Thermo Fisher Scientific CyberScan 6000 Series Meters v.2 - Manual (Page 88)

173
Appendix: pH Theory
PH6000, PC6000, PH6500, PC6500 and PCD6500 meters
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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.