Anatomy & Physiology

Acids, Bases and Buffers INTRODUCTION When a water molecule splits, it splits into a positively charged hydrogen (H+ ) ion and a negatively charge OH- (called hydroxide) ion. In a sample of pure water, the number of hydrogen ions equals the number of hydroxide ions – giving a neutral pH. This equilibrium between ions can shift if we mix other substances with water. When the compound HCl (Hydrogen Chloride) is dissolved in water it separates into 2 ions: a positively charged hydrogen proton and a negatively charged chlorine ion. The positively charged hydrogen ion (H+) increases the concentration of H+ in the water. Our sample becomes acidic because it contains more H+ ions than OH- ions. The original compound that we added, HCl, is said to be an acid because it donates H+. But how do we measure the concentration of an acid or base? The acidity of a solution is measured using the pH scale. The pH scale corresponds to the concentration of hydrogen ions in a solution. For example, the pH of a solution of pure water is 7 (neutral). The pH scale ranges from 0 to 14, below 7 is acidic and above 7 basic. The further from 7 you are on the pH scale, the more acidic or basic the solution. The table below illustrates the pH of some common substances.

Acid-base chemistry is important for your body to function properly. The body continually works to maintain homeostasis – in this case maintain a stable internal pH - in order to survive. Most cells and body fluids maintain a pH close to neutral even though biochemical activities of living tissues frequently tend to change the pH. So how then, do living tissues maintain homeostasis of pH? Many naturally occurring substances are capable of stabilizing pH. These chemicals – called Buffers - allow changes in pH to be “absorbed”. In this way, the tissues don’t experience drastic changes in pH. One of the most important buffers in your body is the one found in blood. Your blood buffering system allows your blood to stay in a very narrow pH range even if acids or bases are introduced. Living cells are also equipped with internal buffers because metabolism results in the formation of acids and bases as waste products. In this lab you will first practice testing the pH of common acids and bases. Then, you will measure the effect of an acid and a base on different living and non-living substances to determine their buffering abilities. You will use water as a control for comparison.

OBJECTIVES In this experiment, you will     

Review tests that determine pH of a substance. add an acid and a base to water and note the extent to which it affects pH. add an acid and a base to a buffered solution and note the extent to which it resists changes in pH. Add acid and base to living cells and note the extent to which natural buffers resist changes in pH. work with classmates to compare the ability of different materials to resist changes in pH

MATERIAL 0.10 M HCl (acid) with dropper pH indicator/paper stirring rod various common acids and bases for testing Various homogenates of living cells

0.10 M NaOH (base) with dropper Small cups/beakers distilled water Various non-living buffers

SAFETY: You must wear proper eye protection. Optional: a chemical apron. In order to get accurate data – you must pay attention and NOT cross contaminate the solutions. Use the droppers that are specifically labeled for specific solutions. CAUTION: Handle the hydrochloric acid with care. It can cause painful burns if it comes in contact with the skin. Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing. Wash your hands immediately and wipe of any spills if you get any on yourself or the table.

Exploration 1: pH 1. 2. 3. 4. 5. 6. 7. 8. 9.

You will use a spot plate for these tests. Rinse your spot plate off with water before starting. Select a known acid from the available samples. Place 3 drops in one of the wells. Using pH paper or Universal Indicator, determine the pH of the substance. Now drop 15 drops of water in the sample. Stir it with a toothpick and touch the tip of the toothpick to a dry spot on your pH paper. Reach the pH. Did it change? Repeat steps 1-5 above with a second acid sample. Select a known base from the available samples. Repeat steps 3-6 with your base samples. Rinse out your plate with lots of water when you are finished.

Exploration 2: Part I: Testing the Buffering ability of living substances against acids and bases. Write a Hypothesis: On the data table sheet – write a hypothesis for this lab. Predict how you think living substances will compare to non-living substances in their ability to buffer again changes in pH. A. Testing the effect of acid on water 1. Put 20 mL of water in a small cup.

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2. Dip the end of a toothpick into the water and touch the wet end to a spot on the pH paper. Record the pH of the water in Table 1, in the column for “0” drops. 3. Add 5 drops of acid to the beaker. Stir the solution thoroughly with a toothpick. 4. Using the tip of the toothpick as before, determine the pH of the solution and record it in Table 1. 5. Repeat Steps 3-4, adding 5 more drops each time until a total of 30 drops has been added to the beaker. Be sure to stir and record after each 5 drops. 6. Rinse the beaker out thoroughly. B. Testing the effect of base on water 1. Put 20 mL of distilled water in your beaker. 2. Test the pH of the water and record it in Table 1, in the column for “0” drops in the “base” row for Water. 3. This time, use 0.1 M Sodium Hydroxide (NaOH) and repeat Steps 3-6 as you did for the acid, recording your results. C. Testing the effect of acid and base on living materials 1. Be sure your beaker is rinsed out well. 2. Choose a sample of “living” substance from those available (yeast, potato, orange juice, spinach, liver, etc.) 3. Place 20 mL of test sample in the beaker. Record its initial pH in the “0” column. 4. Repeat Steps 3-6 (Part A) above, using acid (HCl). Record your results. 5. Now repeat using the same substance and the base (NaOH). 6. Select another sample to test and repeat the acid/base test on that sample. Be sure to record all your results in Table 1. 7. Find another lab group who has tested different materials and fill out your remaining row in Table 1 using their data (you should now have date for water and 3 other substances). D. Compute the change in pH (Δ pH) by subtracting the lowest pH number from the highest. Record this in Table 1.

