SOLUBLE REACTIVE SILICA

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     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. 
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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 o/oo 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

    Niskin bottles and rosette/CTD unit
    acid-washed, 125 ml polyethylene bottles 
    Autoanalyzer (Technicon Corp.) and accessories


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.

    Glass distilled deionized water (DDW)
    Ammonium molybdate solution:  Dissolve 10 g of ammonium molybdate 
      into 1 liter of sulfuric acid (0.1 N).  Filter and store in an  
      amber plastic container.
    Oxalic acid (0.56 M):  Dissolve 50 g of oxalic acid into 900 ml 
      of DDW and dilute to 1 liter.      
    Ascorbic acid solution (1.76% wt/vol):  Dissolve 17.6 g of ACS 
      quality ascorbic acid in 500 ml of DDW containing 50 ml of 
      acetone.  Mix and dilute to 1 liter with DDW.  Add 0.5 ml  
      of Levor V per liter of reagent.


9.  References

    Grasshoff, K., M. Ehrhardt, and K. Kremling.  1983.  Methods 
    of Seawater Analysis.  Verlag Chemie. 

    Macdonald, R. W., F. A. McLaughlin and C. S. Wong.  1986.  
    The storage of reactive silicate samples by freezing. Limnology 
    and Oceanography, 31, 1139-1142.
                           
    Standard Methods for the Examination of Water and Wastewater, 
    15th Edition.

    Strickland, J. D. H. and T. R. Parsons.  1972.  A Practical 
    Handbook of Seawater Analysis. Fisheries Research Board of 
    Canada, 167 p.   

    Technicon Industrial Systems.  1977.  Silicates in Water and 
    Seawater.  Autoanalyzer II R Industrial Method  No. 155-71W.  
    W. Tarrytown, New York 10591.