Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
pH and Buffers ILO of the current topic: By the end of this topic, the student will be able to: Identify different pH solutions. 1 Our Goals for today 1 .Dissociation of Water: • Apply Kw 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 Dissociation Of Water 1-Kw 2-pH Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Dissociation Of Water H2O H+ + OH- Water has limited capacity to dissociate (ionize) into H+ & OH- Ka Is The Acid Dissociation Constant Of Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Water [H+involving ] [OH-] such –ve Since those values Ka = powers of 10 which[His2O] difficult to use in Since water is too little dissociated calculations [H+] [OH-] So, Kw So Sorensen (1909) suggested that the + Kw = [H ] [OH ] [H+]concentration of DILUTE aqueous -7 -7 10 mol/L Kw = 10 X 10 solutions Kw =beThe Ionexpressed Product Of would better asWater PH value -14 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 2 3 4 5 acidic (H+) > (OH-) ranges from 0 – 14 pH < 7 acid pH = 7 neutral pH > 7 base 6 7 8 neutral @ 25oC (H+) = (OH-) distilled water 9 10 11 basic or alkaline (H+) < (OH-) 12 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 [OH-] may be represented as pOH value P= -Log OH= [OH- Concentration] pOH= -log of [OH- Concentration] Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Since [H+] x [OH-] =10 -14 By –log of both sides –log [H+] + -log [OH-] =14 i.e. pH + pOH =14 Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Problem 2 Q- What is the pH of 0.0001 N HCL sol.? A- [H+] = 0.0001 = 10-4 mol/L pH = - log 10-4 = 4 Q - What is the pH of 0.001 N NaOH sol.? A- [OH-] = 0.001 = 10-3 mol/L pOH = - log 10-3 = 3 Since pH+ pOH=14 So, pH = 14 - pOH= 14 -3= 11 Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Problem 3 What is the pOH of a 0.010 M solution of Ba(OH)2: ? A. 1.0 B. 2.0 C. 4.0 D. 5.0 Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department pH + pOHProblem = 14.00 4 The pH of a sample of human blood pOH = 14.00-7.4 = 6.6 was measured to be 7.41 at 25°C. + [H ] = antilog(-pH)= antilog (-7.41) = + Calculate pOH, [H ] and [OH ] for -8M 3.9x10 the sample. [OH-] = antilog(-pOH) = antilog (-6.6) Answer = 2.51 x 10–7 M Acids and bases 1-Definition 2-Properties 3-Classification 4-Neuteralization Acids and bases Acids • Taste sour • React w/ metals & carbonatesproduce H gas • Turn blue litmus paper red • form hydrogen ions (H+) in water Bases • Taste bitter • Feel slippery • Don’t react w/ carbonates • Bases turn red litmus paper blue. • form hydroxide ions in water Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Acids cause H+ to increase Bases cause OH- to increase 2-17 Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Amphoteric Substance They can act as Proton DONORS with bases & Proton ACCEPTORS with acids Eg. Amino acid –H2O Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Problem 5 In dilute aqueous solution, as [H+] increases: A. pH decreases B. pOH increases C. [OH–] decreases D. all of the above Neutralization HCl + NaOH → H2O + NaCl acid base water salt • reaction between an acid & base • produces water & a salt Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department 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 What happens to the HA molecules in solution? Let’s examine the behavior of an acid, HA, in aqueous solution. Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department 100% dissociation of HA HA H+ Strong Acid A- Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Partial dissociation of HA HA H+ Weak Acid A- Would the solution be conductive? HA H+ + A- HA H+ Weak Acid A- At any one time, only a fraction of the molecules are dissociated. HENDERSON-HUSSELBALCH Equation 1-Calculation 2-Uses Faculty of Medicine Ain Shams University How can you calculate the pH of Medical Biochemistry & Molecular Biology Department 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 HENDERSON-HUSSELBALCH Equation pH = pKa [A-] + Log [HA] Faculty of Medicine Ain Shams University USES OF Medical Biochemistry & Molecular Biology Department 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 When [A-]= [HA] pH = pKa [A-] +Log [HA] So PKa Of an acid is the pH at which [The Acid conc.] = [Its Conjugate base conc.] Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Summary The smaller the value of pKa The stronger the acid The greater the value of Ka The more it dissociates Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Strong Acid Weak Acid Transfers all of its protons to water Transfers only a small fraction of its protons to water Completely Ionized Partly Ionized Ka large Ka is small smallerpKA larger pKA Eg. HCl and H2SO4 Eg. Organic acids e.g. H2CO3 33 Determining pH 1-Indicators 2-pH Meter 3-Titeration 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 In basic solutions • indicator act as acid and loss protons • ionized form • Blue color In acidic solutions • indicator act as bases and accept protons • mostly nonionized form • Red color In intermediate pH • ionized form+nonionized form • Intermediate color (green) Name of indicator Colour acid Colour base pH range Red Yellow 3-4 Bromothymol Blue Yellow Blue 6-8 Phenolphthalein Clear Red 8 - 10 Methyl Orange Universal indicators Universal indicators made by mixing several different indicators Paper soaked in universal indicator pH paper 2-pH meter 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 BUFFER 1-Definition 2-Principle of buffering 3-Application Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Buffers It is a solution of A Weak Acid (H2CO3) & its Conjugate Base (NaHCO3) Or Weak Base (NH4OH) & its Conjugate Acid (NH4CL) 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 OHOH- or bases are added H+ Acid “H2CO3” OH- OH- Conjugate base “NaHCO3” H+ H+ H+ Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Principles of Buffering: AH ↔ H+ + A- Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department The Extent Of Buffer Capacity Depends on 3 factors -]= [HA ] i.e the acid is half If [A Any buffer exerts maximal dissociated (half-neutralization state) buffering capacity Log [A-]/ [HA ] = log 1= when zero pH = pKa + 0 the pH is equal to its pKa. 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. 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. Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department Problem 6 • 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-] Faculty of Medicine Ain Shams University Lab activities include : Medical Biochemistry & Molecular Biology Department 1.To identify a solution is acidic, basic or neutral in the laboratory 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 3.Demonstrate neutralization reaction (in home ) Faculty of Medicine Ain Shams University Medical Biochemistry & Molecular Biology Department