Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department 1 pH and Buffers (B)

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department 2 By the end of this topic, the student will be able to: Identify buffers Demonstrate how buffer can resist changes in pH.

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department 2-Henderson-Hasselbalch Equation Calculation Importance 1-Acid and Bases Definition General properties Classification (strong, weak) 3-Buffer Definition Principles of Buffering Applications

Strength Of Acids & Bases Ionization It depends on the degree of Ionization in an aqueous solutioni.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 H+H+ A-A- Strong Acid 100% dissociation of HA

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department HA H+H+ A-A- Weak Acid Partial dissociation of HA

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department HA H+H+ A-A- Weak Acid HA  H + + A - At any one time, only a fraction of the molecules are dissociated.

1-Calculation 2-Uses HENDERSON-HUSSELBALCH EQUATION

How can you calculate the pH of Strong Acids & Bases ?? From the Molarity or Normality of themi.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 H A ↔ H + + A - [HA] [H + ] [A - ] = KaKa = KaKa [H + ] + [HA] [A - ] -Log Values Of Ka like [H + ] Involve –Ve Powers Of 10 & They are therefore expressed as pK a values pK a = -log Ka

pH pKa = [HA] [A - ] + Log HENDERSON-HUSSELBALCH HENDERSON-HUSSELBALCH Equation

USES OF HENDERSON-HUSSELBALCH Equation 1.For Calculation of pH of a weak Acid. 2.Preparation Of Buffer Of Known pH

[A - ]= [HA] When [A - ]= [HA] So PKa Of an acid is the pH at which The Acid conc.Its Conjugate base conc. [The Acid conc.] = [Its Conjugate base conc.] pH pKa = + [HA] [A - ] Log

Summary The stronger the acid The smaller the value of pKa The greater the value of Ka The more it dissociates

16 Strong AcidWeak Acid Transfers all of its protons to water Transfers only a small fraction of its protons to water Completely IonizedPartially Ionized Ka largeKa is small Smaller pKAlarger pKA Eg. HCl and H 2 SO 4 Eg. Organic acids e.g. H 2 CO 3

1-Definition 2-Principle of buffering 3-Application BUFFER

Buffers (H 2 CO 3 ) It is a solution of A Weak Acid (H 2 CO 3 ) (NaHCO 3 ) & its Conjugate Base (NaHCO 3 ) (NH 4 OH Or Weak Base (NH 4 OH) & (NH 4 CL) its Conjugate Acid (NH 4 CL)

Buffers It resists changes in pH in a solution when moderate amount of strong acids or bases are added Acid “H 2 CO 3 ” Conjugate base “NaHCO 3 ” OH - H+H+ H+H+ H+H+ H+H+

Principles of Buffering: A H ↔ H + + A -

The Extent Of Buffer Capacity Depends on 3 factors Any buffer exerts maximal buffering capacity when the pH is equal to its pKa. If [A - ]= [HA ] i.e the acid is half dissociated (half-neutralization state) Log [A - ]/ [HA ] = log 1= zero pH = pKa + 0

pH= pKa  1 Is Considered as pH range For SATISFACTORY Buffering Capacity

Physiological buffers 1.H 2 CO 3 / HCO 3 (pKa= 6.1) This is the most important buffer system in the body 2.H 2 PO 4 / HPO 4 (pKa= 6.8). “NH 2 +, COO - ” ) 3.Plasm Proteins (20% of buffering capacity “NH 2 +, COO - ” ) “Histidine” 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] p H + 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