Page 103 - Basic physical chemistry for the atmospheric sciences

Acids and bases 89

5.6 The pH scale
For dilute solutions, the molar concentrations of hydrogen ions are
small . For example, the y drogen ion concentration of a 0. 1 M solu­
h
0
tion of HCl in water is . 1 M. As a shorthand notation, the hydrogen
l
ion concentrations of dilute solutions are general y indicated by their
4
pH value, which is defined by
5
( . 1 4 )
where concentrations are measured in moles per liter (M). Thu s , a 0. 1
1
M solution of HCl has a pH of - log(I 0 - ) = 1 . We see from defini­
e
tion (5 . 1 4 ) that ( I ) the greater the hydrogen ion concentration (i. . ,
i
p
the more acidic the solution) the smaller s the H value of the solu­
tion, and (2) a change in the hydrogen ion concentration by an order
2
of magnitude (e.g. , from 1 0 - 1 to 10 - M) changes the pH value by
unity.
At the beginning of this section we defined a solution as being
neutral if [H +(aq)] = [OH - (aq)]. Pure water is neutral ; therefore , from
Eqs. (5. 1 2) and (5 . 1 3)
[ H 30 + (aq)][OH - (aq)] = 10 - 1 4

or,

Therefore, for pure water
[H30+ ( aq)] = [H +(aq)] = 1 0 - 7 M
7
Hence, the pH of pure water is - log(I 0 - 7 ) = . It follows that acidic
solutions have pH < 7 and basic solutions have pH > .
7
Observed H values n nature are generally between 4 and 9. Seawa­
p
i
ter normally has a pH between 8 . 1 and 8 . 3 . Streams in wet climates
generally have a pH between 5 and 6.5 and in dry climates between 7
and 8 . Soil water in the presence of abundant decaying vegetation may
have a pH of 4 or lower. The pH of rainwater can range from quite
acidic (around 4.0) in industrial regions to about 5 . 6 in very clean
regio s . We will discuss the acidity of rainwater in some detail at the
n
end of this chapter, but the following exercise illustrates why even
dean rainwater does not have a pH of .
7
Exercise 5 . 2 . The pH f natural rainwater is about 5 . 6 . Assuming
o
t h at all of this acidity is due to the absorption of C02 by the rain,