pH Scale and
Indicators
pH
• pH stands for “the power of hydrogen” and will tell us
if a solution is acidic, neutral, or basic.
• The pH of a substance depends on the concentration of
hydrogen ions in the solution (H+)
• The pH scale is based on a logarithmic scale meaning
that every change of one unit on the scale represents a
tenfold effect (multiply or divide by 10). Which means
there is a 10x difference between each pH level.
• e.g. the hydrogen ion concentration for…
pH = 1
[H+] = 10-1 = 0.1
pH = 2
[H+] = 10-2 = 0.01
pH = 3
[H+] = 10-3 = 0.001
pH = 14
[H+] = 10-14 = 0.00000000000001
pOH
• We can also calculate the concentration of
hydroxide ions and use the pOH scale.
pOH = 1
[OH-] = 10-1
= 0.1
pOH = 2
[OH-] = 10-2
= 0.01
pOH = 3
[OH-] = 10-3
= 0.001
pOH = 14
[OH-] = 10-14
= 0.00000000000001
pH and pOH
• In every solution the pH and pOH must equal
14.
pH + pOH = 14
For example, if….
a) pH = 3
pOH = 11
b) pH = 8
pOH = 6
c) pH = 7
pOH = 7
(acid)
(base)
(neutral)
Calculating pH
• To calculate the pH use the formula:
pH = - log [H+]
e.g. What is the pH of a solution that has a
hydrogen ion concentration of 0.00548 M.
[H+] = 0.00548 M
pH = ?
pH = -log [H+]
pH = -log (0.00548M)
pH = 2.26
Calculating [H+]
• To calculate the concentration of hydrogen use
this formula:
[H+] = 10-pH
e.g. What is the hydrogen ion concentration of a
solution with a pH of 9.46?
pH = 9.46
[H+] = ?
[H+] = 10-pH
[H+] = 10-9.46
[H+] = 3.47 x 10-10 = 0.000000000347
pH Scale
• The pH scale measure how acidic or basic a
solution is.
• It ranges from 0 to 14.
• A solution with a pH of 7 is neutral.
• A solution with a pH of less than 7 is acidic.
Strong acids have a low pH (close to 0)
• A solution with a pH greater than 7 is basic (or
alkaline). Strong bases have a high pH (close
to 14).
pH Scale
pH Scale
Acid-Base Indicators
• Any substance that changes colour in the
presence of an acid or a base
• They change colour because of the presence
of H+ ions or OH- ions.
Common Indicators
Indicator Name What is it?
Red litmus
paper
Blue litmus
paper
Universal
indicator
A plant extract that is
dried onto strips of
paper.
How does it work?
Turns blue when it is
dipped in a basic
solution and remains the
same colour in a neutral
or acidic solution.
A plant extract that is Turns red when it is
dried onto strips of
dipped in an acidic
paper.
solution and remains the
same colour in a neutral
or acidic solution
A mixture of chemicals Different colours
that changes colour
correspond to different
through a wide range pH values.
of pH values.
Common Indicators
Indicator Name
What is it?
How does it work?
Phenolphthalein
Chemical that
changes colour in
acids and bases.
Turns pink in a base
and clear or cloudy
in an acid or neutral
solution.
Red cabbage juice,
cranberry juice.
Chemicals in these
liquids change
colour depending
on if they are in an
acid or a base.
Turns purple in
neutral, red in acid,
and green-yellow in
bases.