. State what pH measures about a given solution


pH and Homeostasis




1. State what pH measures about a given solution

2. State the ranges for acids, bases, and neutral substances on the pH scale

3. Define homeostasis

4. Discuss how buffers maintain a constant pH

5. Explain the difference between positive and negative feedback mechanisms

Things you should be able to explain to someone else after this lab:

pH positive feedback

acid negative feedback

neutral homeostasis


Homeostasis is the process by which living things, including humans, maintain a constant, stable internal environment. This consistency requires constant regulation by the cells of the body-not an easy task and yet you maintain homeostasis without even thinking about it. Some processes included in homeostasis are maintaining a constant internal temperature and maintaining a balance of salt and water concentrations.

Cells use both positive and negative feedback mechanisms to maintain homeostasis. In negative feedback the cells reverse direction to maintain homeostasis. For example, if the body gets too hot temperature sensors in the blood send signals to the hypothalamus in the brain. The hypothalamus will signal the skin to sweat and the blood vessels to increase blood flow to the skin. These responses counteract the heat and cool the body back to normal temperature levels. In positive feedback cells maintain homeostasis by intensifying the change in the same direction. One example of positive feedback is blood clotting. When someone gets a cut the blood begins to clot. The damaged blood vessel releases chemicals that increase the amount of clotting eventually ceasing the flow of blood. Both negative and positive feedback pathways are vital in maintaining homeostasis.

pH, another aspect of homeostasis, is a measurement of the concentration of hydrogen (sometimes you will see it written as hydronium) ions (H+) in a solution. Because of its mathematical formulation, low pH values are associated with solutions with high concentrations of hydrogen ions, while high pH values occur for solutions with low concentrations of hydrogen ions and high concentration of hydroxide ions (OH-).

The pH scales ranges from 0 through 14. Pure water has a pH of 7.0, and other solutions are usually described with reference to this value. A pH of 7.0 is considered neutral. Acids are defined as those solutions that have a pH less than 7 (i.e. more hydrogen ions than water); while bases are defined as those solutions that have a pH greater than 7 (i.e. fewer hydrogen ions than water). An example of a pH scale is shown on the next page with pH values of common substances.


The values of the pH scale are based on logarithms. Thus each change between values on the pH scale is a magnitude of 10 in hydrogen ions. For example, a pH value of 3 is has 1,000 times more hydrogen ions than a solution with a pH of 6 (10*10*10=1000). The pH equation is shown below.

pH = -log [H+]

There are several methods used for determining the pH of a solution. The two most common methods are using pH test strips and pH meters. A pH meter is an electrical device that contains a glass electrode that can detect and then display the concentration of hydrogen ions using the pH scale. pH meters are the most accurate when properly calibrated.

What is a buffer? A buffer is a solution that resists change. Biological buffers are crucial in preventing changes in the pH of the cell. Why is maintaining proper pH so important for a cell? At very high and very low pH (basic and acidic conditions), the enzymes of a cell will cease to function. Since enzymes are crucial to almost every biological reaction that occurs in a cell, the cell will die if its pH is changed beyond a very narrow range of tolerated pH.


Effectiveness of antacids

Today you will test the effectiveness of three bands of antacids. Someone would take an antacid to alleviate heartburn. One symptom of heartburn is increased acidity in the stomach. An antacid works by increasing the pH or decreasing the acidity. It is trying to balance or buffer the acid to regain homeostasis. The “best” antacid will raise the pH the most.

1. Before you begin, create a hypothesis. Which antacid do you think will be the most effective? Write your hypothesis below:

2. Read through the instructions, and then watch the video (YouTube *Note: May not be compatible with Apple products, i.e. iphones, ipads, etc.)

· Three 50 mL beakers.

· Vinegar

· Water

· Mortar and pestle

· Three different brands of antacids (Rolaids, Tums, Alka-Seltzer)

· One tablet of each brand

· Glass stir rod

· pH testing strips or pH meter

· Timer

Even though you are not actually completing the procedure below, please be familiar with the steps.


1. Fill each beaker with 30 ml of vinegar. The vinegar mimics stomach acid.

2. Use a pH strip or pH meter to measure the pH in one of the beakers of vinegar. This way you know how acidic the environment is before you add the antacid. Record the pH on the next page.

3. Use the mortar and pestle to crush one tablet of one brand of antacid. The mortar and pestle mimics the human teeth chewing the tablet.

4. Pour the crushed tablet into the first beaker of vinegar.

5. Use the glass stir rod to mix the antacid and vinegar for 10 seconds.

6. Wait 60 seconds.

7. Measure the pH of the solution with a pH strip or a pH meter.

8. Repeat steps 3-7 with the other two brands of antacid.

9. Record your results in the table on the next page.

10. Wash out your beakers with soap and water.

3. Watch the following video displaying the performance of this experiment. You will see the pH is measured with both pH strips and a pH meter.

4. Results: Record the pH readings from the pH meter in the table below and any observations.

pH of vinegar:_____

Observations: Use this space to record any observations you made such as change in color or amount of bubbling after you added the antacids.

Table 1: Antacid pH results

Brands of antacids

pH value

pH change from vinegar

5. Answer the following questions about your experimental conclusions:

1. Which antacid was the most effective? The least effective?

2. Do your results support your hypothesis?

3. How does this experiment illustrate homeostasis?

4. Is this experiment an example of positive or negative feedback? Explain.


Simulation: What is the pH of common substances?

For this activity you will complete a simulation from Glencoe/McGraw-Hill textbook publisher.

1. Click on the following link to access the virtual lab: http://www.glencoe.com/sites/common_assets/science/virtual_labs/E22/E22.html.

Read the background information and follow the directions for the simulation to complete the activity.

2. Answer the following questions related to the simulation.

a. What is the range of the pH scale?

b. State the portion of the pH scale that is acidic.

c. State the portion of the pH scale that is basic.

3. Record the names of the solutions you will predict pH value of each solution in the table below. After you make your predictions, test the solutions using the pH paper and record in the table below.


Predicted pH value

Actual pH Value

Type of solution (acidic, neutral, or basic)

4. Check your work by clicking on the check button before continuing and then determine the type of solutions you tested (the last column in the table above).

5. Answer the following questions:

1. What facts did you use to predict the pH values of the solutions?

2. How did your predicted pH values for each of the common solutions compare with the actual pH values for those solutions?

3. Of the six solutions you tested, which one was the most acidic? Which one was the most basic? Which one was the closest to neutral?

4. Milk of magnesia is sometimes used as a remedy for an “acid stomach.” Would you expect the pH of milk of magnesia to be less than 7, more than 7, or 7? Why?

5. What are some real-world applications in which pH is an important factor?

6. What component of our blood resists changes in pH from the foods we eat?


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