Evaluation of sodium omadine for short-term preservation of nitrate-N and orthophosphate in water samples

Maintaining nitrate-N and orthophosphate (dissolved reactive phosphorus) concentrations in a water sample over time is critical for water quality research. This study investigated the ability of current preservation methods to maintain nitrate-N and orthophosphate concentrations in environmental water samples over a period of up to 14 days. The United States Environmental Protection Agency (US EPA) currently recommends adding sulfuric acid for nitrate-N and filtering the water sample at the time of collection for orthophosphate. This study compared these recommended methods to baseline control concentrations in subsurface tile drainage water, pond water, and stream water. In addition, sodium omadine was tested as a potential alternative preservative. Across the three sources of water tested, sulfuric acid was the most reliable preservation method for both nitrate-N and orthophosphate, with percent changes from the baseline concentrations on Day 14 ranging from 2.30-3.27% for nitrate-N and 1.94-8.48% for orthophosphate. The control samples also maintained the baseline concentrations quite well, with percent changes on Day 14 ranging from 0.09-6.68% for nitrate-N and 2.63-11.87% for orthophosphate. When sodium omadine was added to the water samples, the nitrate-N concentrations had percent changes of 0.13-6.10% from the baseline concentration on Day 14. In contrast, sodium omadine was less effective for maintaining orthophosphate concentrations, with percent changes of 8.86-13.06% from the baseline concentration on Day 14. However, there were no significantly different orthophosphate concentrations when sodium omadine was added to stream water, showing a promising alternative for this scenario. Depending on the objective, researchers may need to consider the source water and length of storage of water samples to determine what, if any, preservative is necessary.