US Long-Term Ecological Research Network

Parameter: Dissolved Reactive Silicon

Samples for silicon analysis (as well as dissolved nitrogen and phosphorus, calcium, magnesium, sodium, potassium, iron, and manganese) are collected together with a peristaltic pump and tubing and in-line filtered (through a 40 micron polycarbonate filter) into 120 ml LDPE bottles and acidified to a 1% HCl matrix by adding 1 ml of ultra pure concentrated HCl to 100 mls of sample. For every sample acidification event, three acid blanks are created by adding the same acid used on the samples to 100 mls of ultra pure water supplied from the lab.

Silica

This procedure determines the total silica, as bicarbonate-reactive silica, in filtered and unfiltered samples. The material which cannot pass through a 0.45 micron filter is considered particulate silica and measurable by the difference between the filtered and unfiltered samples. Silica reacts with a molybdate reagent in an acidic environment to form a yellow silicomolybdate complex. This complex is reduced by ascorbic acid to form the molybdate blue color. The color intensity is proportional to the silica concentration.

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