Appendix II
153 APPENDIX II
Determination of the second acid dissociation constant of the hydroxyquinoline The reaction between HQ and NaOH can be expressed by the relationship
HQ + OH - Q- + H2O (II.1)
whose equilibrium constant is
W A K K OH HQ Q K 2 ] [ ] [ ] [ = × = − − (II.2)
where Kw is the ionic product of water and KA2 the dissociation constant of HQ. If the Lambert & Beer law applies, for a wavength where the two species [HQ] and [Q]- only absorb and for a 1 cm path lenght cell, the overall absorbance is given by the equation ] [ ] [ + − − = HQ Q Abs εHQ εQ (II.3)
The mass conservation equations are
] [ ] [ + − = HQ Q CHQ (II.4) If we now define HQ HQC Abs Abs= −ε ∆ (II.5) HQ Q ε ε ε = − ∆ − (II.6)
Appendix II 154 ] [ − ∆ = ∆Abs ε Q (II.7)
and introduction of equation (II.4) and (II.7) into (II.2) yields
] [ 1 1 1 − × ∆ + ∆ = ∆Abs K OH CHQ ε ε (II.8)
which corresponds to equation (4.10) of chapter 4.
A plot of CHQ/∆Abs vs. 1/[OH-] = 1/(CNaOH-CHQ) is a straight line whose slope and intercept are equal to 1/K∆ε and 1/∆ε respectively. Therefore, K is obtained as the intercept/slope ratio and subsequently KA2 is obtained as Kw × K.