Hawaii Ocean Time-series (HOT)
in the School of Ocean and Earth Science and Technology at the University of Hawai'i
|» Home » Field & Laboratory Protocols » Chapter 11|
SUMMARY: Seawater samples are collected at discrete depths in 12-liter Niskin bottles. The water samples are prefiltered (202 µm) and transferred to specially designed, precalibratedfiltration bottles, pressure filtered through combusted acid-rinsed GF/F filters and stored frozen for subsequent analysis. In the laboratory, the filters are combusted at 450-500°C and the concentration of the resulting inorganic phosphorus is determined by colorimetric analysis.
The procedure presented here is a modification of one used by the Hawaii Institute of Marine Biology Analytical Services laboratory at the University of Hawaii. It is a method pioneered and used by soil scientists and marine chemists for particulates which can be homogenized into a fine powder.
The method relies on the release of organically-bound phosphorus compounds as orthophosphate, by high temperature combustion at 450-500°C. The orthophosphate is then extracted with 0.5 N HCl at 90°C. The liberated orthophosphate is reacted with a mixed reagent of molybdic acid, ascorbic acid and trivalent antimony to form phosphomolybdic acid. This heteropoly acid is then reduced to the colored molybdenum blue complex by ascorbic acid and the solution is measured spectrophotometrically.
This procedure measures all forms of phosphorus which can be released by combustion and acid hydrolysis.
Contamination is the primary problem to be avoided with these samples. Combusted acid rinsed filters are used. All sampling bottles, forceps, tubing and filtration bottles are also acid rinsed.
3. Sampling, Filtration and Storage
4. Blank Determination
Standards are corrected for reagent blanks while samples are corrected for field filter blanks.
6. Data Reduction and Calculations