APPARATUS:
Eppendorfpipettes (10-100 µl , 100-1000 µl und 1-5 cm³)
Various volumetric flasks
Various cylinders and beakers
Various bottles to store the reagents
Photometer

REAGENTS:
H₂SO₄ (4.5 M): Mix 250 cm³ conc. H₂SO₄  and 750 cm³ pure water. Let cool and store in PE-bottle. Stable.
Ascorbic acid: Dissolve 10 g ascorbic acid in 50 cm³ pure water and add 50 cm³ H₂SO₄ (4.5 M). Store refrigerated in PE-bottle and replace weekly  (Limit of storage is indicated by the development of a brownish color.)
Tartrate solution: Dissolve 2.5 g potassium antimonyl tartrate  in 100 cm³ pure water and store refrigerated in PE-bottle. Stable.
Heptamolybdate reagent: Dissolve 4 g of ammoniumheptamolybdate-tetrahydrate in 30 cm³ pure water. Add this solution carefully to 90 cm³ H₂SO₄ (4.5 M) and add 5 cm³ tartrate solution. Stable for several months if stored refrigerated in a PE-bottle.
PO₄^3--Standard (MERCK Titrisol ™) 1 g PO₄^3- per liter.
Saltwater 36 g NaCl per liter pure water

STANDARDS:
Stock-Standard:  Dilute (MERCK Titrisol ™) PO₄^-3-Standard with pure water  to exactly 1 dm³. This solution contains 1 g PO₄^3-  per dm^-3 (10.52 mM)
Work standard: Dilute the stock-standard with salt water in different relations to obtain standards with different concentrations for a calibration (5-6 calibration points).
Calibration range: 2.63-26.3 µmol/l (for pore water)  and 0.526-5.260 µmol/l (for seawater samples).

PROCEDURE:
After 2 cm³ of pore water sample or standard were made up to 5 cm³ with pure water, 0.1 cm³ ascorbic acid and  0.1 cm³ heptamolybdate reagent are added. The absorbance can be measured after 10 minutes at 880 nm. Analysis of seawater samples does not require any dilution.

INTERFERENCES:
High concentrations of H₂S (> 1mM) in the sample disturbe the measurements. Therefore these samples should be acified with 10 µl HCl (30%/suprapur) per cm³ sample and then be stored for 1-2 days, without lid, in a cool room.
(Don`t forget the dilution factor of the acid when you calculate the final result!)