Lab 8 - Acids, Bases, and Buffers

Physiological Application of pH - Blood

Acidosis. Carbon dioxide. Bitter. Alkalosis. Sour. Bile. How are all of these things related?You have learned a great deal about acids and bases so far. Now, you get to actually see the physical and chemical properties of a variety of compounds in the lab. pH testing is useful in qualitative screening in medical settings. A quick, qualitative pH test may be appropriate in some situation – such as a urine screening, while a more specific quantitative pH test (using a pH meter) may be useful in a blood test where even 0.1 units of difference can be critical.

Many qualitative pH tests rely on indicators, chemicals that change color at various pHs. These can be used in liquid form or on paper, as you will see in the experiment today. Many deeply colored natural products, such as red cabbage, contain indicators. Red cabbage indicator is made by adding red cabbage to boiling water for a few minutes. Liquid indicators are often useful for a real-time monitoring of pH. Paper indicators are useful because they do not need to be added directly to the solution and are very portable.

Another physiological application of pH is found in blood. Blood pH must be carefully maintained between 7.35-7.45. Chemical systems called buffers aid in this process. Recall that buffers must contain a weak acid and its conjugate base (or a weak base and its conjugate acid), and that they resist changes in pH when small amounts of acid or base are added. The major buffer system in our blood comes from the carbon dioxide that is given off as a waste product of metabolism in the cells. This reaction occurs in the blood.

Carbon dioxide in the blood quickly forms carbonic acid. In the resulting buffer, the carbonic acid (H2CO3) is the weak acid, and the bicarbonate ion (HCO3- ) is the conjugate base. This buffer absorbs small changes in blood pH due to respiratory and metabolic fluctuations. The capacity of buffers to absorb pH change is limited, however. If too much strong acid or base is added, the buffer is used up and can no longer minimize pH changes; it is said to have reached its buffer capacity. All buffers operate on these same chemical principles, as you will see in the experiment today.1 The buffer that you make today will contain acetic acid, a weak acid, and its conjugate base acetate. These chemicals react as shown below.

Topics to Refer in Your Textbook

Topics and terms to refer to in your textbook Acid, base, weak acid, conjugate base, pH scale, neutralization, buffer, equilibrium, respiratory acidosis, respiratory alkalosis.

1. What is the pH of common household chemicals (using pH paper and indicators)?

2. Can a buffer solution minimize pH changes when acid and base are added?38

A.Experiment

B.Chemicals

C.Equipment

D.Baking soda (3% (m/v) sodium bicarbonate) spot plate

E.Milk of magnesia (5% m/v Mg(OH)2)

F.Dropper

G.Clear soda pop

H. graduated cylinders

I. Vinegar (0.1 M acetic acid)

J. pH paper

K. red cabbage indicator (RCI)

L. Glass stirring rod

M. 0.1 M sodium acetate

N. 4 - 100 mL beakers

O. 1.0 M HCl

P. 250 mL beaker

Q. 1.0 M NaOH

Each lab group should bring 2 mostly colorless liquid household chemicals to test for pH, about 5 mL each.

Handle strong acids and bases with care. Rinse skin immediately with plenty of water if skin contacts the acid or base. Do not consume any food or drink that are used in the experiment. WASTE NOTES: All solutions can be poured down the drain.

pH testing using pH paper and Red Cabbage Indicator (RCI)

1. In a 50 mL graduated cylinder, obtain about 25 mL of red cabbage indicator (RCI) to use today.

2. On a spot plate, prepare each well with 5 drops of each household chemical (vinegar, baking soda, soda, milk of magnesia and your samples from home), water, 1 M NaOH,and 1 M HCl. Test the pH of each solution with pH paper (half of a piece per chemical is sufficient). Record your results. IF RCI pH paper is ready, use it to test the pH of thewater, HCl, and NaOH. Next, add 5 drops of RCI to each solution. Record your observations.

3.Optional demo: pH paper is prepared by soaking paper in an indicator solution, and then allowing it to dry. In this way, an indicator can be made more portable and can be used without contaminating a sample. Your instructor will prepare pH paper by soaking filter paper in RCI, and allowing it to dry. Once it is available, test of the pH of water, HCl, and NaOH using this RCI pH paper. Record your observations.

Addition of acid and base to water – Not a buffer 

1.If you did NOT do part A, obtain about 25 mL of red cabbage indicator (RCI) in a 50 mL graduated cylinder.

Ask

2.Place 50 mL of tap water into a clean 250 mL beaker. Add 5 mL of RCI. This is five full droppers. Write your color observations. Stir well after adding indicator, and measure the pH with pH paper. Save this neutral solution for reference.

3. Use a graduated cylinder to measure 50 mL of tap water into a clean 100 mL beaker.

4. Add 5 mL of RCI (5 full droppers). Add 5 drops of 1.0 M HCl to the beaker. Write 39 your color observations. Stir well after adding acid, and measure the pH with pH paper. Save this strong acid solution as a reference.

5.Use a graduated cylinder to measure 50 mL of tap water into a clean 100 mL beaker.Add 5 mL of RCI (5 full droppers). Add 5 drops of 1.0 M NaOH to the beaker. Write your color observations. Stir well after adding base, and measure the pH with pH paper.

6.Can you reverse the indicator color change? To the beaker from step B-3, add 10 drops of 1.0 M HCl and stir. Record your observations. To the same beaker, add 1 mL of 1.0 M NaOH and stir. Record your observations. Save this basic solution as a reference.

Addition of acid and base to a buffer solution (acetic acid/acetate).

1. Prepare a buffer by following this procedure. Use a graduated cylinder to measure about 25 mL of the weak acid 0.1 M acetic acid into a clean 100 mL beaker. Add about 25 mL of 0.1 M sodium acetate (acetate is the conjugate base of acetic acid) to the same beakerand mix well with a glass stirring rod. Add 5 mL of RCI. Compare the color to thesolutions from Part B – is the buffer solution acidic or basic? Use pH paper to measure the pH of this buffer solution.

2. Add 5 drops of 1.0 M HCl to the beaker and stir well. Compare the color to the solutions from Part B – is the solution acidic or basic? Use pH paper to measure the pH of this buffer solution. Record your observations.

3. Return the buffer its original pH by adding 5 drops of 1.0 M NaOH. Stir the solution well.

4. Add 10 more drops of 1.0 M NaOH, and stir well. Compare the color to the solutions from Part B – is the solution acidic or basic? Use pH paper to measure the pH of this buffer solution. Record your observations.

5.To the buffer solution, add about 4 mL of 1.0 M NaOH. By comparing the color to the solutions from Part B, estimate the pH of this solution. Then, using pH paper, measurethe pH of this buffer solution. Record your observations. Did you reach the buffer capacity?

1. Ask a follow-up question to the experiment that you did today that includes only chemicals available in lab. Briefly describe how you could answer it using an experiment and the chemicals that are available in the lab. Write your question/experiment in the

Table 3 in the Results section.

1. Do that experiment, and record your data and observations in Table 3.

2. Dispose of all solutions from today’s experiment in the sink.