Acids-Bases and pH
Background
You encounter acids and bases in everyday life. For example, orange juice is acidic while bleach is basic. It is important to understand acids and bases in biology because both greatly affect chemical reactions in living organisms.
Acids are chemical compounds that increase the amount of hydrogen ions (H+) in a solution by releasing H+. Bases, on the other hand, decrease the amount of H+ in solution by accepting H+. Pure water (H2O), some of the molecules naturally separate into H+ and OH–. Thus, there is a small amount of free H+ in its solution that equals the amount of OH–. This condition is considered neither acidic or basic, but neutral. If there is a surplus of H+ compared to pure water, the solution is acidic. If the amount of H+ is less than that of pure water, the solution is basic.
The degree of acid or base in a solution is measured by the pH scale. The pH scale runs from zero to fourteen. The value of pH depends on the amount of H+ in the solution. pH values below 7 represent an acidic solution. The lower the pH value, the more acidic a solution is and the greater the amount of H+. A pH of 7 is neutral (e.g. pure water). pH values above 7 indicate basic solutions. The higher the pH value, the more basic solution a solution is and the lower the amount of H+. Because each pH value is actually an exponent of 10, for each whole number increment in pH, you change by a factor of 10. For example, a solution that has a pH of 5 is 10 times more acidic because it has 10x more H+ than a solution of pH of 6. A solution with pH of 11 is 10 times more basic because it has 10x less H+ than a solution of pH of 10. The pH value of a solution can be measured by pH meters or by certain dyes. Each of these dyes undergoes a color change at a different point in the pH scale. A careful selection of dyes permits determination of a broad range of pH. These are used to make pH paper. When dipped in a solution, the pH paper will change color depending on the pH of the solution. By comparing the color to a reference color chart, you can indicate the approximate pH of the solution. Acids- Bases and pH
Buffers are important chemicals that keep a stable pH in organisms to maintain homeostasis. A buffer keeps a stable pH by releasing or accepting H+. For example, when acidic substances are added to blood or saliva, the buffer present in blood or saliva will bind to excess H+ to help neutralize the acidity. On the other hand, when basic substances are mixed with them, the buffer will release more H+ to help neutralize the basicity. Acids- Bases and pH
The tests you observe in this exercise will introduce you to the pH scale and demonstrate some pH values encountered in biology.
To review the concepts of acids and bases for this lab, watch this video:What are acids and bases? (Links to an external site.)
Minimize Video(3 min) Note: video uses term “amphoteric” instead of “amphipathic.”
Purpose
- Perform (or observe and analyze) pH tests
- Discuss pH values encountered in biology
Materials (these will be viewed in the videos)
- pH paper with reference color chart
- Hydrochloric acid (HCl) (diluted and concentrated)
- Sodium hydroxide (NaOH) (diluted and concentrated)
- Distilled water
- Samples such as Lemon, Vinegar, Milk, Tap Water, Baking Soda, Soap, Disinfectant (bleach)
Procedure
- Take one strip of pH paper and dip it into the liquid sample.
- Compare the color with the reference color chart.
- Record the pH of the sample in the table below.
- Indicate whether the pH is acid, basic or neutral.
- Repeat for each sample.
For a video of the lab exercise, watchAcids, Bases & pH lab (Links to an external site.)(2 min 58 sec).
Follow procedure above. Observe color patterns in pH strips dipped into specified samples and record pH value in Table 1. Acids- Bases and pH
Results
Table 1. pH Values of Different Samples
| Substance | pH | Acidic, Basic or Neutral |
| HCl diluted | ||
| HCl concentrated | ||
| NaOH diluted | ||
| NaOH Concentrated | ||
| Water | ||
| Lemon | ||
| Bleach | ||
| Vinegar | ||
| Apple juice | ||
| Milk | ||
| Baking soda |
Discussion/Conclusion
- Is the pH of a hydrochloric acid (HCl) solution greater than or less than the pH of water? ______________
- Compare the pH of the two acid solutions (diluted and concentrated).
- HCl (diluted) pH = ______ HCl (concentrated) pH= ______
- Which HCl solution has more H+? diluted or concentrated (circle one)
- Which HCl solution is more acidic? diluted or concentrated (circle one)
- As the solution becomes more acidic, does pH value increase or decrease? Why?
- Compare the pH of two basic solutions (diluted and concentrated).
- NaOH (diluted) pH = ______. NaOH (concentrated) pH = ______
- Which solution has less H+? diluted or concentrated (circle one)
- Which solution is more basic? diluted or concentrated (circle one)
- As the solution becomes more basic, does the pH value increase or decrease? Why? Acids- Bases and pH
- pH levels vary in different parts of your body.
- Your stomach has a pH of 2. Is your stomach acidic, basic or neutral?
- Human blood has a pH of 7.45. Is blood acidic, basic or neutral?
- Your stomach (normal pH 2) drops to a pH of 1.8 because you drank too many cups of coffee. You take an anti-acid pill such as Alka Seltzer. Why does it relieve your upset stomach? Is Alka Seltzer acidic or basic? Acids- Bases and pH
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