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Topic starter 14/07/2022 12:47 pm
Our team conducted spectrophotometic titration of several anions of CH-acids (pKa of conjugated acid ~ 5-10) in DMSO. We proceeded from original paper by Bordwell (10.1021/ja00857a010), which says that several aliquots of acid should be enough, while differences between pKa at different points should not exceed 0.05 pKa units.
Nonetheless, in our numerous attempts to make this type of titration we always ran into big deviation of pKa between first and last points of the run (small and big amount of acid added, respectively), which usually extend up to 0.3-0.4 pKa units, in one case up to 1 unit (pic pinned, titrant = p-nitrobenzoic acid, pKa = 9). The biggest deviation is between the first points, when approx. 0.5-1 eq. of titrant is added.
Maybe someone can explain what is the reason for such spread of values? And how can we fix it? For now, we tried out different devices, different techniques for titrant addition, in some cases different titrants - deviation stays in all cases. Thanks!
Nonetheless, in our numerous attempts to make this type of titration we always ran into big deviation of pKa between first and last points of the run (small and big amount of acid added, respectively), which usually extend up to 0.3-0.4 pKa units, in one case up to 1 unit (pic pinned, titrant = p-nitrobenzoic acid, pKa = 9). The biggest deviation is between the first points, when approx. 0.5-1 eq. of titrant is added.
Maybe someone can explain what is the reason for such spread of values? And how can we fix it? For now, we tried out different devices, different techniques for titrant addition, in some cases different titrants - deviation stays in all cases. Thanks!
