Evaluation of Preservation Methods for Nitrate and Orthophosphate in Water Samples

Abstract.

The variability in nitrate-N and orthophosphate (dissolved reactive phosphorus) concentrations within a water sample over time can be due to many factors related to location, chemical make-up of the water, and the microbiology of the water. The EPA highlights preservation methods of water samples to make testing as accurate and precise as possible. However, even with these techniques, variability can still be an issue. The objective of this study was to determine how well current preservatives maintain nitrate-N and orthophosphate (dissolved reactive phosphorus) concentrations in water samples over a given period of time and how the existing preservatives compare to new preservation techniques. The current preservation methods, filtering followed by refrigeration, were compared with new methods: sodium omadine and sulfuric acid, and a control. Across the sources of water tested, sodium omadine maintained nitrate and phosphate concentrations better than the standard filtration method in some cases and also than sulfuric acid, which resulted almost always in an increase in concentrations that could have been due to undesired bacteria growth. Therefore, the study results indicate that sulfuric acid is not a suitable preservative for phosphate. However, it should be noted that most of the sulfuric acid results were not statistically significant.