1 ILO of the current topic: By the end of this topic, the student will be able to: Identify different pH solutions. pH and Buffers

Our Goals for today 1.Dissociation of Water: Apply K w Calculations pH Define the pH scale 2.Acids & Bases Definition General properties Classification (strong, weak) 3.The Henderson-Hasselbalch Equation Calculation Importance 4.MEASURING THE pH Indicators PH Meter Titration 5.Buffer Definition Principles of Buffering Applications

1-Kw 2-pH Dissociation Of Water

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Dissociation Of Water H2OH2OH + + OH - Water has limited capacity to dissociate (ionize) into H+ & OH-

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department K a Is The Acid Dissociation Constant Of Water too little Since water is too little dissociated [H 2 O] [H + ] [OH - ] = KaKa So, K w [H + ] [OH - ] = KwKw K w = The Ion Product Of Water X = KwKw mol/L Since those values involving such –ve powers of 10 which is difficult to use in calculations So Sorensen (1909) suggested that the DILUTE [H + ]concentration of DILUTE aqueous solutions PH value would be better expressed as PH value

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Problem 1 What is fraction of molecules in a liter of water are dissociated? 1 L of water has a mass of 1000 g, MW of water= 18g. the probability that hydrogen exist in water as ion = 1.8 x Answer The number of moles in 1000 g of H2O is = Weight in g/ Molecular weight So, (1000 g) / (18 g mol –1 ) = 55.5 mol. The molar concentration of [ H + ] = The probability that hydrogen exist in water as ion X molar conc. of water = 1.8 x x 55.5 == 99.9x ~ 10 2 x = mole /L

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department What is the pH? The pH of a solution is the negative of the logarithm to the base 10 of its hydrogen ion concentration in (mol/L) pH = -log[H + ]

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department pH scale o C (H + ) = (OH - ) distilled water acidic (H + ) > (OH - ) basic or alkaline (H + ) < (OH - ) ranges from 0 – 14 pH < 7 acid pH = 7 neutral pH > 7 base

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department P= -Log H= H + Concentration pH= -log of H + Concentration

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department P= -Log OH= [OH - Concentration] pOH= -log of [OH - Concentration] [OH - ] may be represented as pOH value

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Since [H + ] x [OH - ] = –log [H + ] + -log [OH - ] = 14 i.e. p H + pOH = 14 By –log of both sides

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Q- What is the pH of N HCL sol.? A- [H + ] = = mol/L pH = - log = 4 Q - What is the pH of N NaOH sol.? A- [OH - ] = = mol/L pOH = - log = 3 Since pH+ pOH=14 So, pH = 14 - pOH= 14 -3= 11 Problem 2

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department What is the pOH of a M solution of Ba(OH) 2 : ? A. 1.0 B. 2.0 C. 4.0 D. 5.0 Problem 3

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department The pH of a sample of human blood was measured to be 7.41 at 25°C. Calculate pOH, [H + ] and [OH - ] for the sample. Problem 4 Answer pH + pOH =  pOH = = 6.6 [H + ] = antilog(-pH)= antilog (-7.41) = 3.9x10 -8 M [OH - ] = antilog(-pOH) = antilog (-6.6) = 2.51 x 10 –7 M

1-Definition 2-Properties 3-Classification 4-Neuteralization Acids and bases

Taste sour React w/ metals & carbonates- produce H gas Turn blue litmus paper red form hydrogen ions (H+) in water Taste bitter Feel slippery Don’t react w/ carbonates Bases turn red litmus paper blue. oxform hydroxide ions in water Acids Bases

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department 2-17 Acids cause H + to increase Bases cause OH - to increase

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Amphoteric Substance They can act as DONORS Proton DONORS with bases & ACCEPTORS Proton ACCEPTORS with acids Eg. Amino acid –H2O

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department In dilute aqueous solution, as [H + ] increases: A.pH decreases B.pOH increases C.[OH – ] decreases D.all of the above Problem 5

Neutralization HCl + NaOH → H 2 O + NaCl acidbase water salt reaction between an acid & base produces water & a salt an ionic compound

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department 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 Would the solution be conductive?

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

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department 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

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department pH pKa = [HA] [A - ] + Log HENDERSON-HUSSELBALCH HENDERSON-HUSSELBALCH Equation

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department USES OF HENDERSON-HUSSELBALCH Equation 1.For Calculation of pH of a weak Acid. 2.Preparation Of Buffer Of Known pH

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department [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

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Summary The stronger the acid The smaller the value of pKa The greater the value of Ka The more it dissociates

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department 33 Strong AcidWeak Acid Transfers all of its protons to water Transfers only a small fraction of its protons to water Completely IonizedPartly Ionized Ka largeKa is small smallerpKAlarger pKA Eg. HCl and H 2 SO 4 Eg. Organic acids e.g. H 2 CO 3

1-Indicators 2-pH Meter 3-Titeration Determining pH

1-Indicator Compound that changes color when in contact w/ an acid or base They are weak acids ex. litmus paper

Red litmus paper Blue litmus paper universal indicators Red Cabbage Juice

Mechanism of action of the litmus paper indicator indicator act as acid and loss protons ionized form Blue color In basic solutions indicator act as bases and accept protons mostly nonionized form Red color In acidic solutions ionized form+nonionized form Intermediate color (green) In intermediate pH

Name of indicator Colour acid Colour base pH range Methyl OrangeRedYellow Bromothymol BlueYellowBlue6 - 8 PhenolphthaleinClearRed8 - 10

Universal indicators  Universal indicators made by mixing several different indicators  Paper soaked in universal indicator  pH paper

 pH meter  determines the pH of a solution by measuring voltage between two electrodes that are placed in solution  Voltage changes as hydronium ion concentration in solution changes 2-pH meter

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

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department 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)

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department 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+

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Principles of Buffering: A H ↔ H + + A -

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department 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

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department pH= pKa  1 Is Considered as pH range For SATISFACTORY Buffering Capacity

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department 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.

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department 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 Problem 6

pH = -log[H + ] pOH= -log of [OH] p H + pOH = 14 pH= pKa +log [A - ] [HA - ]

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Lab activities include : 3.Demonstrate neutralization reaction (in home ) 2.Demonstrate how indicators can be used to test for the presence of acids or bases and the effect of pH on them and to understand the concept of neutralization reaction as well 1.To identify a solution is acidic, basic or neutral in the laboratory

Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department