بسم الله الرحمن الرحيم ACID-BASE TITRATION-2 UMM AL-QURQ UNIVERSITY Faculty of Medicine Dep. Of BIOCHEMISTRY
OBJECTIVES: • To estimate Ka and pKa values. • To detect the method of estimation of pKa of an acid for further use in the application with Henderson-Hasselbalch equation.
Definitions: • Astandard solution (standard titrant) is a reagent of known concentration that is used to carry out a titrimetric analysis • Titration: is a procedure performed by adding a standard solution from a buret or other liquid-dispensing devices to a solution of the analyte until the reaction between the two is judged complete. • Equivalence point in titration is reached when the amount of added titrant is chemically equivalent to the amount of analyte in the sample • E.g. AgNO3 + Cl- AgCl mol mol • (Equivalent points can’t be determined experimentally) • End point is the point in a titration when physical change occurs that is associated with the condition of chemical equivalence. • Indicator: a chemical compound that change it’s color or other physical property at or near the equivalence point.
What are acids and bases? • An acid:is a substance that increases the concentration of H3O+ (hydronium ion) in aqueous solution. • A base: is a substance that decreases the concentration of H3O+ in aqueous solution. A decrease in [H3O+] requires an increase in [OH-], so we can define the base as a substance that increase the concentration of OH- in aqueous solution. • H+ is called a proton, because a proton is all that remains when a hydrogen atom loses its electron. • Hydronium ion (H3O+) is a combination of H+ with H2O.
Bronsted and Lowry definition of acids and bases • An acid is a proton donor • A base is a proton acceptor. • Example • HCl is an acid because it donates a proton to H2O to form H3O+ • HCl(l) + H2O H3O+(aq) + Cl-(aq) • Salts: • - Any ionic solid such as ammonium chloride is called salt. It can be thought of as the product of an acid base reaction. • Most salts are strong electrolytes, i.e. they dissociate almost completely into their component ions when dissolved in water • NH4+Cl- NH4+ + Cl- H20
The Henderson-Hasselbalch Equation • Weak acids and bases in solution do not fully dissociate and, therefore, there is an equilibrium between the acid and its conjugate base. • This equilibrium can be calculated and is termed the equilibrium constant = Ka.
In the reaction of a weak acid: HA <-----> A- + H+ Ka = [H+][A-]/[HA] But: pKa = -log Ka = -log[H+][A-]/[HA] pKa = -log[H+] -log[A-]/[HA] pKa = pH -log[A-]/[HA] Or : pH = pKa + log[A-]/[HA]
From this equation it can be seen that the smaller the pKa value the stronger is the acid. This is due to the fact that the stronger an acid the more readily it will give up H+ and, therefore, the value of [HA] in the above equation will be relatively small.
What is the pKa: • At the point of the dissociation where the concentration of the conjugate base [A-] = to that of the acid [HA]: pH = pKa + log • The log of 1 = 0. Thus, at the mid-point of a titration of a weak acid: pKa = pH • In other words, the term pKa is that pH at which an equivalent distribution of acid and conjugate base exists in solution.
Experiment – 1: Titration of a weak monoprotic acid with a strong base Generate a curve for the titration of 40ml of 0.1 M acetic acid (Ka= 1.75 x10-5) with 0.1 M sodium hydroxide. Find the pH after adding a) 0.00ml , b) 20.00 ml ,c) 40.00 ml, d) 41.00 ml of titrant. Plot the titration curve.
Experiment – 2:Titration of a strong monoprotic acid with a strong base • Here we will be interested in calculating the hypothetical titration curves of pH versus volume of titrant. • Three types of calculation must be done to construct the hypothetical curve for treating a solution of strong acid with a strong base: 1- pre-equivalence ( we compute the conc. Of the acid from its starting conc. And the amount of the base that has been added) 2- equivalence (H3O+ and OH- are present at equal concentrations, [H3O+] is derived directly from the ion product constant of water) 3- post-equivalence (conc. of excess base is computed. [OH-] is converted to pH by Kw= [H3O+][OH-] pKw=pH + pOH Example:Do the calculations needed to generate the hypothetical titration curve for the titration of 50 mL of 0.05M HCl with 0.1 M NaOH.
ABDULLATIF TAHA ABDULLAMSc. BIOCHEMISTRY سبحانك لا علم لنا إلا ما علمتنا * إنك أنت العليم الحكيم 0