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

Don`t use any glass device, only PE !!!

REAGENTS:
H₂SO₄ (4.5 M): Mix 250 cm³ conc. H₂SO₄  and 750 cm³ pure water. Let cool and store in PE-bottle. Stable.
Heptamolybdate solution: Dissolve 6.3 g ammoniumheptamolybdate-tetrahydrate in 50 cm³ pure water and add to 50 cm³ of the H₂SO₄-solution (4.5 M). Do not add acid to molybdate! Store in light protected PE-bottle. Stable for several months.
Oxalic acid solution: Dissolve 10 g oxalic acid in 100 cm³ pure water. Store in PE-bottle at room temperature. Stable.
Ascorbic acid: Dissolve 2.8 g ascorbic acid in 100 cm³ pure water. Store refrigerated in PE-bottle. Replace weekly.
Si-Standard (MERCK Titrisol ™) 1.000 g Si in NaOH (14%).
Saltwater (36 g NaCl per liter pure water)

STANDARDS:
Stock-Standard: Dilute MERCK Titrisol ™  Si-Standard with pure water  to exactly 1 dm³. This solution contains 1.00 g * dm^-3 (35.605 mmol/dm³) Si and is stable for several months if stored refrigerated in a PE-bottle.
Work standard: Dilute the stock-standard with salt water in different relations to obtain standards with different concentrations used for calibration (5-6 calibration points).
Calibration range: 35.605 µmol/l - 427.26 µmol/l (for pore water)

PROCEDURE:
Aliquots of 0.5 cm³ pore water or standard are diluted to 5.0 cm³ with pure water and 0.2 cm³ heptamolybdate solution are added. Shake well and wait for 30 min. Then add 0.2 cm³ oxalic acid, immediately followed by 0.2 cm³ ascorbic acid.
Only 0.1 cm³ of the reagents are added to seawater samples with low Si concentration. 5 cm³ of undilutes seawater are used for the analysis of these samples.
In any case, the blue silicomolybdic complex takes about 30 minutes to develop before the absorbance can be measured at 810 nm.
 

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!)