This chart represents the NADP/NTN database values for NH4 compared to those same values adjusted with Robin's algorithm.

The procedure to adjust NH4 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. NH4 (8.62/V) * (1 - exp(-.00392*V))