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.
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 there 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.
›H+ ...1000x 100x 10x ↓ 10x 100x 1000x… ›OH-
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.
Part 1: Using Alkacid test paper (pH paper)
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.