1. pH and Acid-Base Indicators

2. The pH Scale “The power of hydrogen” Expresses the concentration of H+
Acidic
Neutral
Basic
[H+] > [OH-]
[H+] = [OH-]
[H+] < [OH-]

3. H2O(l)  H+(aq) + OH-(aq)
Ionization of Water
Pure water is neutral (pH 7)
Why?
You might think that pure water contains no H+ or OH-
However, even pure water ionizes slightly
At SATP, ~2 in every billion water molecules ionize:
H2O(l)  H+(aq) + OH-(aq)
i.e [H+] in pure water is ~ 1.0 X 10-7 mol/L

4. Relating [H+] to pH [H+] = 10-pH pH = -log[H+] Acidic Neutral Basic
[H+] > 1.0 X 10-7 M
pH < 7
i.e. 1.0 X 10-6 M
1.0 X 10-2 M
[H+] = 1.0 X 10-7 M
pH = 7
[H+] < 1.0 X 10-7 M
pH > 7
i.e. 1.0 X 10-8 M
1.0 X M
pH = -log[H+]

5. Acid-Base Indicators
A common method for determining pH of a solution is to use an indicator
Indicators respond to changes in [H+] (change colour)
Universal Indicator
Litmus Paper
Cabbage Juice

6. Bromothymol blue
Phenolphthalein

7. Homework
P. 371 #2-6
P. 371 #9,10
P. 375 #1-4,6cd,12

8. Bromothymol blue
Universal Indicator