ph units constitute a scale which allows scientists to determine the acid or base content of a substance or solution. The ph 0

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1 ACID-BASE TITRATION LAB PH 2.PALM INTRODUCTION Acids and bases represent a major class of chemical substances. We encounter them every day as we eat, clean our homes and ourselves, and perform many other daily activities. Figure 1 shows the ph values of some common substances. Acids can be recognized by their sour taste, while bases are known for their bitter taste and slippery feel. One way to define acids and bases chemically is that acids are substances that donate protons or H + ions to a solution whereas bases accept H + when in solution. ph units constitute a scale which allows scientists to determine the acid or base content of a substance or solution. The ph 0 Lemon juice (~ 2.0) Acid rain (3.5-4) ionization of water is the basis for the ph scale. Ionization of water refers to the breaking apart of water molecules resulting in hydrogen ions ACIDIC (H + ) and hydroxide ions (OH - ). The Milk (6.6) ph scale ranges from 0, the most acid, to 14, the most basic. Neutral ph, 7.0 NEUTRAL (H + = OH - ) Blood ( ) where the concentrations of H + and OH - are equal, is set at 7, in the BASIC Tums (~9.0) Hair remover (~ 12.5) ph 14 middle of the scale. ph is on a logarithmic scale. This means that a change of 1 ph unit is equivalent to a 10-fold change in H + ions. If a solution increases from a ph of 3.0 to a ph of 4.0, then there are 10 times fewer H + ions in the solution. Figure 1. ph values of some common substances A titration is a process used to determine the volume of a solution needed to react with a given amount of another substance. In this experiment, you will titrate hydrochloric acid solution, HCl, with a basic sodium hydroxide solution, NaOH. The concentration of the NaOH solution is given and you will determine the unknown concentration of the HCl. PH2.PALM-1

2 Hydrogen ions from the HCl react with hydroxide ions from the NaOH in a one-to-one ratio to produce water in the overall reaction: H + (aq) + Cl (aq) + Na + (aq) +OH (aq) H 2 O(l) + Na + (aq) + Cl (aq) When an HCl solution is titrated with a NaOH solution, the ph of the acidic solution is initially low. As base is added, the change in ph is quite gradual until close to the equivalence point, when equimolar amounts of acid and base have been mixed. Near the equivalence point, the ph increases very rapidly. The change in ph then becomes more gradual again, before leveling off with the addition of excess base. One consideration in a titration is the ionic strength, or molarity of the NaOH titrant and the molarity of the unknown HCl. If the molarity of the NaOH is higher than that of the HCl, it will be able to accept more H + ions. This means that the volume of NaOH solution necessary to reach the equivalence point will be less than if the NaOH and HCl solutions have the same molarity. Differences in molarity between the NaOH titrant and the unknown HCl are accounted for in the equation given in the Processing the Data section on p. 4. In this experiment, you will use a ph Sensor to monitor ph as you titrate. The region of most rapid ph change will then be used to determine the equivalence point. The volume of NaOH titrant used at the equivalence point will be used to determine the molarity of the HCl. PURPOSE The purpose of this experiment is to find the concentration of a hydrochloric acid solution. To accomplish this, a graph of ph versus volume of sodium hydroxide solution added will be plotted and interpreted. EQUIPMENT/MATERIALS LabPro interface Palm handheld Data Pro program ph Sensor deionized water and wash bottle magnetic stirrer (if available) stirring bar 50-mL burette HCl solution, unknown concentration 0.1 M NaOH solution ring stand utility clamps (2 per station) 10-mL pipette and pipette pump 250-mL beaker goggles gloves PH2.PALM-2

3 SAFETY Always wear an apron and goggles in the lab. 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. PROCEDURE 1. Prepare the ph Sensor for data collection. a. Plug the ph Sensor into Channel 1 of the LabPro interface. Connect the handheld to the LabPro using the interface cable. Firmly press in the cable ends. b. Remove the ph Sensor from the ph storage solution bottle by unscrewing the lid. Carefully slide the lid from the sensor body. c. Rinse the tip of the sensor with distilled water. 2. Titration with a Burette a. Use a pipet bulb (or pipet pump) to pipet 10 ml of the HCl solution into a 250- ml beaker. Add 50 ml of distilled water. CAUTION: Handle the hydrochloric acid with care. It can cause painful burns if it comes in contact with the skin. b. Place the beaker on a magnetic stirrer and add a stirring bar. If no magnetic stirrer is available, you need to stir with a stirring rod during the titration. c. Use a utility clamp to suspend a ph Sensor on a ring stand as shown in Figure 2. Position the ph Sensor in the HCl solution and adjust its position so that it is not struck by the stirring bar. d. Obtain a 50-mL burette and rinse the burette with a few ml of the ~0.1 M NaOH solution. Before pouring the NaOH into the burette, be sure that the stopcock is closed (perpendicular to the glass burette). Dispose of the rinse solution in the extra beaker provided. e. Use a utility clamp to attach the burette to the ring stand as shown in Figure 2. Fill the burette a little above the 0.00-mL level of the burette with ~0.1 M NaOH solution. Drain a small amount of NaOH solution so it fills the burette tip and leaves the NaOH at the 0.00-mL level of the burette. Record the precise concentration of the NaOH solution in your data table. CAUTION: Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing. f. Proceed to Step 3. Figure 2. Clamping ph sensor and burette 3. Turn on the handheld. To start the Data Pro program, tap the Data Pro icon on the Applications screen. Choose PH2.PALM-3

4 New from the Data Pro menu or tap to reset the program. 4. Set up the handheld and interface for the ph Sensor. a. On the Main screen, tap. b. If the handheld displays ph in CH 1, proceed directly to Step 5. If it does not, continue with this step to set up your sensor manually. c. Tap to select Channel 1. d. Press the Scroll buttons on the handheld to scroll through the list of sensors. e. Select PH from the list of sensors. 5. Set up the data-collection mode. a. While still on the Setup screen, tap, then choose Events with Entry from the list. b. Enter the x-axis label (Volume). For the unit, enter ml. You can enter this information using the onscreen keyboard (tap abc ). c. Tap twice to return to the Main screen. 6. You are now ready to perform the titration. This process is faster if one person adds NaOH titrant, while another person operates the handheld and enters volumes. a. Tap on the Main screen to begin data collection. b. Before you have added any of NaOH solution, tap and enter 0 as the burette volume (using the numerical keyboard displayed on the screen). Tap to store the first data pair for this experiment. Record this in your lab handout as well. c. Add the next increment of NaOH titrant (enough to raise the ph about 0.15 units). When the ph stabilizes, tap and enter the current burette reading (to the nearest 0.01 ml) and then tap. You have now saved the second data pair for the experiment. Be sure to record each data point in the Data Table provided in the Data Sheet portion of this lab handout. This information will be used to create a graph. d. Continue adding NaOH solution in increments that raise the ph by about 0.15 units and enter the burette reading after each increment. When a ph value of approximately 5.5 is reached, change to a one-drop increment. Enter a new burette reading after each increment. Note: It is important that all increment volumes in this part of the titration be equal; that is, one-drop increments. Read the burette as accurately as possible; two numbers past the decimal for both the ph and the volume are acceptable in this experiment. e. After a ph value of approximately 10 is reached, again add larger increments that raise the ph by about 0.15 ph units, and enter the burette level after each increment. Continue adding NaOH solution until the ph value remains constant. 7. Tap when you have finished collecting data, then tap to view a graph of ph vs. volume. PH2.PALM-4

5 8. Use the data recorded on your Data Sheet to plot the titration curve on the provided graph paper. You may compare the graph you plot to the graph displayed on the Palm. 9. Use your graph to find the equivalence point that is the largest increase in ph upon the addition of 1 drop of NaOH solution. Find the NaOH volume just before this jump. Record this value in the Calculation Table. Then record the NaOH volume after the drop producing the largest ph increase was added. To examine the data pairs on the displayed graph on the Palm, tap or any data point. As you move to examine the line, the ph and volume values of each data point are displayed to the right of the graph. 10. (Optional) Print a copy of the graph of ph vs. volume from the Palm. 11. Dispose of waste solutions as directed by your instructor. Rinse the ph Sensor and return it to the ph storage solution. PH2.PALM-5

6 PROCESSING THE DATA 1. Use your printed graph and data table to confirm the volume of NaOH titrant you recorded before and after the largest increase in ph values upon the addition of 1 drop of NaOH solution. 2. Determine the volume of NaOH added at the equivalence point. To do this, add the two NaOH values determined above and divide by two. 3. Determine the concentration of the hydrochloric acid solution (in M or mol/l), using the formula: M acid = M base X V base V acid where M acid is the concentration of the acid (in M or mol/l), M base is the concentration of the base, V base is the volume of the base, and V acid is the volume of the acid. Use the precise concentration of the sodium hydroxide solution provided by your instructor (it is ~0.10 M). The volume of sodium hydroxide solution at the equivalence point was determined from the experiment. Recall that you initially added 10.0 ml of the unknown HCl. References: Introduction and Laboratory Protocol adapted from: Gastineau, J.; Holmquist, D. D.; DeMoss, G.S. Science with Handhelds: Acid-Base Titration; Vernier Software & Technology: Beaverton, OR, 2002; pp Starr, Cecie. Biology: Concepts and Applications. Chemical Foundations for Cells. Brooks/Cole-Thomson Learning, Inc. Fifth Edition pp PH2.PALM-6

7 DATA SHEET ACID-BASE TITRATION Name Name Period Class Date CALCULATIONS TABLE Concentration of NaOH (M) NaOH volume added before the largest ph increase NaOH volume added after the largest ph increase Volume of NaOH added at equivalence point Concentration of HCl (M) QUESTIONS 1. What is the concentration of the unknown HCl stock solution? (The original 10 ml of HCl)? 2. What is the final concentration of the HCl solution (10 ml + 50 ml water)? 3. What is the difference in the concentration of the NaOH and the final HCl solutions? How does this affect the titration? (Consider the molarity of the two solutions.) PH2.PALM-7

8 4. Normal rainwater has a ph of approximately 5.5. In western Pennsylvania, though, rain with a ph of is recorded regularly. Does this change represent an increase or decrease in H +? By how much does the H + ion level increase or decrease? PH2.PALM-8

9 DATA TABLE ph Volume NaOH (ml) PH2.PALM-9

10 Data Table, continued ph Volume NaOH (ml) PH2.PALM-10

11 PH2.PALM-11