Spectrophotometric determination of pKa values for fluorescein using activity coefficient corrections

The absorbance of the organic water tracer compound fluorescein is known to be pH dependent but differences between the reported pK a values make it difficult to predict these absorbance changes. A new pK a determination method, which incorporated activity corrections, was used to calculate the pK a values of fluorescein. Several published pK a values were re-evaluated and were in agreement once activity corrections were applied.