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 PKa values make it difficult to predict these absorbance changes. A new pKa determination method, which incorporated activity corrections, was used to calculate the pKa values of fluorescein. Several published pKa values were re-evaluated and were in agreement once activity corrections were applied.