Measuring pH of Common Foods

Introduction

The pH of a solution is a measure of the concentration of hydrogen ions. In water, molecules dissociate into ions. The two ions produced are expressed as the hydrogen ion (H+) and the hydroxide ion (OH-). If there are more H+ ions than OH- ions, the solution is acidic. If there are more OH- ions than H+ ions, the solution is basic. If the concentration of H+ ions and OH- ions are equal, the solution is neutral. By definition, an acid has a pH below 7; a base has a pH above 7; and the pH of 7 is neutral. In reality, most solutions with a pH range of 6-8 are neutral solutions.

What You Will Investigate?

The pH of common foods

Goals

  • Measure pH
  • Compare the pHof various foods
  • Determine if various foods are acidic, basic, or neutral

Equipment

  • Nova5000 data logger
  • pH sensor
  • pH electrode
  • sensor cable
  • 50mL beakers, 9-oz. Styrofoam cups, or other containers to hold tap water and various “foods”
  • sharpie or marker to label containers which hold tap water and various “foods”
  • tap water
  • various “foods” such as coke, milk, apple juice, coffee, crushed aspirin dissolved in 100 mL of distilled water, crushed Mylanta dissolved in 100 mL of water

Safety Precautions

  • Follow standard safety procedures for laboratory activities in a science classroom.
  • Do not touch the end of the pH electrode or bump the end into anything, as it may break.
  • If you are not using your pH electrodefor an extended period, place the end of the electrode in your container of tap water. The bulb at the end of the electrode will damage if it dries out.
  • All “foods” used in this activity may be disposed of by pouring them down the sink drain.

Equipment Setup Procedure

1.Turn on the Nova5000.
2.Connect the pH sensor and pH electrode to each other.
3.During this activity, you will not be plotting a graph, but only using the Meter View to determine the pH of various “foods.” Therefore, you will NOT enter Setup to determine a rate nor number of samples. There is NO Data Logger Setup in our directions. In addition, you will NOT click on Runin the upper toolbar.

Experimental Procedure

1.Predict the pH of tap water, coke, milk, apple juice, coffee, aspirin and Mylanta;record your predictions in the data table.
2.Carefully remove the pH electrode from the buffer storage container and thoroughly rinse the bulb (end of the electrode) and rinse the end of the sensor in running tap water.
3.Connect the pH sensor to input 1 (I/O-1) of the data logger, using the sensor cable.
4.Launch MultiLab (StartProgramsScience & MathMultiLab)
5.Click Meter view on the upper toolbar and then click Digital view on the lower toolbar.
6.Place the pHelectrode into the container filled with tap water. When the reading stabilizes (about 2 minutes), record the pH of the tap water in your data table. Indicate in the data table whether water is an acidic, basic, or neutral “food.”
7.Remove the pH electrode from the tap water and dip it into the coke. When the reading stabilizes (about 2 minutes), record the pH of the coke in your data table. Indicate if coke is acidic, basic, or neutral.
8.Remove the pH electrode from the coke and dip it in the tap water again, stirring gently without bumping the end of the electrode. Rinse the electrode for about 30 seconds.
9.Remove the pH electrode from the tap water and dip it in the milk. When the reading stabilizes (about 2 minutes), record the pH of the milk. Indicate whether milk is acidic, basic, or neutral.
10.Remove the pH electrode from the milk and dip it in the tap water again, stirring gently without bumping the end of the electrode. Rinse the electrode for about 30 seconds.
11.Repeat step 9 and 10 using applejuice, coffee, aspirin, and Mylanta. Be certain to rinse the end of the electrode in tap water between each food.
12.Carefully rinse your pH electrode in running water, and return your electrode to the buffer storage container.

13.Clean up. All “foods” used in this activity may be poured down the sink. Run plenty of tap water into the sink after disposing of all liquids.

14.Answer the analysis questions located beneath your data table.

Data Table

Type of Food

/

Predicted pH

/

Actual pH

/

Acid, Base or Neutral

Tap water

Coke

Milk

Apple juice

Coffee

Aspirin

Mylanta

Questions and Conclusions

Analyze the data table and answer the following questions.

