7 12 95 pH and Buffers by Richard B Brandt Ph D Professor of Biochemistry Molecular Biophysics and Preventive Medicine MCV Station Box 980614 Richmond VA 23298 0614 Tel 804 828 0104 Richard B Brandt Ph D Copyright 1973 1995 10 th Revision 1995 All Rights Reserved 1 INTRODUCTION Man is a creature that only exists within certain narrow limits of hydrogen ion concentration acidity The normal pH a measure of acidity of blood and many other 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 The body is equipped with physical and chemical means to maintain its pH within the physiological 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 The many cellular processes involving membranes biochemical structures and enzymatic reactions that you will learn in Biochemistry will involve pH and will very often be strongly dependent on pH The quantitative nature of the process of pH change and protection against change is important in understanding the biological processes The chemical buffer system provides a partial explanation for the control of the body pH within the narrow limits required for life OBJECTIVES The overall objective is to define and explain pH change and its control through the actions of buffers The Sub objectives are define and explain qualitatively and quantitatively the action of H3O in water define and explain qualitatively and quantitatively pH weak acids strong acids and buffers solve problems involving buffers and pH explain the importance of buffers in biological systems 2 PRE 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 b c d e pH strong acid weak acid salt buffer 4 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 pKa 5 and its salt 3 PRE TEST ANSWERS 1 log 1 0 log 0 1 1 Whoops If you have forgotten this please start the package at Appendix Section A 2 10 2 x 10 2 10 4 10 10 10 3 10 7 If 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 the Henderson Hasselbalch equation SALT pH pKa log 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 4 5 pH for 2 x 10 4 H log 2 log 10 4 0 30 4 3 7 Therefore in doubling H there is only a change of 0 3 pH units 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 X2 and 10 5 0 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 1 pH 5 log 1 and pH 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 through the packet If you find your time exceeding three hours QUIT See an instructor If you get most of the questions but want a brush up see Appendix E supplemental problems 5 PRACTICE CYCLE 1 Water Dissociation INPUT A lean 154 lb man 70 kilos is about 70 water This is most fortunate since water has excellent solvent properties that provide the medium for the physicochemical and biochemical reactions occurring in the body H3O 2H2O water OH eq 1 hydrated proton hydroxyl ion hydronium ion The reaction is directed so that the predominant form at equilibrium is undissociated water Eq 1 is important for solving problems involving the addition or removal of hydronium ions or protons H Expressed as a special chemical equilibrium with an equilibrium constant Kw Kw H3O OH 1 x 10 14 at 24 C eq 2 What is shown in both eq 1 and eq 2 is that an increase in H3O would result in 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 an increase in OH concentration since again the product of H3O OH must remain a constant 1 x 10 14 PRACTICE 1a When an acid this will be further defined but for now assume an acid donates H3O is added to water what is the general effect on OH Use equation 1 in answering the question STOP You should check FEEDBACK 1a after writing the answer and before you continue to the next practice question 6 FEEDBACK 1a From equation 1 the effect of increasing H3O would be to decrease 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 equilibrium so that any change in H3O must also have a corresponding but opposite change in OH PRACTICE 1b Now we can be specific and have a H3O …