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16. EVALUATION OF LEMONADE AS A BUFFER

IntroductionBuffers prevent rapid changes in pH when acids or bases are added to a system. Living cells must be buffered in order to maintain constant pH for cellular processes. Consumer products, especially pharmaceuticals and food products, are oftentimes buffered to maintain their activity and enhance consumer preference. In this lab, you will test the effects of buffering by a store-bought lemonade drink mix on a pH titration curve. The results will be compared to a non-buffered citric acid solution. The inquiry will extend to investigate the effect on titration curves of different concentrations and/or brands of lemonade mixes.

Concepts Buffer Buffer capacity pH Dissociation constant Weak acids and bases Conjugate acids and bases Ka and pKa

BackgroundWhen a consumer purchases a lemonade drink mix from a store, it is reasonable to expect a consistent flavor profile unique to a specific brand. This is a factor that drives consumer preferences in commercially available products. Unfortunately, tap water from different geographical regions varies greatly in pH: some sources are acidic, while others are basic. Many powdered lemonade mixes are buffered to ensure flavor consistency for the consumer regardless of the pH of the local tap water.The ability of buffers to resist pH changes when either hydroxide ions or protons are added is attributable to its chemical composition. A buffer is composed of a weak acid or weak base and its salt. Commercial lemonade mixes are made with citric acid and its conjugate salt, sodium citrate.The pH of a buffered solution is calculated using the Henderson-Hasselbach equation:

pH = pKa + log ¿¿The buffering capacity of a solution indicates the amount of protons or hydroxide ions the buffer is able to absorb without causing a large change in pH. It is determined by the magnitude of [HA] and [A-] whereas the pH of a solution is determined by the ratio of [A-]/[HA]. It is the concentration of a buffer that is directly correlated to its buffering capacity. Simply put, buffering capacity can be manipulated simply by adjusting the total concentration of the buffer. The pH of a buffer solution can be changed by altering the ratio of the acid or base and its salt.In a buffer made with from a weak acid and its conjugate base, H+ will be present in solution due to the ionization of the weak acid

HA ⇄ H+ + A-and the equilibrium constant for this reaction is the Ka of the weak acid. When hydroxide ions are added to the system, they combine with dissociated protons in solution to form

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16. EVALUATION OF LEMONADE AS A BUFFER / STUDENT HANDOUT

water. This causes the equilibrium of the ionization to shift right thereby causing additional weak acid to dissociate and reestablish the equilibrium.Citric acid (H3C6H5O7) is triprotic. The Ka and pKa of each of the protons are listed below.

Proton Ka pKa

1 7.1 x 10-4 3.152 1.7 x 10-5 4.773 6.4 x 10-6 5.19

The buffer range for a citrate buffer is 3 – 6.2. Country Time® Lemonade Flavor Drink Mix contains 1.6 g/L citric acid. When a consumer purchases a lemonade drink mix from a store, it is reasonable to expect a consistent flavor profile unique to a specific brand. This is a factor that drives consumer preferences in commercially available products. Unfortunately, tap water from different geographical regions varies greatly in pH: some sources are acidic, while others are basic. Thus, many powdered lemonade mixes are buffered to ensure flavor consistency for the consumer regardless of the pH of the local tap water.

Pre-Lab Question

Define equivalence point. The titration curve of a weak acid with a strong base is represented in the graph below. Identify the equivalence point on the graph. Is the pH at the equivalence point acidic, basic or neutral? Identify the point on the graph where pH equals pKa.

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Materials and EquipmentUse the following materials to complete the initial investigation. For conducting an experiment of your own design, check with your teacher to see what materials and equipment are available. Data collection system Wireless pH sensor Wireless drop counter 0.008 M Citric acid, H3C6H5O7 Sodium citrate dihydrate, Na3C6H5O7∙2H2O 0.1 M Sodium hydroxide, NaOH, 200 mL Wash bottle with distilled water Distilled water Graduated cylinders, 50-mL (2) Beaker, 150-mL (1) Beaker, 250-mL (1) Magnetic stir plate and stir bar Volumetric flask, 500-mL Ring stand Country Time®Lemonade dry mix Variety of fruit-flavored dry mixes, optional (e.g. Kool-Aid®, Crystal Light®, Monster Energy®, Mountain Dew®, etc.)

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SafetyFollow these important safety precautions in addition to your regular classroom procedures: Wear safety goggles and gloves at all times. This lab uses a weak acid and a strong

base. Sodium hydroxide is caustic and should be handled with special care. In case of contact with your skin, wash off acid and base solutions with a large amount of

water. Wash hands thoroughly with soap and water before leaving laboratory. Review chemical handling and disposal instructions as directed by Material Safety Data

Sheet.

DisposalIf your drain system is connected to a sanitary sewer system, the following instructions apply. Base solutions, citric acid and drink mixes may be rinsed down the drain with an excess of water.

