Method / Description Silicate

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

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

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)

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)

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.

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!)