Agenda

1. Agenda • Collect Pre-lab #3 • Lab quiz #2 • Samples that may be used for Microworlds today may include cabbage cells • Doing Lab #3: pH and buffers today • (JV, set up the pH 11 buffer & safety seal it.)

2. Lab #3: pH and Buffers

3. pH ACIDS BASES Removes H+ (hydrogen ions) from solution pH higher than 7 Sodium Hydroxide (NaOH) Magnesium Hydroxide (Mg(OH)2) Potassium Hydroxide (KOH) • Releases or causes the release of H+ (hydrogen ions) into solution • pH lower than 7 • Hydrochloric acid (HCl) • Sulfuric acid (H2SO4) • Acetic Acid (vinegar) • Carbonic acid

4. pH • Each step of the pH scale = 10-fold decrease in H+ concentration • What fold difference in H+ concentration is there between pH: • 7 and 9? • 11 and 4?

5. Safety Precautions • Wear GOGGLES at all times when there are people at your table working with hazardous chemicals • If using anything higher than pH 10 goggles must be warned. • (Vote to skip pH 11?)

6. A: pH Indicators • Change color depending on the pH. • Phenol red turns RED when solution is basic and turns YELLOW when solution is acidic (near neutral it is orange). • In Exercise 1 you will use cabbage extract as pH indicators.

7. A: pH Indicators - anthocyanins • Red cabbage has anthocyanins • Boiling will extract the anthocyanins which can then be used as a pH indicator • Ex.1, create a set of standards at pH 3, 5, 7, 9 (Skip 11)

8. A: pH Indicators - anthocyanins

9. B: Comparing pH of Beverages and Medicines • Ex.2: Determine the pH value of various household solutions • Pay attention to how the parts of this experiment fit the Scientific Method • But be quick, divide and conquer!

10. Buffers Control pH • In Exercise 3 you will study buffers. Buffers resist changes in pH (see next slide). • Do NOT make a solution NEUTRAL. • Allow biological molecules to maintain their structure, shape, characteristics. • Buffers are important for life.

11. Buffered vs non-buffered solution Add HCl to buffer Add HCl to water (low buffering capacity)

12. Bicarbonate Buffer System • Carbon dioxide in blood forms carbonic acid, which separates into H+ and bicarbonate Excess OH- ions combine with H+ to form H2O Excess H+ ions combine with bicarbonate to form carbonic acid

13. Bicarbonate Buffer System • Organs assist in maintaining blood pH homeostasis • Lungs remove excess CO2 • Kidneys remove excess HCO3- • Other buffers in the body

14. Buffers in the environment • Some SOILS are an excellent buffer • LIMESTONE

15. Ex 3: Using pH meter to determine buffering capacity • Why would we use a pH meter instead of red cabbage extract? • BUFFER: resists pH changes when a small amount of acid or base is added • Must have an acid component to add in H+ and a base component to take out H+

16. Ex 3: Using pH meter to determine buffering capacity • How can you tell the pink line is a good buffer while the blue line is not?

17. Ex 3: Using pH meter to determine buffering capacity • At what pH(s) is this buffer useful?

18. Ex 3: Using pH meter to determine buffering capacity • At what pH(s) is this buffer useful?

19. Ex 3: Using the pH meter • dilute mystery buffers? IMPORTANT NOTES: • pH electrodes are fragile, handle gently • After being in ANY solution, rinse the electrode with distilled water, & keep it wet! • Immerse the electrode fully and swirl to get accurate reading

20. Ex 3: Using the pH meter • Add 20mL of your mystery buffer • Add 1mL HCl, swirl, & record pH (1ml ≈ 2 dropper fulls) • Add 1mL HCl (2mL total), swirl, record pH on Fig. 3 • Continue adding 1mL and recording pH until you’ve added 10mL total HCl • Rinse electrode WELL • REPEAT experiment with 1mL increments of NaOH • Record in Fig 4, zero is in middle of x-axis • How to divide & conquer: two groups choose the same mystery buffer, one adds HCl, the other NaOH, now share your data!

21. Extra Credit: Design an Experiment • Design an original experiment to investigate an aspect of pH or buffering capacity • Use pH meter to measuring buffering capacity and pH • Be quick on other exercises to have time to do a good job here! • Worth 1 pt f/ each section on right (6 pts) Scientific Process – turn in one per group • Question (see next slide) • Hypothesis & Prediction • Dependent, independent and constant variables, • Control & experimental groups • Brief materials and methods • Results • Discussion/conclusions (brief) • (Skip the lab report introduction and references!) • If your group does not do this you may join one that is.

22. Questions for Designing your Experiment Design your experiment to test a hypothesis to one of these two questions: • Is red cabbage extract good at buffering water? (Only use 10 ml of cabbage extract to conserve.) Speculate on why or why not it is in the discussion section. • What is the pH ranking of the following aqueous solutions: ocean water (if available), cabbage extract, tap water, distilled water, pond water, spit (if someone has some drinks, you may add these, but no sipping in the lab)? Speculate on why their pH differs in the discussion section. (Only use 10 ml of cabbage extract to conserve.)

23. Notes to Exercise 3: • Clean all glassware & put it away! • Return the trays and all materials used to the place you obtained them from in a better condition than you got them in!