style="font-size:15px;"> 2 KOH, 0.5 N3 HCl, 0.5 N
4 HCl, 0.001 N
5 Sprite® (Coca Cola Company) or comparable lemon‐flavored soda
6 Selected vegetables, e.g. fresh and canned tomatoes
7 Calibration buffers, pH 2 and 4
1.5 Procedures
1.5.1 Determining the pH of a Solid Food [5]
1 Cut a fresh tomato into small cubes and blend in a blender until a uniform slurry is formed, measure the temperature of the slurry.
2 Calibrate your pH meter.
3 Measure the pH of the slurry.
4 Centrifuge an aliquot of the slurry for 10 minutes at maximum speed.
5 Measure the pH of the supernatant.
6 Repeat Steps 1 through 5 using canned tomatoes.
1.5.2 Preparation of a Buffer and Determination of Buffer Capacity
1 Calculate the amounts of citric acid monohydrate and 0.5 N KOH required to prepare 200 ml of 0.05 M citrate buffer, pH 3. Note: Even though citric acid is a triprotic acid, calculations for this pH range are made using pKa = 3.06.
2 Prepare 200 ml of the buffer.
3 Measure the pH of your buffer. Is it 3.0? If not, can you explain why?
4 Determine the buffer capacity of your buffer in the alkaline direction by titrating a 100 ml aliquot with 0.5 N KOH. Express buffer capacity as the number of moles of OH− required to increase the pH of 1 l of the buffer by 1 pH unit.
5 Repeat Step 4 using 0.001 N HCl in place of the citrate buffer, i.e. determine the buffer capacity of 0.001 N HCl.
6 Determine the buffer capacity of your buffer and 0.001 N HCl by calculation. Compare your experimental results with your calculated answers. Explain any discrepancies between experimental and calculated values.
7 Determine the buffer capacity of Sprite® in the same way you did for your citrate buffer (Step 4 above).
1.6 Problem Set
1 The Ka for the weak acid HA is 4.0 × 10−6. What is the pH of a 0.01 M solution of this acid? What is its pKa?
2 How many grams of acetic acid and sodium acetate are required to prepare 1.0 l of 0.5 M acetate buffer, pH 4.5? The pKa for acetic acid is 4.75.
3 Explain carefully how to prepare 1.0 l of 0.05 M phosphate buffer, pH 7.0 from NaH2PO4 .H2O and 1.0 N NaOH or 1.0 N HCl. The molecular weight of sodium phosphate monohydrate monobasic is 138 g mole−1. The pKa2 for H3PO4 is 7.21. Hint: To determine whether you will need to add NaOH or HCl, you need to calculate the pH of a 0.05 M solution of NaH2PO4.
4 Preparation of buffers using published tables.Using Tables III.2a, III.2b, and III.2c in Appendix III, describe carefully how you would prepare 1 l of 0.1 M acetate buffer, pH 5.2 and 1 l of 1/15 M phosphate buffer, pH 7.6.Using the Henderson–Hasselbalch equation (shown below), calculate the theoretical pH of these 2 buffers. Explain why the calculated pHs are not exactly the same as the pHs shown in the Appendix table.
1.7 References
1 1 FDA (2019). Acidified & low‐acid canned foods guidance documents & regulatory information [Internet]. FDA [cited 2020 Mar 5]. http://www.fda.gov/food/guidance‐documents‐regulatory‐information‐topic‐food‐and‐dietary‐supplements/acidified‐low‐acid‐canned‐foods‐guidance‐documents‐regulatory‐information (accessed 5 March 2020).
2 2 Bartek Ingredients, Inc (2020). Self teaching guide for food acidulants ‐ Google search [Internet]. [cited 2020 Mar 5] p. 37. https://www.google.com/search?q=Self+teaching+guide+for+food+acidulants&rlz=1C1EJFC_enUS825US876&oq=Self+teaching+guide+for+food+acidulants&aqs=chrome..69i57j69i60.2887j0j4&sourceid=chrome&ie=UTF‐8 (accessed 5 March 2020).
3 3 USDA ARS (2020). pH of selected foods [Internet]. [cited 2020 Mar 5]. https://pmp.errc.ars.usda.gov/phOfSelectedFoods.aspx (accessed 5 March 2020).
4 4 Bennion, M. (1980). The Science of Food . San Francisco: Harper & Row. 598 p.
5 5 AOAC Official Method 981.12 (1982). pH of Acidified Foods . AOAC International.
1.8 Suggested Reading
1 Lindsay, R.C. (2017). Food additives. In: Fennema’s Food Chemistry , 5e (eds. S. Damodaran and K.L. Parkin ), 803–864. Boca Raton: CRC Press, Taylor & Francis Group.
2 Segel, I.H. (1976). Biochemical Calculations: How to Solve Mathematical Problems in General Biochemistry , 2e. New York: Wiley. 441 p.
Answers to Problem Set
1 pH = 3.7; pKa = 5.4.
2 14.68 g sodium acetate; 19.26 g acetic acid.
3 Use NaOH to adjust pH; dissolve 6 g NaH2PO4 (or 6.9 g NaH2PO4·H2O) in water (~900 ml); using pH meter, titrate to pH 7.0 with 1.0 N NaOH; transfer to a volumetric flask and dilute to 1 l.
4 a)pH 5.2 acetate buffer: mix 768 ml 0.1 M sodium acetate and 232 ml acetic acid. pH 7.6 phosphate buffer: mix 128 ml 1/15 M KH2PO4 and 872 ml 1/15 M Na2HPO4.b)Calculated pHs: Acetate buffer = 5.27; Phosphate buffer = 8.04.
2 Chemical Leavening Agents
2.1 Learning Outcomes
After completing this exercise, students will be able to:
1 Write balanced chemical equations showing the production of CO2 in various chemical leavening systems.
2 Determine, experimentally, the rates of CO2 release from selected leavening systems.
3 Select a chemical leavening system suitable for making biscuits, muffins, and other baked products.
4 Use the ideal gas law to calculate the volume of available CO2 in a leavening system.
5 Calculate the weight of a given leavening acid required to neutralize a given amount of NaHCO3.
2.2
