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Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department pH and Buffers (B) 1 Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department By the end of this topic, the student will be able to: • Identify buffers • Demonstrate how buffer can resist changes in pH. 2 1-Acid and Bases Medical Biochemistry & Molecular Biology Department •Definition •General properties •Classification (strong , weak) Faculty of Medicine Ain Shams University 2-Henderson-Hasselbalch Equation • Calculation • Importance 3-Buffer • Definition • Principles of Buffering • Applications Strength Of Acids & Bases It depends on the degree of Ionization in an aqueous solution i.e. The efficacy with which an acid acts as a proton donor & a base acts as a proton acceptor Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department HA Let’s examine the behavior of an acid, HA, in aqueous solution. What happens to the HA molecules in solution? Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department HA 100% dissociation of HA H+ Strong Acid A- Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department HA H+ Partial dissociation of HA Weak Acid A- HA Faculty of Medicine + H Ain Shams University + A Medical Biochemistry & Molecular Biology Department HA H+ Weak Acid A- At any one time, only a fraction of the molecules are dissociated. HENDERSON-HUSSELBALCH EQUATION 1-Calculation 2-Uses How can you calculate the pH of Strong Acids & Bases ?? From the Molarity or Normality of them i.e. pH of Acid = - log of acid conc [H+] pH of Base = 14 - pOH Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department HA ↔ H+ + AKa = -Log Ka [H+] [A-] [HA] -] [A +] +-Log [H -Log = [HA] Values Of Ka like [H+] Involve –Ve Powers Of 10 & They are therefore expressed as pKa values pKa= -log Ka HENDERSON-HUSSELBALCH Equation pH = pKa [A-] + Log [HA] USES OF HENDERSON-HUSSELBALCH Equation 1. For Calculation of pH of a weak Acid. 2. Preparation Of Buffer Of Known pH When [A ]= pH = pKa [HA] [A-] +Log [HA] So PKa Of an acid is the pH at which [The Acid conc.] = [Its Conjugate base conc.] Summary The smaller the value of pKa The stronger the acid The greater the value of Ka The more it dissociates Strong Acid Transfers all of its protons to water Weak Acid Transfers only a small fraction of its protons to water Completely Ionized Partially Ionized Ka large Ka is small Smaller pKA larger pKA Eg. HCl and H2SO4 Eg. Organic acids e.g. H2CO3 16 BUFFER 1-Definition 2-Principle of buffering 3-Application Buffers It is a solution of A Weak Acid (H2CO3) & its Conjugate Base (NaHCO3) Or Weak Base (NH4OH) & its Conjugate Acid (NH4CL) Buffers It resists changes in pH in a solution when moderate amount of strong acids OHOH- or bases are added H+ Acid “H2CO3” OH- OH- Conjugate base “NaHCO3” H+ H+ H+ Principles of Buffering: AH ↔ H+ + A- The Extent Of Buffer Capacity Depends on 3 factors -]= [HA ] i.e the acid is half If [A Any buffer exerts dissociated (half-neutralization state) maximal Log [A-]/ [HA ] =buffering log 1= zero pH = pKa + 0 capacity when the pH is equal to its pKa. pH= pKa 1 Is Considered as pH range For SATISFACTORY Buffering Capacity Physiological buffers 1. H2CO3/ HCO3 (pKa= 6.1) This is the most important buffer system in the body 2. H2PO4/ HPO4 (pKa= 6.8). 3. Plasm Proteins (20% of buffering capacity “NH2+ , COO-”) 4. Hb. (60% of buffering capacity “Histidine”) 5. Free amino acids But Theoretically Bicarbonate buffer is < efficient than Phosphate buffer in intracellular fluid. The pKa of acetic acid is 4.76, its buffer mixture is most effective at pH: a) 2.3 b) 4.5 c) 3.7 pH = -log[H+] pOH= -log of [OH] pH + pOH =14 pH= pKa +log [A-] [HA-] Lab activities Demonstrate how buffer can resist changes in pH. Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department 27 Extended Modular Program 28