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.

1. Principle

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".

2. Precautions

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)

3.1. Rinse the nutrient sample bottle (acid-washed, 125 ml polyethylene bottle) 3 times before filling. Fill to approximately 2/3 full, tighten cap and freeze.
3.2. Record cruise, cast and Niskin bottle number on the bottle and data sheet.

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

5.1. Calibration stocks and regression standards

The calibration of dissolved inorganic nutrients in the autoanalysis of seawater samples is performed using standard solutions containing N, P and Si. Nutrient stock solution "A" is prepared by dissolving dried (65°C, 72 hours) analytical grade reagent chemicals with distilled-deionized water in 1 liter glass volumetric flasks containing 1 ml of chloroform. Once dissolved, this stock solution is immediately transferred into 1 liter amber polypropylene bottles and stored at 4°C. The reagent chemicals and concentrations are: phosphate (KH2PO4, 1 mM), nitrate (KNO3, 5 mM) and silica (Na2SiF6, 4 mM).

Working standards are prepared daily by volumetric dilutions of the stock using glass pipettes and a plastic (polymethylpentene; PMP) volumetric flask. All pipettes and PMP flasks are acid-washed (1 M HCl) and gravimetrically calibrated prior to use. The daily regression standards are prepared by diluting the working standard with low nutrient natural seawater diluent (SWDIL). The SWDIL is filtered open ocean surface seawater that is stored in a carboy at room temperature. By using this technique all standards are matrix-matched with the seawater samples and any cross-nutrient interference effect should be accounted for.

Cross-nutrient interference and reagent contamination was evaluated by preparing separate solutions, as above, but with one of the three standards omitted. Only phosphate showed a slightly measurable increase (+0.014 µM) in the presence of 40 µM-NO3 and 160 µM-Si. The linear regressions of the standards were applied to all seawater sample peaks for calculating each batch of cruise samples. Typical correlations produced r2 values that were between 0.9999 and 0.99999.

5.2. Blank corrections

All seawater standard absorbance peaks were corrected for the absorbance of the seawater diluent (SWDIL). All seawater sample peaks were corrected for the refractive index absorbance for each unique nutrient detection system. The refractive index corrections (in apparent µM units) ranged from approximately 0.13 (for P), 0.23 (for N) to 2.41 (for Si), and represent the increase in absorbance that is due strictly to the presence of dissolved salts in seawater when compared to the distilled-deionized water baseline. These corrections were measured by sampling seawater (35 ‰ salinity) with DDW only in reagent lines and also with all reagents except the color producing reagent. The Levor surfactant used routinely in the phosphate channel was omitted from the DDW lines during the refractive index measurement.

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%.

7. Equipment/Supplies

8. Reagents

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.

9. References