Uni Bremen Home MARUM - Sediment Geochemistry - Leobener Str - D-28359 Bremen - Germany
Marine Geochemistry - Laboratory Methods

Ferrous iron detection - onboard
method Ferrous iron forms a violet color complex with Ferrospectral (Merck # 111613) Detection is performed on board using a Merck SQ 118 portable photometer at 565 nm
solutions needed  
solution 1 oxygen free pure water pure water (Seradest cartridge) stripped with Argon 4.6 or better for 10 min
solution 2 fresh ascorbic acid 1% in oxygen free pure water(solution 1)
solution 3 1000 ppm Fe(II) stock solution freshly prepared from 4987 ppm (17.9mM) fresh iron (II) sulfate 7 hydrate with 3154 ppm (17.9 mM) ascorbic acid in oxygen free pure water
solution 4 100 ppm Fe (II) work solution: 5mL/L 1% ascorbic acid (solution 2) + 100 mL/L Fe(II) stock solution (solution 3) in oxygen free pure water (solution 1)
solution 5 oxygen free artificial seawater
solution 6 0.05 , 0.1, 0.25, 0.5, 1 ppm calibration standards are being prepared in artificial seawater: 5mL/L 1% ascorbic acid (solution 2) plus 0.5, 1, 2.5, 10 mL/L Fe(II) work solution (solution 4) in artificial seawater (solution 5)
solution 7 color reagent prepared from 5 ppm Ferrospectral (C20H12N4Na2O6S2, Merck#111613) in oxgen free pure water (solution 1)
blank 1mL artificial seawater (solution 5) + 50 無 color reagent (solution 7)

exp.conc < 1ppm (~20 痠ol/L)

Put 50 無 color reagent (solution 7) and 1 mL pore water sample into a test tube or directly into the cuvette, mix and wait for 10 min to form violet color complex
exp.conc >1ppm (~20 痠ol/L) 1 mL of the the diluted (!) sample is added to 50 無 of color reagent (solution 7) as above mix and wait for 10 min to form violet color complex
tips + tricks  
test tubes All test tubes and flasks have to be rinsed with hydrochloric acid (approx. 0.5M) followed by pure water prior to analysis. We use 2 mL Eppendorf test tubes (#0030.120.094)
pore water extraction The pore water extraction from sediments is performed by pressure filtration through 0.2 痠 cellulose acetate filters (i.e.Sartorius # 11107-142) in a glovebox under argon atmosphere
colored samples If the filtered samples are colored (i.e. due to high DOC) the filtered sample without color reagent is used as a blank
sample preservation Ferrous iron is preserved by filling reaction vials whithin the glovebox adding fresh ascorbic acid solution. Put 10 無 ascorbic acid (solution 2) and 1 mL pore water sample into a test tube and mix (sufficient to preserve iron(II) concentrations up to approx. 30 ppm or 500 然/L. For higher concentrations a more concentrated ascorbic acid solution should be used.The sample may than be diluted in the lab i.e. 1 in 10 dilution : 10 無 ascorbic acid (solution 2) 100 無 sample 890 無 artificial seawater (solution 5)
sample dilution Do not dilute the sample / color reagent mixture, reagent concentration might be too low.
dynamic range Extinction must be well within the dynamic range of the instrument. If the concentration range is unknown try different dilutions first.

glovebox set up in cool lab on RV Meteor

instruments for pH and Eh measurement are on top. bottled Argon is used for both, keeping inert atmosphere and for pressure filtration inside the box.

pressure distributors allow simultaneous extraction of 12 sediment samples

precut core segments are fed through the shutter and opened inside the box

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