Unit 4

Acids, Bases and pH

• Water molecules dissociate and ionize to form

• H20 H+ + OH-

• Water hydrogen ion + hydroxide ion

• In pure water, there is a small but equal concentration of hydrogen ions and hydroxide ions.

• The concentrations of H+ and OH- in pure water are equal.

• The actual concentrations are– 10-7 moles/Liter

• A solution with this concentration is said to be neutral.

• Acid– A substance that releases H+ when dissolved

in water.

• Base– A substance that reduces the H+

concentration– Donates hydroxide ions to interact with

hydrogen ions to form water, OR– Directly accept hydrogen ions to from another

molecule.

pH

• pH is the measure of the hydrogen ion concentration in a solution.

• pH is the negative log of the hydrogen ion concentration– Hydrogen ion concentration of 10-7, pH 7– Hydrogen ion concentration of 10-2, pH 2– Hydrogen ion concentration of 10-9, pH 9

• pH below 7, acidic• pH above 7, basic

pH Indicators

• Plant pigments– Change color in response to pH– Made from boiled red cabbage

• Make a set of standards using known pH buffers– Add 5 ml of pH buffer, then 3 ml cabbage juice to

each tube.– Record color

• Compare your unknowns to your standard to determine pH of unknown.

• Exercise 4.1 and 4.2

Using pH meter and pH paper

• We are using red hand-held meters, NOT the Pasco probes

• Calibrate pH meter using the small screwdriver on cart. Follow directions at your table.

• Test pH of several household substances with both the meter and the paper

• Exercise 4.3

Buffering capacity of a solution

• Buffer- a substance in a solution that resists dramatic changes in pH.

• Buffering capacity- extent to which a solution can resist changes in pH

• Acid- Hydrochloric acid (HCl)

• Base- Sodium hydroxide (NaOH)

Exercise 4.4

• Solution A– Test buffering capacity of solution A by adding

acid to the solution (1 mL at at time) and testing the pH.

– Then test Solution A by adding a base (1 ml at a time.)

• Solution B– Add acid to the solution and test pH– Add base to the solution and test pH

• Graph your results.

• Which is the better buffer, A or B?