣

∣

¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

a

a

pH

=

−

log

(

[

H

3

O

+

]

)

a

a

∣

∣

−−−−−−−−−−−−−−−−−−−−−−−−  

Now, we use the negative log base  

10

because the concentration of hydronium cations is usually significantly smaller than  

1

.

As you know, every increase in the value of a log function corresponds to one order of magnitude.

Explanation: