Caution Acetic acid is corrosive and sodium hydroxide is caustic. Purpose To determine the concentration of a common acid by acid-base titration. Introduction Analytical chemistry is a branch of chemistry concerned with sample analysis what is present and how much is present. Research in this field focuses on both developing new analysis methods, as well as regenerating older methods for new applications. More recently, applications in this field have focused on in situ methods, in which analysis is performed on molecules present in a system that is as near to reality as possible. Instrumentation also plays an important role in this field by reducing the time required for analysis as well as improving the quality. Before an analysis can be performed, chemical properties of the sample must be known in order to choose the proper method (although with modern developments there are some exceptions). Simple chemical tests can be performed to determine the nature of a substance. Some of the most widely used methods in analytical chemistry are wet-chemical methods, which involve solutions and known chemical properties, e.g. if the substance is an acid or base. Titration of an acid or base was first introduced by Guy-Lussac, who invented Guy-Lussac s tower, which is referred to as a buret in these modern times, as well as other volumetric glassware. Because acids and bases encompass a wide variety of both inorganic and organic compounds, they are found in a wide variety of commercial products. For quality-control purposes, acid-base titrations are still in wide use. Acid-base titrations rely upon the neutralization reaction just enough base is added to a sample of acid for a complete reaction. Typically one solution is of known concentration (standard solution or titrant) while the other is of an unknown concentration. The equivalence point of the titration is the point at which just enough standard has been added to completely react the unknown. At this point, the number of moles of standard has been added in the exact stoichiometric ratio to the number of moles of unknown present. This point can be detected by using an acid-base indicator or by using a potentiometric probe. An acid-base indicator is a dye molecule that changes color depending on whether it is present in acidic or basic solutions. A potentiometric probe measures the electrical potential (voltage) difference across a membrane. A ph meter is one of the most common potentiometric probes, as it measures the activity of hydrogen ions outside the glass membrane as compared to a standard. To determine the concentration of an unknown as accurately as possible, carefully measure the amounts of unknown and standard solution. Special glassware has been designed for just this purpose; burets, transfer pipettes and volumetric flasks. It is also important to perform multiple titrations to attempt to eliminate any user errors that may occur. In this experiment the concentration of acetic acid (HC2H3O2) in vinegar will be determined by acid-base titration. Your instructor will tell you whether you will be titrating vinegar using an indicator or a ph meter. The vinegar concentration determined by titration will be compared to the listed value on the vinegar container. Revision Su15 Page 1 of 9
Procedure for Use of an Indicator (phenolphthalein) 1. Obtain one 10 ml volumetric pipette, one 50 ml buret, two small beakers and three 250 ml Erlenmeyer flasks plus one large beaker for the collection of waste solutions. 2. Label one of the small beakers Vinegar, and place about 50 ml of commercial vinegar in it. Make sure to record the brand of vinegar and its listed concentration on the data sheets. 3. Label the other small beaker NaOH, and add about 60 ml of standard base solution. Make sure to record the concentration of the standard base from its container on your data sheets. 4. Make sure the buret stopcock is closed. Use a buret funnel to carefully add a small portion (5-10 ml) of the NaOH solution to the buret. Rinse the buret allowing the entire portion to drain into the waste beaker after each rinse. Repeat this step for a total of two rinses. 5. Fill the buret up to near the 0.00 ml mark with the standard NaOH solution. Open the stopcock quickly and completely to allow the base to fill the tip of the buret. Close the stopcock and record the initial volume of base (which may or may not be 0.00 ml) to the nearest 0.01 ml on your data sheets. 6. Use a pipette bulb to draw some vinegar into the volumetric pipette. Rinse the inside of the pipette with this solution. Drain this solution into the waste beaker. 7. Weigh a 250 ml Erlenmeyer flask, and record this mass on the data sheets. Transfer 10.00 ml of vinegar into this flask using a volumetric pipette and weigh it again. (This mass and the delivered volume will be used to determine the density of the vinegar solution). 8. Add about 50 ml of de-ionized water and a few (2-5) drops of phenolphthalein indicator to the vinegar sample; swirl to mix. 9. Titrate this solution, continuing to swirl, until a faint pink color persists for ~ 20 seconds. It may be helpful to place a piece of white paper under the flask to see the endpoint. The color should not be deep pink this means too much base was added. Record the new volume of base remaining in the buret to the nearest 0.01 ml on your data sheets. (Be sure that the level of the base does not drop below the 50.00 ml mark of the buret. If the level of the base is approaching the 50.00 ml mark of the buret during a titration, stop the titration and consult your instructor.) 10. Repeat the titration (steps 7-9) two additional times. Be sure to record the mass of your samples each time. Procedure for Use of a ph meter (potentiometric titration) 1. Obtain one 10 ml volumetric pipette, two small beakers, three 200 ml beakers, and one large beaker for the collection of waste solutions. You will need to use a drop counter and ph meter as well. This will already be set up for you and should look like the assembly in Figure 1. Do not touch the assembly until you have read the rest of the directions below. Figure 1. Assembly required for potentiometric titration. The reagent reservoir should allow drops to fall through the drop counter into your solution. The probe should not touch the beaker. Revision Su15 Page 2 of 9
2. Label one of the small beakers Vinegar, and place about 50 ml of commercial vinegar in it. Make sure to record the brand of vinegar and its listed concentration on the data sheets. 3. Label the other small beaker NaOH, and add about 60 ml of standard base solution. Make sure to record the concentration of the standard base from its container on your data sheets. 4. The reagent reservoir is not as accurate as a buret that was used for the procedure above. As such, a drop counter is utilized to determine delivered volume of NaOH. Carefully add a small portion (5-10 ml) of the NaOH solution to the reservoir. Rinse the reservoir by TURNING the BOTTOM STOPCOCK. Do not touch the top stopcock. The drop counter has been calibrated for you. If you turn the top stopcock by mistake, you will change the size and rate of the drops that are dispensed, and you will no longer have an accurate reading of volume. You will then need to re-calibrate the drop counter which takes time, so DO NOT touch the top stopcock. Let your instructor know if you accidently do. Allow the entire portion of NaOH to drain into the waste beaker after each rinse. Repeat this step for a total of two rinses. 5. Fill the reservoir with the standard NaOH solution. Refill for each trial. Open the bottom stopcock quickly and completely to allow the base to fill the tip of the reservoir. Close the bottom stopcock. Place the reservoir over the drop counter with a waste beaker below the drop counter. You will want to make sure the drops are going through the opening on the drop counter, indicated by the star in Figure 2. When running, the red light will light up if drops are falling in an appropriate place. 6. Wash the ph probe with de-ionized water. Blot dry with a Kimwipe. Be careful with the glass on the end of the sensor as it is fragile. Place the ph probe through the opening in the drop counter. Make sure it does not come into contact with the glass beaker. You will either use the stir bar that comes with the ph sensor or a separate little magnetic stir bar as instructed by your instructor. If you use a magnetic stir bar, make sure you do not wash the stir bar down the drain. 7. Use a pipette bulb to draw some vinegar into the volumetric pipette. Rinse the inside of the pipette with this solution. Drain this solution into the waste beaker. 8. Weigh a 200 ml beaker, and record this mass on the data sheets. Transfer 10.00 ml of vinegar into this flask using a volumetric pipette and weigh it again. (This mass and the delivered volume will be used to determine the density of the vinegar solution). 9. Add about 50 ml of de-ionized water; swirl to mix. Place the solution under the drop counter with the ph sensor submerged in your solution. Slowly turn on the stir plate. You want to make sure the stir bar is not causing the solution to splash and also not hitting the side of the beaker or the ph sensor. The solution is now ready to be titrated. Figure 2. Drop counter shown with opening where drops should pass indicated by a star. The red light will light up if drops are falling in an appropriate place. 10. On the computer, LoggerPro software will already be open and ready to use. The ph sensor and drop counter will have been calibrated prior to lab. Make sure the ph reading in the bottom left hand corner of the screen is reading acidic. If it is basic, neutral, or not reporting a value, contact your instructor. Revision Su15 Page 3 of 9
11. You are now ready to begin. Press the green collect button before starting your titration. 12. Start the titration by opening the BOTTOM stopcock. You will start to see the program generating a graph that looks like the curve shown in Figure 3. Stop the titration (press the stop button) after the graph has leveled off at a basic ph. You will want a couple of minutes of data collected at this basic ph. Your data should be shown in a graph very similar to the graph shown in Figure 3. 13. Repeat the titration (steps 8-12) two additional times. Between trials, make sure to rinse and blot dry your ph sensor and refill your NaOH reservoir. Be sure to record the mass of your samples each time. When you press the play button to begin your second and third trials, a dialogue box will open with saving options. Choose, Store Latest Run. 14. Place the ph sensor back into its storage solution. 15. You have now collected three titration curves and need to determine the equivalence volume for each curve. The equivalence volume is the volume of base required to neutralize your acid. It is the volume at the inflection point of your curve. In order to find the inflection point of your curve, you need to look at the graph of the first derivative. Steps 16 21 will take you through the steps required to determine this equivalence volume. You will need to print off one graph for each trial with the equivalence volume marked on each. See Figure 4 as an example of what you need to turn in (times 3). 16. Right click on your graph and choose, Graph Options. 17. In the Graph Options Window under Axes Options you will be able to check the box for the information you wish to display on the titration curve. 18. In the Graph Options Window under the Graph Options tab, make sure the following boxes in the Appearance Section are checked: Connect Points, Y Error Bars, and X Error Bars. 19. Now switch to the Axes Options tab of the Graph Options Window. Under Y-Axis Columns, check the boxes for ph and 1 st Derivative for the trial you wish to view first. Click Done. This will generate a graph that looks like Figure 4. 20. Move your cursor over the maximum point of your first derivative curve and record the equivalence volume. (#9 on your data table) Figure 3. Example of a completed acid-base titration curve. 21. Right-click on your graph and select copy. Then open a word document and paste the image into your document. You can resize these graphs to fit three figures on one page. Repeat these instructions (Steps 16 21) for each of your three trials so your word document has 3 graphs, one for each trial. 7.35 ml Figure 4. Example of a completed acid-base titration curve (red) and its corresponding first derivative (green). The tallest data point in green correlates with the equivalence volume. Revision Su15 Page 4 of 9
Calculations: 1. Determine the moles of base delivered for each trial. 2. Use stoichiometry to relate the moles of base to moles of acid present in the vinegar sample for each trial. 3. Use the mass of acid present and the mass of the solution before titrating to determine the %(w/w) concentration. Waste Disposal Pour all solutions into the waste beaker. If the result is pink, it may be poured down the sink. If it is not pink, add just enough base to make it basic, then pour it down the sink. Clean-Up Wash all glassware with soap then rinse 3 times with tap water, and once with de-ionized water. Revision Su15 Page 5 of 9
Data Sheet Show all work for calculations on a separate piece of paper. Name: Lab Partner: Identity of vinegar sample: ***If you are performing the potentiometric titration, you will leave numbers 7 8 blank. Write the balanced chemical reaction occurring in the titration: 1. Volume of vinegar transferred (ml) Trial 1 Trial 2 Trial 3 2. Mass of empty flask (g) 3. Mass of flask and vinegar sample (g) 4. Mass of vinegar transferred (g) 5. Density of vinegar (g/ml) 6. Concentration of NaOH standard solution (mol/l) 7. Final buret reading (ml) 8. Initial buret reading (ml) 9. Volume of NaOH transferred (ml) (Equivalence Volume) 10. Number of moles of NaOH used 11. Number of moles of HC 2H 3O 2 in sample 12. Mass of HC 2H 3O 2 in sample 13. Molar concentration of HC 2H 3O 2 in sample (mol/l) 14. Percent by weight concentration [% (w/w)] of HC 2H 3O 2in vinegar sample 15. Average % (w/w) concentration of HC 2H 3O 2in vinegar sample Listed Concentration of HC2H3O2 in vinegar (% w/w): Revision Su15 Page 6 of 9
Post-Lab Questions Name: Lab Partner: 1. Explain why adding distilled water to the vinegar sample prior to titration does not affect the concentration determination. 2. Answer one of the following depending on which type of titration you completed: a. Indicator Titration: Why is it important not to add so much NaOH that the vinegar solution becomes bright pink (or magenta)? b. Potentiometric Titration: Why was the ph at your equivalence point basic? Use the balanced chemical reaction on page 6 to help answer this question. 3. Considering the reported concentration, calculate a percent error in your determination. 4. How well does your experimental value for the concentration of acetic acid in vinegar match that on the label? List some possible sources of error. 5. How many ml of 0.101 M HCl are needed to neutralize 1.00 gram of potassium hydroxide? Revision Su15 Page 7 of 9
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Pre-Lab Questions Name: 1. Define the following in your own words: a. standard solution b. equivalence point c. indicator 2. Hydrogen peroxide, H2O2, is used as a disinfectant to treat minor cuts or scrapes. The label claims this solution is 3.0 % (w/w); that is there are 3.0 grams of H2O2 in every 100 grams of solution. What is the molar concentration of this solution? (Assume a density of 1.00 g ml -1 ) 3. A titration is performed to determine the amount of sulfuric acid, H2SO4, in a 12.00 ml sample taken from car battery. About 50 ml of water is added to the sample, and then it is titrated with 48.62 ml of a standard 0.6403 M NaOH solution. What is the concentration of H2SO4 in the sample? Revision Su15 Page 9 of 9