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Initial Investigation

pH Titration curve for 0.008 M Citric Acid

1. Start a new experiment on the data collection system from your Chromebook, computer or mobile device.

2. Set up the wireless drop counter apparatus on a magnetic stir plate and connect to the data collection system. Open lab file 16 Evaluation of Lemonade as a Buffer or create a graph of pH vs. Volume of NaOH (mL).

3. Close stopcock assemblies of the syringe reservoir and fill with approximately 60 mL of 0.1 M NaOH.

4. Set a 250-mL beaker and open both stopcocks. Allow a small volume (approximately 2 mL) of NaOH to flow through and remove air bubbles. Close the top stopcock. Repeat as necessary to remove all trapped bubbles.

5. Gradually open the top stopcock and adjust to a drop rate of approximately 2 drops/second.

6. Close the bottom stopcock. The flow rate of your drop counter is now set and titration will be controlled by opening and closing the bottom stopcock.

7. Calibrate the drop counter (Reference Guide 013-16026A at pasco.com) and set the number of drops/mL for the apparatus.

8. Rinse wireless pH meter with distilled water and calibrate if necessary.

9. Transfer 20.0 mL of 0.008 M citric acid into a clean 150-mL beaker and set on the magnetic stirrer with a stir bar.

10. Adjust the pH meter so that the electrode is completely immersed in the acid, turn on magnetic stirrer and adjust to spin the stir bar gently.

11. Open the bottom stopcock and begin data collection. Your program will display pH on the y-axis and Volume (mL of NaOH) on the x-axis.

12. When titration is completed (pH should read 11.5 or greater), close bottom stopcock and stop data collection. Rinse pH electrode with distilled water in preparation for next titration.

13. Dispose of waste solutions according to the Safety Guidelines. 

pH Titration curve of Lemonade

14. Start a new run on the data collection system. Open page 2 of the lab file or create a new graph of pH vs. Volume.

15. Transfer 20.0 mL of lemonade into a clean 150-mL beaker and set on the magnetic stirrer with a stir bar.

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16. Follow Steps 10 to 13.

17. Compare the titration curves of citric acid and lemonade.

Titration Curves of Citric Acid and Lemonade

18. How are the two graphs generated in this Initial Investigation similar? How are they different? Relate the appearance of the two graphs to the similarities and differences between the lemonade and citric acid solutions.

19. How would your curves differ if the concentration of citric acid solution and lemonade solutions were increased? Make a prediction before performing the Advanced Investigation.

Advanced Investigation

Lemonade as a buffer

1. Design a guided inquiry experiment to investigate the titration curves using different concentrations of lemonade. Record data on page 2 of the lab file. Does the concentration of lemonade mix change the titration profile with 0.1 M NaOH as a titrant? Explain.

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Comparison of titration curves of citric acid, 1X lemonade and 2X lemonade

Titration curves of various buffered and unbuffered drink mixes

2. Examine the labels of at least two alternative drinks or drink mixes and identify the components that make up a buffer. Design a guided inquiry experiment to investigate the titration curves of different drink mixes. Record data on page 3 of the lab file or create a new graph of pH vs. Volume. Do the samples display characteristics of buffered or non-buffered solutions? Explain.

Titration Curves of Commercial Drink Mixes

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Extended Inquiry Investigation

Real world connections

Design an experiment that illustrates how oceans buffer acid rain. What do you believe are the major buffers found in the ocean ecosystem? What chemical reactions can you present to show how buffering occurs?

Synthesis Questions

1. The acidity constant of formic acid (HCOOH) is Ka = 1.8 x 10–4. Would you expect a higher or lower pH if formic acid were used instead of your unknown at the half-titration point? Explain your answer! The acidity constant of formic acid (HCOOH) is Ka = 1.8 x 10–4. Would you expect a higher or lower pH if formic acid were used instead of your unknown at the half-titration point? Explain your answer!

2. If you had to determine the acidity constants of oxalic acid, which is a diprotic acid (Ka1 = 6.5 x 10–2, Ka2 = 6.1 x 10–5), what differences would you expect to find in the titration curve?

AP® Chemistry Review Questions

1. A buffered solution contains 1.0 M HA (Ka = 1.8 x 10-5) and a 1.0 M NaA. A titration of 20.0 mL of the solution was performed using 100.0 mL of 1.0 M NaOH. The following graph was generated.

Add the curve you would expect if the buffered solution contained 2.0 M HA and 2.0 M NaA and explain why this difference occurs.

Add the curve you would expect if the buffered solution contained 0.5 M HA and 0.5 M NaA and explain why this difference occurs.

What is the most noticeable difference in the graphed data if the buffer were made with twice as much HA as NaA?

What would be the most noticeable difference in the graphed data if the buffer were made with twice as much NaA as HA?

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