Unit 3, Lesson 9

Lesson 9 - Acids, Bases and the Concept of pH

Student Performance Objectives
1. Define the meaning of pH and explain the pH scale. 
2. Explain why each unit of change of pH is a 10-fold change in hydrogen ion concentration. 
3. Explain the importance of the low pH of gastric juice.
4. Explain the importance of the stomach's mucus coating and the importance of sodium 
    bicarbonate secretion by the pancreas in the normal course of digestion.
5. Explain what a buffer is and give 2 examples.

Lesson Outline
A. The concept of the hydrogen ion and hydroxyl ion as ionization products of water.
B. An acid as a hydrogen ion (proton) donor.
C. A base (alkali) as a proton accepter.
D. The pH scale
    1. Extending from 0-14, as logarithmic scale so that one unit difference in pH is a 10 fold 
        difference in hydrogen ion concentration (or acidity). pH 7 as the neutral point. To 
        see the pH of natural substances, go to http://www.johnkyrk.com/pH.html
    2. DEMONSTRATION: the point is to demonstrate the differences in pH of natural 
        substances. Using a pH meter and/or pH paper determine the pH of freshly distilled
        water, distilled water, vinegar, dilute HCl, lemon juice, soda of some type, milk, sea 
        water, milk-of-magnesia, Clorox, soap solution, Windex, ammonia. The students keep 
        a chart of the tested substances. Ask the students how we survive since we ingest 
        foods and beverages of such widely varying pH. 
            Explain that the stomach is an acid vat much more powerful than any acid content 
        in foods we eat and that this acid destroys most microorganisms and dissolves 
        minerals out of our food. Explain how important it is for the stomach to be coated with 
        mucus to protect it from our own secreted acid. Then in the small intestine, sodium 
        bicarbonate from the pancreas neutralizes the stomach's hydrochloric acid.
    3. If the students have a background from previous chemistry you can explain pH more 
        deeply.
        a. Since one in every 10 million water molecules, in pure water, is ionized into 
            H+ and OH-, then the actual concentration of H+ in pure water is 0.0000001 
            molar or 10-7 M.
        b. Since pH is technically defined as the negative logarithm of the hydrogen ion
            concentration, then -log [.0000001] = 7. 
    4. Concept of a buffer - chemical solutions that resist changes in pH and which help to 
        establish the pH of the solution. Examples: proteins, bicarbonate buffer, phosphate 
        buffer, hemoglobin buffer.