  1. Is the tap water acidic, basic, or neutral?

______

  1. What quantitative evidence indicates whether the tap water is acidic, basic or neutral?

______

______

  1. What can you say about the concentration of H+ ions and OH- ions in tap water?

______

______

______

  1. How does the pH of the milk compare with the pH of tap water?

______

______

  1. How does the pH ofcoke compare to the pH of milk?

______

  1. What was your predicted pH of coffee?

______

  1. As the pH increases, does the concentration of H+ ions increase or decrease?

______

  1. Are there more H+ ions or more OH- ions in coffee?

______

  1. Explain how your predicted pH of coffee compares to the actual pH of coffee.

______

  1. As you go from a pH of 4 to a pH of 3, does the concentration of OH- increase or decrease?

______

  1. As you go from a pH of 5 to a pH of 2, does the concentration of H+ increase or decrease?

______

  1. What inferences can you make (concerning pH) about someone who drinks 5-7 cokes a day?

______

  1. Does coffee or milk have a greater concentration of H+ ions?

______

  1. Explain your answer to the previous question concerning the concentration of H+ ions in coffee and milk.

______

  1. Aspirin is acidic. Why should you NOT take an aspirin with coke?

______

  1. Of the foods you tested, which one or ones would be best to drink while taking an aspirin?

______

  1. Explain your answer to the last question in detail.

______

Teacher Notes

To prolong the life of the pH electrode, rinse in distilled water after use and store in equal parts pH buffer of 4 and 1 molar KCl. A mold inhibitor should also be added. If mold does form on the sensor, rinsethe mold off in tap or distilled water. Never store the pH electrode in distilled water; use tap water if no electrode storing solution is available. During the activity, the end of the electrode should not be allowed to dry out. Students should place the end of the electrode in a container of tap water if they are not actively using the electrode.

When using the Meter View , without clicking Run, data is cannot be saved. If you want to project data as a line graph and/or save data, Run must be clicked. Suggested Set Up: Rate- every second; Samples- 1000 for a run time of 16 minutes 40 seconds.

Answers to Questions and Analysis

  1. Is the tap water acidic, basic, or neutral?

The pH should be neutral. (City water systems often add a buffer to the water supply.)

  1. What quantitative evidence indicates whether the tap water is acidic, basic or neutral?

Answers may vary. (A reading of approximately 7 on the pH meter.)

  1. What can you say about the concentration of H+ ions and OH- ions in tap water?

The concentration of H+ ions and the concentration of OH- ions are equal in tap water, since the pH of tap water is around 7.

  1. How does the pH of the milk compare with the pH of tap water?

Answers may vary, depending of the pH of local tap water. The pH of both tap water and milk should be around pH 7 or neutral.

  1. How does the pH ofcoke compare to the pH of milk?

The pH of coke is much lower than the pH of milk. Coke has a pH of approximately 3 while milk’s pH is around 7. Milk is neutral while coke is acidic.

  1. What was your predicted pH of coffee?

Answers will vary, but should match what was predicted in the data table.

  1. As the pH increases, does the concentration of H+ ions increase or decrease?

Decreases

  1. Are there more H+ ions or more OH- ions in coffee?

More H+ ions

  1. Explain how your predicted pH of coffee compares to the actual pH of coffee.

Answers will vary, depending upon individual predictions.

  1. As you go from a pH of 4 to a pH of 3, does the concentration of OH- increase or decrease?

OH- ions decrease

  1. As you go from a pH of 5 to a pH of 2, does the concentration of H+ increase or decrease?

H+ ions increase

  1. What inferences can you make (concerning pH) about someone who drinks 5-7 cokes a day?

There may be too much acid in their stomach

  1. Does coffee or milk have a greater concentration of H+ ions?

Coffee

  1. Explain your answer to the previous question concerning the concentration of H+ ions in coffee and milk.

The pH of coffee is around 6 while the pH of milk is around 7. The lower the pH, the greater the amount of H+ ion concentration.

  1. Aspirin is acidic. Why should you NOT take an aspirin with coke?

You would be taking an acidic aspirin with an acidic coke. This may result in too much acid in the stomach.

  1. Of the foods you tested, which one or ones would be best to drink when taking an aspirin?

Milk or tap water.

  1. Explain your answer to the last question in detail.

Milk or tap water will tend to neutralize the low pH of the aspirin.