Hawaii Ocean Time-series (HOT)
in the School of Ocean and Earth Science and Technology at the University of Hawai'i
|» Home » Field & Laboratory Protocols » Chapter 9|
SOLUBLE REACTIVE SILICA
SUMMARY: Seawater is collected from known depths using CTD- rosette sampling protocols. Subsamples are drawn and stored in acid-washed polyethylene bottles.Soluble reactive silica is measured spectrophotometrically following the formation of silico- molybdic acid from the reaction of ammonium molybdate and silica at acidic pH.
Silicon is the second most abundant element in the Earth's crust. Subaerial weathering processes produce orthosilicic acid Si(OH)4 which eventually is deposited in the oceans. In seawater, various groups of organisms (diatoms, radiolarians, silicoflagellates, sponges and some fungi) utilize silica primarily as a structural component.
The analysis of soluble reactive silica is based upon the formation of yellow silicomolybdic acid from the reaction of ammonium molybdate and silica at low pH. Phosphate also reacts to produce a positive interference due to the formation of molybdophosphoric acid. The addition of oxalic acid eliminates the phosphate interference. The sensitivity of the analysis is increased by a further reduction of the yellow silicomolybdic acid using ascorbic acid, in order to produce "molybdenum blue".
Contamination is the primary concern with these samples. This is particularly true with samples collected from the euphotic zone, where inorganic nutrient concentrations are extremely low. In order to avoid contamination, all sample bottles must be meticulously cleaned with dilute HCl and rinsed with deionized distilled water (DDW) before use. It is important to realize that silica is leached from glass at seawater pH. Therefore, plastic should be used for sample handling and storage. Finally, special care must be taken when performing dissolved Si analyses on frozen seawater samples (Macdonald et al., 1986).
3. Sample Collection and Storage (also see "NOTE" in Chapter 7, section 3)
4. Sample Analysis
Currently, GOFS and WOCE nutrient samples collected during the Hawaii Ocean Time Series cruises are analyzed by the Hawaii Institute of Marine Biology Analytical Facility. Mr. Ted Walsh has provided us with the following procedure for the analysis of reactive Si.
Si analyses are performed on a four-channel Technicon Autoanalyzer II continuous flow system. The automated wet chemistries generally follow the standard methods of seawater analysisas given by Technicon (1977). This method involves a reaction of the sample with oxalic acid, molybdate, and ascorbic acid. The absorbance is read at 660 nm using a 15 mm pathlength flowcell.
5. Calibration, Data Reduction and Calculations
6. Accuracy and Precision
The detection limit for dissolved Si is approximately 0.3 µM. The coefficient of variation of field-collected replicates is 6%.
Because Si is the principle component of glass, all solutions should be made up and contained in plastic. Glass distilled deionized water will have minimal silica leaching due to the low pH of distilled water.