DETERMINATION OF pKa AND pKb USING ACID-BASE TITRATIONS
Introduction
Acid-base titrations can be used to determine pKa of a weak acid and pKb of a base. A weak acid and a weak base incompletely dissociate in water. For this reason, the concentration of hydrogen ions (H+) and hydroxide ions (OH-) depend on equilibrium coefficients of acid (Ka) and base (Kb). Equation 1 shows the chemical dissociation equation of a weak acid. Furthermore, Equation 2 describes the equilibrium constant of a weak acid. On the other hand, Equation 3 and Equation 4 illustrate the chemical equation for the dissociation and equilibrium constant of a weak base.
HA ↔ H+ + A-…………………………………………………. Equation 1
Ka=[H+][A-]/[HA]……………………………………………… Equation 2
BH+ + OH-↔ B + H2O………………………………………… Equation 3
Kb= [B+][OH-]/[BOH]…………………………………………. Equation 4
The equilibrium constants (Ka and Kb) are intrinsic properties of a particular weak/base pair. Ka defines the manner in which conjugate base holds on to the H+ (proton) while Kb describes the ‘stickiness’ of a weak acid for hydroxide ions(OH-). Taking the logarithm of …show more content…
For acetic acid-sodium hydroxide titration, 10ml of distilled water was added to 250ml Erlenmeyer flask. 20ml of diluted acetic acid was pipetted into that 250ml Erlenmeyer flask. Titration system that contained 0.100M NaOH in the burette was set up. The pH meter was used to monitor pH of the system when sodium hydroxide was being added. NaOH was added in an increment of 1ml until the pH of the solution reached 4.8. After the solution reached pH of 4.8, NaOH was added in increment of 0.1ml until the solution passed its equivalence point. NaOH was added in an increment of 1ml after equivalence point until there was an insignificant change in solution pH when titrant was added. Values of pH and titrant used were recorded and used in drawing titration
Introduction
Acid-base titrations can be used to determine pKa of a weak acid and pKb of a base. A weak acid and a weak base incompletely dissociate in water. For this reason, the concentration of hydrogen ions (H+) and hydroxide ions (OH-) depend on equilibrium coefficients of acid (Ka) and base (Kb). Equation 1 shows the chemical dissociation equation of a weak acid. Furthermore, Equation 2 describes the equilibrium constant of a weak acid. On the other hand, Equation 3 and Equation 4 illustrate the chemical equation for the dissociation and equilibrium constant of a weak base.
HA ↔ H+ + A-…………………………………………………. Equation 1
Ka=[H+][A-]/[HA]……………………………………………… Equation 2
BH+ + OH-↔ B + H2O………………………………………… Equation 3
Kb= [B+][OH-]/[BOH]…………………………………………. Equation 4
The equilibrium constants (Ka and Kb) are intrinsic properties of a particular weak/base pair. Ka defines the manner in which conjugate base holds on to the H+ (proton) while Kb describes the ‘stickiness’ of a weak acid for hydroxide ions(OH-). Taking the logarithm of …show more content…
For acetic acid-sodium hydroxide titration, 10ml of distilled water was added to 250ml Erlenmeyer flask. 20ml of diluted acetic acid was pipetted into that 250ml Erlenmeyer flask. Titration system that contained 0.100M NaOH in the burette was set up. The pH meter was used to monitor pH of the system when sodium hydroxide was being added. NaOH was added in an increment of 1ml until the pH of the solution reached 4.8. After the solution reached pH of 4.8, NaOH was added in increment of 0.1ml until the solution passed its equivalence point. NaOH was added in an increment of 1ml after equivalence point until there was an insignificant change in solution pH when titrant was added. Values of pH and titrant used were recorded and used in drawing titration