Part II: Testing the buffering ability of Commercial substances against acids and bases. You can purchase commercial buffers to control fluctuating pH. They act similar to the natural buffers found in your cells and blood. In this series of tests, you will use the same testing procedure as you followed for Part I above, but you will test non-living substances for their ability to buffer against pH changes. 1. Choose 2 substances to test. 2. Complete the tests as described above for Part I using acid and base. 3. Fill in Table 2 as you complete the tests. When you are finished. Rinse out all beakers, clean up your area, and return any materials back to their lab bucket or the proper place on the counter. Be sure to wipe up any water that may have spilled on your work area.

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DATA Table 1 – impact of Acids and Bases on various living substances Substance tested

Add

pH, after adding this many drops of acid (HCl) 0

5

10

15

or base (NaOH) 20

25

30

pH

Buffer

acid WATER base acid base acid base acid base

Table 2 – impact of Acids and Bases on various commercial substances Substance tested

Add

pH, after adding this many drops acid (HCl) 0

5

10

15

base (NaOH) 20

acid

(same data as Table 1 above)

base

(same data as Table 1 above)

25

30

pH

Buffer

WATER acid base acid base acid base

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PROCESSING THE DATA 1. Make two, colored, line graphs of the data in Table 1 (biological buffers). Construct the graphs using these guidelines: a. b. c. d.

The X-axis (horizontal) has Volume scaled from 0 to 30 drops. The Y-axis (vertical) has pH scaled from 0 to 14. The acid and base data you obtained for water should be included on the graphs. Construct one graph with your water data and the acid data, construct the second graph with the water and the base data in Table 1. e. Be sure you use different colors or line patterns and provide a key or label each line. 2. Calculate the overall change in pH for each material and record in Table 1. The change in pH can be found using the equation: pH = pH at 30 drops – pH at 0 drops 3. Determine the buffering ability of each substance listed in Table 1. To do that, subtract the ΔpH of your acid test from the ΔpH of base test for each substance. Record in the “Buffer” column of Table 1. 4. Repeat steps 1-3 above for the data in Table 2 (commercial buffers). 5. In Table 3, make a list of each substance you tested (including water). Place the most acidic material (look at the pH for “0” drops column) at the top of the list and the most basic material at the bottom of the list. This value represents the natural pH of the material. Table 3 Substance

Initial pH

Rank most acidic 2 3 4 5 6 least acidic

6. Put the substances tested into the following three categories:

Biological organisms (tissues or cells)

Biological chemicals

Non-biological chemicals water

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7. List each substance tested with its buffer value in Table 3. Order the materials from worst buffering ability (largest buffer number) to best buffering ability (smallest buffer number). Table 3 Substance

Buffer

Rank worst 2 3 4 5 6 best

ANALYZING THE DATA Provide the answers to these questions in your written lab report as described in the “Final Write Up” section below. 1. Summarize the effects of HCl and NaOH on tap water. Does there seem to be a steady increase or a sudden jump? Is there any correlation between the number of drops and the change in pH? 2. How should the pH of a substance in the data table’s “acid” row compare to that in the table’s “base” row before any acid or base is added? Why? 3. Refer to your data. Does your data support your hypothesis? If not, what might cause the differences? 4. Generally, what was the effect of adding HCl to each solution? Was this true for every solution? Why do you think this happened the way it did? 5. Generally, what was the effect of adding NaOH to each solution? Was this true for every solution? Why do you think this happened the way it did? 6. Compare the various graphs of each substance. Why was it beneficial to include the plot of water in acid and water in base with each graph? 7. Which class of materials (biological organisms, biological chemicals, or non-biological chemicals) reacted most dramatically (had the greatest fluctuation) to the addition of acid or base? How does this relate to their complexity? 8. Which of the materials in Table 3 is the best buffer? The poorest buffer? 9. Look at the total pH change for the acid and base added to the liver. Compare this to what happened with tap water. How do you account for the difference in the liver and tap water? 10. Does the potato act more like the liver or tap water? Describe what may account for this. 11. Write a short paragraph and describe what you think would be the advantage of having organic buffers in cells.

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FINAL LAB WRITE UP You will be writing a formal lab report for this experiment. You can discuss results and answers to questions with your lab group – but ultimately, you written report is your own work. It must be typed and include all the information described below. Formal Lab Report Format Writing Your writing should be in full sentences and easily understood. It should conform to the conventions of standard written English (sentence form, grammar, spelling, etc.). While this is a science paper, not English writing, you should still strive for presentations that are free of spelling errors and grammatically correct. Presentation The first page of a lab report should include the title of the lab, the names of the students in your group (be sure your name is clearly identified as belonging to the report), the date, the name of the class and class period. There is no minimum required number of pages as long as all the required information is included and addressed thoroughly. Be sure to attach your data and analysis sheets to the report. Include the following components in your report: Introduction What is the problem being studied? Write a paragraph that defines the problem studied by the experiment. Provide some background information to help the reader understand the importance of the experimental question/problem. Materials and Methods Describe, in general, the experimental procedure. What did you do? You do not need to list all the materials and the procedures, step by step as long as you cite the source of the procedures you followed. You need to provide enough information that the reader could repeat the experiment exactly as you did it. In this case the citations should be: “Followed protocol as described in “Acids, Bases and Buffers”, Mrs. Whittaker, THS, October 2011.” You must be sure to identify any changes you made from the original lab instructions. Data and Results Include a heading for this in your report. Refer the reader to your data tables and graphs that you will attached to the end of the report. Discussion This is where you will write the answers to the 11 analysis questions from the lab. When you are finished answering the questions, be sure to write a closing paragraph that summarizes the results of the experiment and discusses whether the results helped answer the original experimental question and your hypothesis. References and plagiarism If you use outside sources, be sure to cite the sources!

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