Preservation of seawater samples for soluble reactive phosphate, nitrite, and nitrate plus nitrite analyses by the addition of sodium hydroxide

Abstract

AbstractSeawater storage at ambient temperature is necessary when freezing facilities are unavailable for the preservation of samples collected for the determination of soluble reactive phosphate (SRP), nitrite ( ), and nitrate plus nitrite (N + N)—notably, during small boat operations. Here, we report that samples may be preserved at ambient temperature by the addition of NaOH at sampling to induce the precipitation of Mg(OH)2, and then returned to a shore base laboratory for further processing. In the laboratory, SRP, which has been sequestered in the precipitate, is analyzed by the MAGnesium‐induced co‐precipitation method. and N + N are quantitatively retained in the supernatant liquid and they are analyzed by the standard methods after a pH adjustment. The samples may be stored for up to at least 2 months and they yield concentrations that are indistinguishable from those in frozen samples. Internally added SRP, , and N + N to seawater samples can be recovered quantitatively after storage. This storage method also offers clear advantages over the preservation of samples at ambient temperature by the addition of HgCl2 as NaOH is a less toxic and less environmentally hazardous chemical and it does not interfere in the determination of N + N. It also allows low level SRP to be determined as part of the analytical routine as a pre‐concentration of SRP has been built into the storage scheme.