pH Lab

Background:

          The concentration of hydrogen ions in solution, as expressed in terms of pH, is of great importance to living systems. Hydrogen bonds- and other weak forces generated by small differences in charge- play a crucial role in shaping large, biologically important molecules. How molecules fold or interact with one another often depends on the concentration of hydrogen ions. So it is not surprising that changes in the balance of positive and negative ions in the watery environment of the cell can affect the shape, and thus the function, of biological molecules. For example, the pH of your blood can change with strenuous activity. Among other things, a change in blood pH affects the shape of hemoglobin molecules, which can increase or decrease their capacity to deliver oxygen to cells.

          Although hemoglobin functions by responding to changes in pH, as with all biological molecules, hemoglobin can function properly only within a limited range of pH. Levels of pH that are too high or too low can damage the structure and thus interfere with the function of hemoglobin. The body has mechanisms for maintaining a range of pH that allows all molecules, cells and organs to function properly.

          Changes in pH can have major effects not only on molecules and cells, but also on entire ecosystems. For example, rain is usually slightly acidic; unfortunately, because of increased levels of carbon dioxide and other pollutants in our atmosphere, some rain, especially in industrialized regions, has a very high concentration of hydrogen ions, that is, it is very acidic. Because everything from the uptake of nutrients by roots to the delicate membranes surround the eggs of fish and amphibians is affected by pH, “acid rain” is slowly destroying our forests and depleting the fish and frog populations in our lakes.

Overview:

          Molecules that are dissolved in water may separate (dissociate or ionize) into charge fragments or ions. Often one of these fragments is a hydrogen ion (H⁺). The pH of a solution is a measure of the concentration of hydrogen ions. Because enormous variations in ion concentrations are possible, pH is calculated in powers of 10, using the mathematical device of logarithms (base 10). The alkalinity or acidity of a solution is determined by its concentration of H⁺ ions, that is, by its pH.

          A water molecule ionizes when one of its two hydrogen atoms leaves its electron behind and, as a hydrogen ion (H⁺), joins a different water molecule. Two ions are produced by this reaction, a hydroxide ion (OH^-) and a hydronium ion (H₃O⁺). We can express this reaction as follows:

2 H₂O ↔ H₃O⁺ + OH^-

          Convention, however, allows us to express the ionization of water more simply as

H₂O ↔ H⁺ + OH^-

          If there are more H⁺ than OH^- ions per mole, the solution is acidic. If ther are more OH^- ions than H⁺ ions per mole, the solution is basic. If the concentration of H⁺ ions equals the concentration of OH^- ions, the solution is neutral.

pH scale:

0-----1-----2-----3-----4-----5-----6-----7-----8-----9-----10-----11-----12-----13-----14

                             ›H+         ^{...1000x      100x     10x}         ↓        ^{10x        100x     1000x…} ›OH-

                                                                  neutral

          Having conquered pH, you are now ready to apply your knowledge. How acidic or basic are the soft drinks you drink or the water you bathe in? How acidic or basic is the soil in your front yard? What is the pH of the rain in your area? All of these questions can be answered by a few simple tests.

Purpose: In your own words, what is the purpose of this lab?

Hypothesis: Read the exercises below, create an if then statement about acids and bases, then predict the pH of each solution.

Procedure:

Part 1: Using Alkacid test paper (pH paper)

Exercise A

1. You will be testing the following substances: Solution A (dionized/distilled water), Solution B (milk), Solution C (orange juice), and Solution D (sparkling water).
2. Place a small amount (a few drops) of each solution in separate wells in your well tray, don’t forget which one is which!
3. Use forceps to dab a piece of pH paper into the first sample (DO NOT TOUCH pH PAPER WITH YOUR FINGERS!), while the paper is still wet, compare its color with the standard pH color scale on the side of the pH paper dispenser. Record the pH value in your data table (you need to make one!).
4. Repeat for other solutions, be sure to wipe the forceps off between substances.

Exercise B

5. You will be testing the following substances: Solution E (apple juice), Solution F (black coffee), Solution G (7-up), and Solution H (sports drink).
6. Place a small amount (a few drops) of each solution in separate wells in your well tray, don’t forget which one is which!
7. Use forceps to dab a piece of pH paper into the first sample (DO NOT TOUCH pH PAPER WITH YOUR FINGERS!), while the paper is still wet, compare its color with the standard pH color scale on the side of the pH paper dispenser. Record the pH value in your data table (you need to make one!).
8. Repeat for other solutions, be sure to wipe the forceps off between substances.

Part 2: Testing the pH of common medicines using cabbage juice (pink for acidic, green for basic, your teacher will prepare 8 test tubes that will be used as your “pH color chart”).

          Background: Anthocyamins, the plant pigments responsible for red, blue and purple colors in flowers, fruits and autumn leaves, can be used as a pH indicator. At low pH they turn red, at high pH they turn blue.

Exercise C

9. You will be testing the following substances: Solution I (aspirin dissolved in deionized water), Solution J (milk of magnesia/ Mg(OH)₂), Solution K (Alka-Seltzer/ NaHCO₃), and Solution L (Maalox).
10. Place a good amount of cabbage juice (3ml or so) in four separate test tubes.
11. Add another 3 ml or so aspirin solution to one of the test tubes (add enough for it to change color). Record the pH value in your data table (you need to make one!).
12. Repeat for other solutions.
13.  Pour the contents of each tube down the sink and flush with water. Clean all glassware and you station and place materials back into bin for the next class.

Observations:

1. In exercise A, which solution was the most basic? The most acidic? How could you tell?
2. In exercise B, which solution was the most basic? The most acidic? How could you tell?
3. In exercise C, which solution was the most basic? The most acidic? How could you tell?
4. Make a diagram of the first 8 substances you tested (with the pH paper) in order of increasing H⁺ ion concentration.
5. Make a diagram of the last 4 substances you tested (with the cabbage juice) in order of increasing H⁺ ion concentration.
6. Which substances turned out to be far more acidic than you thought? Which ones were far more basic?
7. Please make your own observations of this experiment (at least 3 sentences).

Conclusions:

1. It is often recommended that aspirin be taken with a large glass of milk or water. Based on your results in these lab exercises, do you agree with this suggestion? Explain.
2. Would apple juice or orange juice be a good accompaniment to aspirin? Explain.
3. Enzymes function best at particular pH values. In the normal human stomach, a pH of 2.0 to 3.0 provide the environment required for proper functioning of the digestive enzymes found there. Which of the medicines you tested are often used for treatment of “acid indigestion”? Based on your results, how would you explain the action of these medicines?
4. What, if any, effect would these medicines have upon digestion? Explain.
5. In some flowers, soil pH affects the uptake of certain metals that can complex (join) with the anthocyamin pigment and prevent its normal color expression, pink. For instance, in hydrangeas, high soil pH values prevent the uptake of aluminum, and the flowers appear blue. Is the soil more acidic or basic in this situation? What color would hydrangeas be if grown in a heavily industrialized region? Explain.
6. Was your hypothesis correct? Which parts were/weren’t? Are there any sources of experimental error involved in this experiment? What else would you like to test if given the opportunity to repeat the experiment?