This chart represents the NADP/NTN database values for SO4 compared to those same values adjusted with Robin's algorithm.

 

The procedure to adjust SO4 is as follows:

Model equations of the lid seal concentration for the conservative ions.
Volume (V) is in milliliters. 'P' is pH. Concentration is in mg/l.

To adjust NADP data before 1994, subtract this result from the lab concentration. SO4 (23.41/V) * (1 - exp(-.00316*V))