بسم الله الرحمن الرحيم

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

2. OBJECTIVES: • To determine the concentration of a given acid by its titration against alkali.

3. What are acids and bases? • An acidis 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.

4. 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

5. 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.

6. Acid / Base Indicator: • Acid base indicator is a week organic acid or a weak organic base whose un-dissociated form differs in color from its conjugate form. • It could be natural or synthetic compound which display colors that depends on the pH of the solution in which they are dissolved.

7. Structures of some of the acid - base indicator e.g. Bromocresol green Yellow Blue pH < 3.8 pH > 5.4

9. Color Range of some Acid/Base Indicators:

10. Treating strong acid with 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

11. Example-1: 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.

12. Calculating the pH value in weak acid titration • Four distinctly different types of calculations are needed to derive a titration curve for a weak acid or a weak base. 1- At the beginning, the solution contains only a weak acid or a weak base, and the pH is calculated from the concentration of that solute and its dissociation constant. 2- After various increments of titrant have been added (in quantities up to, but not including equivalent amount), the solution consists of a series of buffers. The pH of each buffer can be calculated from the analytical concentration of conjugate base or acid and the residual concentrations of the weak acid or base. 3- At the equivalence point, the solution contains only the conjugate of weak acid or base being titrated (that is a salt), and the pH is calculated from the concentration of this product. 4- Beyond the equivalence point, the excess of strong acid or base titrant represses the acidic or basic character of the reaction product to such and extent that the pH is governed largely by the concentration of excess titrant

13. Example-2: 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. (use phenolphthalein as indicator). HPr : hypothetical acid, Ka = 1x 10-5

14. ABDULLATIF TAHA ABDULLAMSc. BIOCHEMISTRY سبحانك لا علم لنا إلا ما علمتنا * إنك أنت العليم الحكيم 0