7/12/95pH and BuffersbyRichard B. Brandt, Ph.D.Professor of Biochemistry & Molecular Biophysics, and Preventive MedicineMCV Station Box 980614Richmond VA 23298-0614Tel#(804) 828-0104Richard B. Brandt, Ph.D., Copyright © 1973, 199510 th Revision, 1995 All Rights Reserved1INTRODUCTIONMan is a creature that only exists within certain narrow limits of hydrogen ionconcentration (acidity). The normal pH (a measure of acidity) of blood and manyother biological fluids is about 7.4. At a blood pH of 7.0, death in acidic coma results,while at blood pH of 7.8, death occurs from convulsive contractions (tetany). Thebody is equipped with physical and chemical means to maintain its pH within thephysiological range. These controls of pH in healthy humans include respiration,digestion, the kidney and blood but will not be presented in detail in this package. Themany cellular processes involving membranes, biochemical structures andenzymatic reactions that you will learn in Biochemistry will involve pH and will veryoften be strongly dependent on pH. The quantitative nature of the process of pHchange and protection against change is important in understanding the biologicalprocesses. The chemical buffer system provides a partial explanation for the controlof the body pH within the narrow limits required for life.OBJECTIVESThe overall objective is to define and explain pH change and its control throughthe actions of buffers.The Sub-objectives are:define and explain qualitatively and quantitatively the action of H3O+ in waterdefine and explain qualitatively and quantitatively pH, weak acids, strong acids and bufferssolve problems involving buffers and pHexplain the importance of buffers in biological systems2PRE-TEST(Please complete this prior to using the instructional package.)1. What is the log of 1? What is the log of 0.1?2. What is 10-2 x 10-2 ? What is 10-10/10-3?3. Define: (a) pH(b) strong acid(c) weak acid(d) salt(e) buffer4. What is the pH of 1.0 x 10-4 [H+]?5. What is the pH change found by doubling the hydrogen ion concentration referred to in question 4?6. What is the change in H+ concentration by increasing the pH 1.0 unit from that of question 4?7. Is the solution in question 6 more acid than a 0.1 M solution of acetic acid (pKa = 5)? Show proof.8. What is the pH of a buffer composed of equal amounts of a weak acid (pKa5) and its salt?3PRE-TEST ANSWERS1. log 1 = 0 log 0.1 = -1Whoops!! If you have forgotten this, please start the package at:Appendix Section A.2. 10-2 x 10-2 = 10-410-10/10-3 = 10-7If you did not get the above answer, start the package at:Appendix Section B.3. (a) pH = -log [H+], a measure of H+ concentration(b) A strong acid is completely ionized in dilute solution so that the H+ concentration is equal to the strong acid concentration.(c) A weak acid is only partly dissociated in solution. The concentration of H+ in solution will not be equal to the weak acid concentration, but is dependent on the Ka and the weak acid concentration. (d) A salt is the product of an acid and a base. Most common salts are completely ionized in dilute solutions.(e) A buffer is a mixture of a weak acid and its salt. For the effective range of a buffer (near the pKa of the weak acid) the addition of small amounts of acid or base will have little effect on the pH. A common biochemical and clinical equation that is used for the calculation of pH is theHenderson- Hasselbalch equation:pH = pKa + log [SALT][WEAK ACID] This package will treat many qualitative and quantitative properties of buffers.4. pH for 1 x 10-4 [H+].pH = -log [H+].pH = -(log 1 + log 10-4).log 1 = 0 and log 10-4 = -4. So that pH = -(-4) and pH = 4.45. pH for 2 x 10-4 [H+]- log 2 + -log 10-4-0.30 + 4 = 3.7Therefore, in doubling [H+], there is only a change of 0.3 pHunits.6. pH of 5 has a [H+] of 10-5.7. A 0.1 M solution of HAc has a pH of about 3. Ka = [H+] [Ac-][HAc]and 10-5 = X20.1M so that X = √10-6 = 10-3. [H+] must equal [Ac-]. The [H+] is therefore 10-3 and the pH = -log [H+] which is 3.The answer to the question is no, the solution from #6 with a pH of 5 is not more acid.8. A solution of equal amounts of a weak acid and its salt is a buffer. The Henderson-Hasselbalch equation applies. (See 3c). So that using a pK of 5,and [salt] = [weak acid]:pH = 5 + log 11andpH = 5 + log 1. Since the log of 1 is zero, then pH = 5.If you have not been able to readily answer these questions, work throughthe packet. If you find your time exceeding three hours -- QUIT!! -- See aninstructor!If you get most of the questions, but want a brush-up, see: Appendix E,supplemental problems.5PRACTICE CYCLE #1Water DissociationINPUT: A lean 154 lb man (70 kilos) is about 70% water. This is mostfortunate since water has excellent solvent properties that provide the mediumfor the physicochemical and biochemical reactions occurring in the body.2H2O H3O+ + OH- eq. 1water hydrated proton + hydroxyl ion(hydronium ion)The reaction is directed so that the predominant form at equilibrium isundissociated water. Eq. 1 is important for solving problems involving theaddition or removal of hydronium ions or protons (H+). Expressed as a specialchemical equilibrium with an equilibrium constant, Kw:Kw = [H3O+] [OH-] = 1 x 10-14 (at 24°C) eq. 2What is shown in both eq. 1 and eq. 2 is that an increase in H3O+ would resultin a decrease in OH-. In order to maintain the Kw at 1 x 10-14, when H3O+increases then OH- decreases. A decrease in H+ would, of course, result in anincrease in OH- concentration, since again the product of [H3O+] [OH-] mustremain a constant (1 x 10-14).PRACTICE 1a: When an acid (this will be further defined, but for now assumean acid donates H3O+) is added to water what is the general effect on [OH-]?Use equation 1 in answering the question. !!STOP!!** You should checkFEEDBACK 1a after writing the answer and before you continue to the nextpractice question.6FEEDBACK 1a: From equation 1, the effect of increasing [H3O+] would be todecrease [OH-] since equation 1 is written as an equilibrium and the product of[H3O+] [OH-] is a constant at a particular temperature.Equation 1 with H2O predominantly undissociated, represents an equilibriumso that any change in [H3O+] must also have a corresponding but oppositechange in [OH-].PRACTICE 1b: Now - we can be specific and have a [H3O+] of 1 x 10-2moles/liter. What is the [OH-]? Write your answer with