Accumet XL 20, User Manual, Page: 184 / 199

173
Appendix:
pH Theory
XL15, 20, 25, 50 and 60 meters
At 25°C in pure water, the concentration of hydronium ions is extremely small, 1 x 10
-7
Moles/liter,
and balanced by an equal concentration of hydroxyl ions. The equilibrium constant, Kw of water is
the product of the hydronium ion and hydroxyl ion concentrations:
Kw = [H
3
O
+
] [OH
–
] = [1x10
-7
] [1x10
-7
] = 1x10
-14
Since the hydronium ion concentration is 1 x 10
-7
Moles/liter, the pH of pure water is 7 , the
neutral pH, as stated above:
pH = -log [1 x 10
-7
] = 7
In aqueous solutions at 25°C, the product [H
3
O
+
] [OH
–
] or the Kw of water must remain constant.
Therefore, an increase in concentration of either term, [H
3
O
+
] or [OH
–
], will result in a decrease in
the other. For example, if a strong base, sodium hydroxide (NaOH) is added to water to the
extent that its final concentration is 0.01 Moles/liter, the concentration of the OH
–
ion is 0.01
Moles/liter, and the concentration of the H
3
O
+
must decrease according to the Kw equation as
follows:
[H
3
O
+
] = Kw/ [OH
–
] = 1 x 10
-14
/ 1 x 10
-2
= 1 x 10
-12
The pH of this solution is:
pH = -log [1 x 10
-12
] = 12
This high pH indicates that the 0.01 M NaOH solution is strongly alkaline; the concentration of
hydronium ions is extremely small.