University:
California State University, Northridge
Course:
CHEM 321/L | Chemical Analysis I and Lab
Academic year:

2009
Views:

342

Pages:

3
Author:

gewoekerdodx

Chemistry 321L Manual Appendix C Determination of Ka1 and Ka2 of a Diprotic Acid by pH Titration with Strong Base The stepwise dissociation of a diprotic acid H2A is represented by: ø H3O+ + HAH2A + H2O õ HA- + H2O õ ø H3O+ + A2Expressions for Ka1 and Ka2 are given by: (1) (2) if the solutions are very dilute so that activity coefficients are unity and [X] • ax. During the course of the titration of this acid with strong base (NaOH), material balance and charge balance are maintained. The total acid concentration [H2A]T is given by: (3) The sum of all cation and anion concentrations must be equal. Thus, (4) Determination of Ka1 Ka1 will be calculated from pH titration data near the midpoint between the initial pH and the first equivalence point. In this buffer region, [HA-] >> [A2-] and [H3O+] >> [OH-], so equations (3) and (4) can be simplified and written as: (3a) (4a) Rearrangement of equation (3a) and substitution of the value for [HA-] from equation (4a) gives: (5) Chemistry 321L Manual Substitution of the expressions for [H2A] (eq. 5) and [HA-] (eq. 4a) into equation (1) yields: (6) Equation (6) can be used to calculate Ka1 using data from the buffer region prior to the first equivalence point. The [Na+], [H2A]T and [H3O+] can be calculated at any point in the maleic acid titration as follows. Recall that the initial volume is 85 mL (10.00 mL of the maleic acid solution and 75 mL of water). For the maleic acid titration, the ionic strength is fixed at 0.1 M, thus γH+= 0.83. Select three pH values near the middle of the first buffer region (about one milliliter apart) and calculate three corresponding values of Ka1. Report these values and the average Ka1 value. Determination of Ka2 In the buffer region between the first and second equivalence points, [HA-] >> [H2A]. Thus, equation (3) can be simplified and written as: (3b) Chemistry 321L Manual Also, in this second buffer region, [Na+] >> [H3O+] and [H3O+] >> [OH-]. Therefore, equation (4) can be simplified and written as: (4b) Substitution of the value for [HA-] from equation (4b) into equation (3b) and rearrangement gives: (7) Substitution of the value for [A2-] from equation (3b) into equation (4b) and rearrangement gives: (8) Substitution of the expressions for [A2-] (eq. 7) and [HA-] (eq. 8) into equation (2) yields: (9) Select three pH values near the middle of the second buffer region (about one milliliter apart) and calculate three corresponding values of Ka2. Report these values and the average Ka2 value.