Unit 3 Acids, Bases, pH This lab uses the following hazardous chemicals: I. 0.1 M Hydrochloric Acid (HCl) II. 0.1 M Sodium Hydroxide (NaOH) As a result students are required to wear, at minimum, Goggles & Gloves. Abstract: This lab examines the concept of pH and will help you to understand what changes in pH mean at the molecular level. You will learn three methods of determining pH and will decide which of these methods is most accurate and therefore most useful to you in the lab. You will then utilize your knowledge of pH and the methods available for measuring pH to investigate the pH of bottled water and a “real life” application of this knowledge for potential clinical use. 3.1 Visualizing pH Using Gum Drops. Note LAB PREP: DO THIS EXERCISE AT HOME BEFORE COMING INTO LAB Introduction This interactive exercise is designed to help you visualize water (H2O) and the result of its dissociation into hydroxide ions (OH-) and hydrogen ions (H+, or “protons”). Protons will associate with water molecules to form Hydronium ions (H3O+) in aqueous solutions, but this dissociation can be simply expressed as: H2O ↔ OH- + H+ Pure water at neutral pH (pH=7) exists as an oxygen atom covalently bound to two hydrogen atoms (H2O) plus hydroxide ions (OH-) and hydrogen ions which each exist individually at concentrations of 10-7moles/liter. When additional hydrogen atoms are added to pure water (such as what occurs when adding acid), the concentration of hydrogen atoms increases. This means that the negative exponent used to express concentration of hydrogen gets smaller…10-6, 10-5, 10-4, 10-3 ,10-2 moles/liter. When additional hydroxide atoms are added to pure water (such as what occurs when adding a 35 base), the concentration of hydrogen atoms decreases as the hydroxide ions associate with hydrogen atoms to form H2O. This decrease in hydrogen atoms means that the negative exponent used to express hydrogen atom concentration will increase…10-8, 10-9, 10-10 , 10-11 ,10-12moles/liter, etc). In this assignment, you will use two colors of gum drops to form H2O, to form hydrogen and hydroxide ions, and to demonstrate to yourself what changes in pH look like at a molecular level. Materials and Methods Access the interactive PP tutorial for Exercise 3. 1 1. Access the PP from the student lab manual website, see resources from unit 3. 2. Read the instructions for the tutorial. 3. Read each slide carefully and follow the prompts. 4. Answer all the questions to assess your level of comprehension. Note The remainder of this lab will be completed in your laboratory section. 3.2 Accurately determining pH Introduction There are several methods that can be employed to determine the pH of a solution. In this exercise, you will determine the pH of a solution using three different methods. Materials and Methods Method 1 In method 1, you will take advantage of the fact that anthocyanins (plant pigments) change color in response to changes in pH. These pigments are responsible for some of the color variations found in fruits and flowers. These anthocyanins, were extracted from red cabbage by boiling the red cabbage for three minutes and then filtering the cabbage extract. You will use the anthocyanins to determine the color at each of the indicated pH to construct a set of standards to use for comparison to various unknowns. 1. Label seven test tubes: 2, 4, 6, 7, 8, 10, and 12. 36 2. Place 5 ml of each buffer (2, 4, 6, 7, 8, 10, and 12) into each test tube. 3. Place 3 ml of cabbage extract into each test tube and mix well Note You should observe a color change throughout the entire solution, not just the top or bottom of the solution). 4. Record the color of each tube in figure 1: pH Color 2 4 6 7 8 10 12 Figure 1. Colorimetric scale of pH buffers ranging from pH 2 to pH 12. 1. Label three test tubes: A, B, and C. 2. Place 5 ml of unknown solution A, B, and C in the appropriately labeled test tube. 3. Add 3 ml of cabbage extract and mix well. 4. Compare the color of each solution to your standards, and determine the approximate pH of that solution. 37 5. Allow your sample buffers the remainder of the lab to incubate on your desktop. At the end of lab, before you dispose of them, check to see if any of the solutions have changed color further, and note these changes in Figure 1. Results A: __________ B: __________ C: ___________ Method 2 Method 2 will utilize pH paper. pH paper is specially design by impregnating the paper with pH indicators. You will submerge a test strip of this paper and compare the color it turns in response to the solution to the color key provided. Using one piece of pH paper for each sample, measure the pH of each of the three unknown solutions directly from their respective beakers at the back of the room. DO NOT record the pH from the test tubes that have the unknowns mixed with the cabbage juice. Record the color and the approximate corresponding pH from the colorimetric scale on the pH paper beaker in figure 2: pH Paper Scale Results Sample Color pH Unknown A Unknown B Unknown C Figure 2. Approximate colorimetric and pH analysis of unknowns using pH paper. 