If dissolved phosphorus forms are to be differentiated, filter sample immediately after collection. Preserve by freezing at or below -10°C. In some cases 40 mg HgCl2/L may be added to the samples, especially when they are to be stored for long periods before analysis. CAUTION: HgCl2 is a hazardous substance; take appropriate precautions in disposal ;use of HgCl2 is not encouraged. Do not add either acid or CHCl3 as a preservative when phosphorus forms are to be determined. If total phosphorus alone is to be determined, add H2SO4 or HCl to pH