Thermo Fisher Scientific CyberScan PC 5000 - Manual (Page 161)

Instruction Manual CyberScan PC5500/ 5000
156
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
-
] = [1 X 10
-7
] [1X 10
-7
] = 1 X 10
-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 [1X 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 H3O+ 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.