38 Method 3 In method 3 you will use a pH meter. Standard pH meters have a glass electrode that is sensitive to hydrogen ions and a reference electrode which completes the electrical circuit. On some pH meters a combination electrode performs both functions. Before a pH meter can be used, it must first be calibrated using solutions of known pH. Usually two buffers (one buffer at pH 7.0 and another at either pH 4.01 or 10.01) are used to calibrate the pH meter before use. These buffers should be at the same temperature as the sample, and should be near the expected pH (acidic or basic) of the sample. Calibrate your red pH meter at the back of the room according to the instructions on the piece of paper at the calibration station. Once you have calibrated your meter, measure the pH of the three unknown samples, and record the values in the figure 3: Results Sample pH Unknown A Unknown B Unknown C Figure 3. Unknown pHs as calculated by electronic pH meters. Conclusions When working in a science lab, accuracy of the data recorded is usually preferred over the quickness with which the data can be recorded. In your group, discuss the three methods of determining pH that you have just applied. Points for Discussion: 1. Would you have been able to determine the pH of the three unknowns using the cabbage juice if you had not made the test tubes containing each of the buffers (pH 2-12) and the cabbage juice? 39 2. If the colorimetric scale for the pH paper, had been lost would you have been able to determine the pH of the unknowns effectively? Why or why not? 3. Which of these three methods (anthocyanins, pH paper, or pH meter) took the shortest amount of time to yield a pH reading for the unknowns? 4. For both the anthocyanins and the pH paper, what do you notice about the colors yielded for a pH of 6 compared to a pH of 7? Are they very similar or very different? 5. Suppose that you were given a sample that had a pH of 6.5, but you were unaware of the pH. Can you perceive a problem that would arise if you attempted to determine the pH of this sample using either anthocyanins or pH paper? 6. Do you think you would have the same problem from question E if you were instead given a pH meter to measure the pH? Based on your above answers, which of those techniques yielded results fastest, and which technique yielded the most accurate results. Record your answer below: _____________________________________________________________________ Once you have come to an agreement, check your answers with your instructor before progressing. 3.3 Determining the buffering capacity of a solution Introduction Buffering capacity is measured by how much the solution resists change in its pH when acid or base is added to the solution. A solution has a high buffering capacity when it is 40 observed to resist changes in pH when an acid or base is added. A solution has a low buffering capacity when it is observed that pH changes fairly quickly when acid or base is added. How does buffering work? Buffering is effected by the presence of a weak acid/weak base pair present in the solution. By weak acid/weak base, we are referring to a solution where there is not complete ionization of the acid when dissolved in water. An example is carbonic acid/bicarbonate in your blood. H2CO3 + H2O ↔ H3O + + HCO3-In this reaction, you can see that the reaction is occurring in both directions. H2CO3 (carbonic acid) acts as an acid, able to donate protons as base is added. This results in no net change in the pH of the solution. When acid (protons) is added, this solution is able to absorb the protons using the conjugate base HCO3- (bicarbonate ion), forming H2CO3. In other words, buffering takes place by having both an acid and its conjugate base present in solution such that each species is capable of combating a change in pH due to addition of either acid or base. Materials and Methods Scientific Method The Independent Variable in this experiment is:_______________________________. The Dependent Variable in this experiment is:________________________________. 1. Pour 40 ml of solution A into a 100 ml beaker. Put a stir bar in the beaker and place the beaker on a magnetic stir plate. 2. Determine the pH of the solution. Record the value. Initial pH of solution A: ________________ In the next steps you will be adding Hydrochloric Acid and Sodium Hydroxide, which are hazardous substances. Goggles and Gloves must be worn from this point on! 3. Add 1.0 ml of 0.1 N HCl to solution A. Allow the acid to mix thoroughly. 4. Read the pH value and record the value at “1 ml HCl added” in the Figure 4. 41 5. Add another 1 ml of HCl and record the new pH at “2 ml HCl added” in the table below. 6. Continue to add 1 ml of HCl at a time and record the pH value until you have added 10 ml. 7. Rinse the electrode with distilled water. Dispose of your solution and rinse the beaker and stir bar. 8. Put another 40 ml of solution A into the beaker and repeat the procedure using 0.1 N NaOH instead of HCl. Record your values in the “ml of NaOH added” portion of Figure 4. Results ml of acid or base added 1 2 3 4 5 6 7 8 HCl NaOH Figure 4. Varying pHs of Solution A with the addition of HCl or NaOH 9. Repeat the procedure using solution B. Record your results in Figure 5. Initial pH of solution B: ____________________ 42 9 10 Results ml of acid or base added 1 2 3 4 5 6 7 8 9 HCl NaOH Figure 5. Varying pHs of Solution B with the addition of HCl or NaOH. Conclusions 1. Please draw a graph of your data for both Solution A and Solution B in the graph provided on the next page. 43 10 2. Which of the two Solutions, A or B, was a buffer? Please explain your answer. 44