بسم الله الرحمن الرحيم UMM AL-QURQ UNIVERSITY Faculty of Medicine Dep. Of BIOCHEMISTRY

To estimate Ka and pKa values. 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. To detect the method of estimation of pKa of an acid for further use in the application with Henderson-Hasselbalch equation. OBJECTIVES:

Definitions: A standard solution (standard titrant) is a reagent of known concentration that is used to carry out a titrimetric analysis A standard 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. 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 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. AgNO 3 + Cl -  AgCl E.g. AgNO 3 + Cl -  AgCl mol mol mol mol (Equivalent points can ’ t be determined experimentally) (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. 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. 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 H 3 O + (hydronium ion) in aqueous solution. An acid: is a substance that increases the concentration of H 3 O + (hydronium ion) in aqueous solution. A base: is a substance that decreases the concentration of H 3 O + in aqueous solution. A base: is a substance that decreases the concentration of H 3 O + in aqueous solution. A decrease in [H 3 O + ] requires an increase in [OH - ], so we can define the base as a substance that increase the concentration of OH - in aqueous solution. A decrease in [H 3 O + ] 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 (H 3 O + ) is a combination of H + with H 2 O.

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 H 2 O to form H 3 O + HCl (l) + H 2 O H 3 O + (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 NH 4 + Cl -  NH Cl - H20H20

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. 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 = K a. This equilibrium can be calculated and is termed the equilibrium constant = K a.

From this equation it can be seen that the smaller the pK a 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.

Acid Dissociation Constants

Acid Dissociation Constants (continue )

What is the pKa : At the point of the dissociation where the concentration of the conjugate base [A - ] = to that of the acid [HA]: At the point of the dissociation where the concentration of the conjugate base [A - ] = to that of the acid [HA]: pH = pK a + log[1] pH = pK a + log[1] The log of 1 = 0. Thus, at the mid-point of a titration of a weak acid: The log of 1 = 0. Thus, at the mid-point of a titration of a weak acid: pK a = pH pK a = pH In other words, the term pK a is that pH at which an equivalent distribution of acid and conjugate base exists in solution. In other words, the term pK a is that pH at which an equivalent distribution of acid and conjugate base exists in solution.

Experiment – 1: Experiment – 1: Titration of a weak monoprotic acid with a strong base Titration of a weak monoprotic acid with a strong base Generate a curve for the titration of 40ml of 0.1 M acetic acid (K a = 1.75 x10 -5 ) with 0.1 M sodium hydroxide. Find the pH after adding a) 0.00ml, b) ml,c) ml, d) ml of titrant. Plot the titration curve. Generate a curve for the titration of 40ml of 0.1 M acetic acid (K a = 1.75 x10 -5 ) with 0.1 M sodium hydroxide. Find the pH after adding a) 0.00ml, b) ml,c) ml, d) ml of titrant. Plot the titration curve.

HPr : hypothetical acid, K a = 1x 10 -5

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. 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: 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 (H 3 O + and OH - are present at equal concentrations, [H 3 O + ] 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 K w = [H 3 O + ][OH - ]  pK w =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.

Experiment – 3: Titration of a strong polyprotic acid with a strong base

ABDULLATIF TAHA ABDULLA MSc. BIOCHEMISTRY