Biogenic silica measurement

To measure the biogenic silica content of marine sediments, hot (85°C) alkaline solutions are used to dissolve biogenic silica over a period of 5-6 hours. The silicate concentration in the leaching solution is measured colorimetrically on a spectrophotometer... 

Figure 37.3. The smallest measurable particle sizes for biogenic silica in the branches of Equisetum arvense [black bars], silica produced in a model system from a silicon catecholate complex [striped bars], and silica produced in a model system in the presence of a biosilica protein containing extract from Equisetum arvense [hatched bars].

NaOH (or other reagents) attacks readily soluble clays in addition to biogenic silica. Conversely, not all of the biogenic silica is dissolved. Thus, extractable Si may be a poor measure of biogenic silica. 

Samples are extracted with a suitable solvent (i.e., a solvent with no C-H bonds), and biogenic polar materials are removed with silica gel. Some polar petroleum constituents may be removed as part of the silica gel cleanup. The absorbance of the silica gel eluate is measured at the specified frequency and compared to the absorbance of a standard or standards of known petroleum... 

Biogenic silicon (BSI) was determined, with minor modifications, by the method of DeMaster (17). As adapted, the technique involved time-course leaching of <20-mg samples of particulate matter in 30 mL of 1.0% Na2C03 in a water bath at 85 °C. Silica in leachates was quantified either colorimetrically (Technicon autoanalyzer procedure) or by nitrous oxide flame atomic absorption. A high-temperature catalytic-combustion technique (Perkin Elmer 240C) was used for particulate organic carbon determinations. Particulate inorganic (carbonate) carbon was measured on the same instrument by CO 2 evolution after treatment of the particles with phosphoric acid. 

Earliest measurements of the oxygen isotope composition of fossilized diatom silica suffered fi om problems with analytical reproducibility (e.g. Labeyrie 1979) because biogenic opal contains significant amounts of loosely-bound water, up to c. 12... 

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