1. Acids, Bases, & pH
Section 2.6

2. Learning Objectives
Contrast acids and bases & discuss their properties.
Convert the hydrogen ion concentration of a solution to a pH value.
Describe how buffers help minimize changes in pH.

3. Pure Water is Neutral
Water dissociates into hydrogen ions (H+) and hydroxide ions (OH-)
One water molecule breaks down into one of each ion
H+ and OH- are equal in concentration
Since H+ and OH- are equal in concentration pure water is neutral
pH = 7

4. Acids & Bases
Acid = a solution that has more hydrogen ions (H+) than hydroxide ions (OH-)
pH < 7
Donates H+ ions
Base = a solution that has more hydroxide ions (OH-) than hydrogen ions (H+)
pH > 7
Accepts H+ ions

5. (Higher concentration of H+) (Lower concentration of H+)
pH scale
The pH Scale
Each pH unit represents a tenfold change in concentration of H+
Logarithmic scale H+
OH–
Lemon juice; gastric juice
(Higher concentration of H+)
Increasingly ACIDIC
Grapefruit juice
Acidic solution
Tomato juice
Urine
NEUTRAL
[H+] = [OH–]
PURE WATER
Human blood
Seawater
Neutral solution
(Lower concentration of H+)
Increasingly BASIC
Milk of magnesia
Household ammonia
Household bleach
Oven cleaner
Basic solution
Figure 2.15

6. pH Indicators
Boiled cabbage creates a bluish juice that can be used to test pH of substances
Turns blue in bases
Turns red in acids

7. pH Indicators Other indicators: pH paper Blue litmus paper
Turns red in acids
Red litmus paper
Turns blue in bases
phenolphthalein
turns pink in presence of bases

8. Calculating Ion Concentrations
[H+] [OH-] = 1 x 10-14
Ex. If [H+] = 0.001, then what is the [OH-]?
Answer: [OH-] = 1 x 10-11
pH 7 is 1 x 10-7
Practice Problem: What pH is a solution that has a [H+] of 1 x ?
Answer: pH 10

9. Buffers Buffer = substance that resists pH change
Accepts H+ ions when they are in excess and donates H+ ions when they are depleted
Cells need a stable pH in order to function properly
Most cells have a pH close to 7