Hydrogen Ions and
Acidity
The Ionization of Water and pH
Hydrogen Ions in Water
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We are used to thinking of water as a pure liquid that
contains only H2O molecules.
However, as a pure liquid, water ionizes into
hydrogen ions and hydroxide ions:
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H2O(l) → H+(aq) + OH-(aq)
H2O(l) + H2O(l) → H3O+(aq) + OH-(aq)
Hydrogen Ions in Water
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The symbol for the concentration of hydrogen ions is
[H+] and for hydroxide ions is [OH-].
We can measure the concentration of the ions in pure
water.
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[H+] = 1.0×10-7 M
[OH-] = 1.0×10-7 M
By stoichiometry, H2O(l) → H+(aq) + OH-(aq), we see
that [H+] = [OH-] in pure water.
Hydrogen Ions in Water
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We also know that the product of the concentrations
is a constant, Kw.
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Kw = [H+][OH-] = 1.0×10-14
This means that as we change [H+], we change [OH-].
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If [H+] goes up, [OH-] goes down.
If [H+] goes down, [OH-] goes up.
Hydrogen Ions in Water
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The higher the value of [H+] and lower the value of
[OH-], the more acidic the solution.
The lower the value of [H+] and higher the value of
[OH-], the more basic the solution.
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For example: [H+] = 1.0×10-6 is acidic.
For example: [H+] = 1.0×10-3 is more acidic.
For example: [H+] = 1.0×10-8 is basic.
For example: [H+] = 1.0×10-11 is more basic.
Hydrogen Ions in Water
Example 1:
Colas are slightly acidic. If a cola solution has [H+] =
1.0×10-5 M, what is the [OH-]?
[H+] = 1.0×10-5 M
Kw = [H+][OH-] = 1.0×10-14
Kw = [H+][OH-]
[OH-] = 1.0×10-9

-14
K
w
1.0×10
[OH-] =
=
-5
+
1.0×10
[H ]
The pH Concept
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pH is another way to measure [H+] and/or [OH-].
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pH = -log[H+]
pOH = -log[OH-]
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If [H+] = 1.0×10-7
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then pH = -log(1.0×10-7) = 7.00
If [OH-] = 1.0×10-7
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then pOH = -log(1.0×10-7) = 7.00
The pH Concept
To find pH on the TI-83 or TI-84 Plus ...
• press the “(-)” button
• press the “LOG” button
• enter the value for [H ]
• press the “)” button
• press “ENTER”
+
The pH Concept
To find pH on the TI-Nspire ...
• press the “(-)” button
• press the “ctrl” button
• press the “10 ” button
• enter the value for [H ]
• press the “)” button
• press “ENTER”
x
+
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The pH Concept
If [H+] = 1.0×10-5
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then pH = -log(1.0×10-5) = 5.00
If [OH-] = 1.0×10-8
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then pOH = -log(1.0×10-8) = 8.00
If [H+] = 3.1×10-2
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then pH = -log(3.1×10-2) = 1.51
If [OH-] = 4.5×10-11
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then pOH = -log(4.5×10-11) = 10.35
The pH Concept
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We can also convert from pH and pOH to [H+] and
[OH-] respectively.
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[H+] = 10-pH
[OH-] = 10-pOH
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If pH = 7.00
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then [H+] = 10-7.00 = 1.0×10-7
If pOH = 3.25
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then [OH-] = 10-3.25 = 5.6×10-4
The pH Concept
To find [H+] on the TI-83 or TI-84 Plus ...
• press the “2ND” button
• press the “LOG” button
• press the “(-)” button
• enter the pH value
• press the “)” button
• press “ENTER”
The pH Concept
To find [H+] on the TI-Npire ...
• press the “10 ” button
• press the “(-)” button
• enter the pH value
• press the “)” button
• press “ENTER”
x
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The pH Concept
If pH = 2.556
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then [H+] = 10-2.556 = 2.78×10-3
If pOH = 9.27
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then [OH-] = 10-9.27 = 5.4×10-10
If pH = 12.12
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then [H+] = 10-12.12 = 7.6×10-13
If pOH = 6.678
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then [OH-] = 10-6.678 = 2.10×10-7
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The pH Concept
pH + pOH = 14
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if pH = 2.0,
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then pOH = 14.0 - 2.0 = 12.0
if pH = 3.250,
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then pOH = 14.000 - 3.250 = 10.750
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The pH Concept
Solutions with pH = 7 (pOH = 7) are neutral.
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Litmus paper remains colorless.
Solutions with pH < 7 (pOH > 7) are acidic.
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Litmus paper turns red.
Solutions with pH > 7 (pOH < 7) are basic.
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Litmus paper turns blue.
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Measuring pH
Indicators may be added to a solution to tell us the
range of pH of the solution.
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Measuring pH
Indicators are most often used where pH may
change, such as in a titration.
But they are also used to tell us where we are in a
particular range of pH.
For example, if we wanted to tell
if a solution has a pH greater
than or less than 7 ...
we might use Bromthymol
Blue (which changes color at
about pH = 7).
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Measuring pH
Indicators are most often used where pH may
change, such as in a titration.
But they are also used to tell us where we are in a
particular range of pH.
For example, if we wanted to tell
if a solution has a pH greater
than or less than 7 ...
we would not want to use
Methyl Red (which changes
color near pH = 5).
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Measuring pH
Some meters provide quick and accurate readings of
pH.
They vary from the simple to the very complex.