US Long-Term Ecological Research Network

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.

Interferences from phosphate, which forms a phosphomolybdate complex, and from tannin are eliminated by the oxalic acid introduced in the sample stream before the addition of the ascorbic acid reagent. Hydrogen sulfide interference is removed by boiling during the autoclaving process.
 
Reagents:
 
For sample preparation:

Sodium Bicarbonate-Digestion Reagent:

          Sodium bicarbonate, NaHCO3                                       50 g
          Milli-Q water,                                                           1000 ml
 
          Dissolve 50 g. of sodium bicarbonate in Milli-Q and dilute to 1 L.
 
1 N Sulfuric Acid-Neutralization Reagent:
Sulfuric Acid,H2SO4                                                    28 ml
Milli-Q water,                                                           1000 ml
Add 28 mL H2SO4 to 900 mL MQ and dilute to 1 L.
 
For Autoanalyzer:
 
Ammonium Molybdate reagent:
Ammonium Molybdate reagent,(NH4)6Mo7O24.4H2O 10 g
Sulfuric Acid,conc.                                                       2.8 ml
Milli-Q, water                                                              1000 ml
 
Add 2.8 mL sulfuric acid to 800 mL of Milli-Q water. Dissolve 10 g ammonium molybdate in the H2SO4 solution and dilute to 1 L. Filter, if needed, and store in an amber plastic container at 4° C.
 
Ascorbic Acid:
 
Ascorbic acid                                                            8.8 g
Acetone                                                                    25 ml
Milli-Q water                                                            500 ml
LEVOR V                                                                0.25 ml
 
Dissolve ascorbic acid in 250 ml. Milli-Q water containing 25 mls acetone and dilute to 500 ml. with Milli-Q. Store in an amber plastic container at 4° C. 
 
Oxalic Acid:
 
Oxalic acid                                                               25 g
Milli-Q water                                                            500 ml
 
Dissolve oxalic acid in Milli-Q water and dilute to 500 mls. Store in a plastic container at 4° C.
 
 
 
Standards
 
The concentrations of silica in the Northern Temperate Lakes LTER site vary greatly from lake to lake. Thus, three ranges of standards are run for the lakes. 
 
                             Low range 2-70 ppb(Stds. 5-8 & blank)
                             Medium range 70-1000 ppb (Stds.1-5, & blank)
                             High range 1000-20,000 ppb (Stds.10-14,1 & blank)
 
Big Muskellunge, Crystal, Crystal Bog and Trout Bog samples are analyzed as medium range. Sparkling, Trout, Allequash samples and the bottom depth samples of Trout Bog are diluted 1:10 (1 ml sample plus 9 ml. milli-Q water) and are run as the high range along with the high range standards diluted 1:10. Trout Lake station acid blanks are treated in an identical manner as the samples and are analyzed with each concentration range. Low BRSi samples (below 70 ppb) generally are not reanalyzed at a higher sensitivity because of the high background produced by the sample preparation procedure.
 

Silica Stock Solution: (145,000 µg/L)

 
Sodium silicofluoride, Na2SiF6                                0.9719 g
Milli-Q water                                                            1000 mL
 
Dissolve 0.9719 g. of sodium silicofluoride in Milli-Q water and dilute to 1 liter. Store in a dark polyethylene bottle at 4° C. Stability: Indefinite.
 
Working Standards
Diluent: 1%(v/v) Ultrex HCl in Milli-Q. Use a polyethylene volumetric flask to prepare the diluent! All volumes are recorded as weights by preparing the standards on the AE200 Mettler balance. See labbook for further details.
          High Range: (made from Silica Stock)
          Std. 10: 10 mls. Silica Stock, dilute to 100 mls with diluent.       Std 11: 6 mls. Silica stock, dilute to 100 mls. with diluent.
          Std. 12: 4 mls. Silica Stock, dilute to 100 mls. with diluent.
          Std. 13 2 mls. Silica Stock, dilute to 100 mls. with diluent.
 
          Medium Range: (made from Std. 10)
          Std 1: 6 mls. Std. 10, dilute to 100 mls. with diluent.
          Std. 2: 4 mls. Std 10, dilute to 100 mls. with diluent.
          Std. 3: 2 mls. Std.10, dilute to 100 mls. with diluent.
          Std. 4: 1 mls. Std. 10, dilute to 100 mls. with diluent.
 
          Low Range:(made from Std 2)
          Std. 5: 12 mls. Std 2, dilute to 100mls. with diluent.
          Std. 6: 7 mls. Std. 2, dilute to 100 mls. with diluent.
          Std. 7: 4 mls. Std. 2, dilute to 100 mls.with diluent.
          Std. 8: 1 ml. Std. 2, dilute to 100 mls. with diluent.
 
          A standard blank is prepared from the 1% Ultrex HCl used to
          dilute the standards.
 
 
Sample Preparation:
--Both unfiltered and filtered samples are run.
 
1. Aliquot 10 ml sample to polysulfone “Oak Ridge” style centrifuge tubes. (Shake samples before aliquoting.)
2. Add 3.5 mL 50g/L NaHCO3 to each tube. Cap with polypropylene cap. Vortex
3. Loosen caps to vent. Tighten caps. Repeat after 30 minutes.
4. Autoclave 90 minutes at 115-120° C.
5. Cool to room temperature.
6. Add 2 mL 1 N H2SO4. Vortex. Wait 30 minutes, and vortex again. (Want to insure that all CO2 is blown off to avoid complications while running on the autoanaylzer.) After each vortex, loosen caps to vent. Tighten caps.
7. Run 10% replicates through procedure.
8. Run 10% blanks through procedure
 
 
Run Set-up
 
1. Fill up tray with flat-bottomed cups.
2. Medium range is run first:
                   Positions 1-5 are Standards 1-5
                   Position 6 is standard blank
                   Positions 8-10 are TLBL’s (if necessary)
3. High level is run second:
                   Positions 1-6 are Standards 10-14 & 1
                   Position 7 is Blank
4. Low level is run third:
                   Positions 1-4 are Standards 5-8
                   Position 5 is Blank
 
Instrument conditions:
 
The Silica module is installed after the pump and the 660 nm. filters are inserted into the colorimeter. The standard calibration setting on the colorimeter is typically around 1.0 for BRSi analysis.
The Houston Omniscribe recorder settings are as follows:
                   chart speed: 5 cm/min
                   toggle switch: ÷10
                   range: .01 V
  
Dissolved Reactive Silica (DRSi) Standard:
 
This procedure determines the reactive silica in filtered 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.
Interferences from phosphate, which forms a phosphomolybdate complex, and from tannin are eliminated by the oxalic acid introduced in the sample stream before the addition of the ascorbic acid reagent. 
 
Reagents:
 
Ammonium Molybdate reagent:
Ammonium Molybdate reagent,(NH4)6Mo7O24.4H2O 5 g
Sulfuric Acid,conc.                                                   1.4 ml
Milli-Q, water                                                            500 ml
 
Add 1.4 mL of sulfuric acid to 400 mL of MQ. Allow solution to mix then dissolve 5 g of ammonium molybdate in this solution and dilute to 1 L. Filter, if needed, and store in an Erlenmeyer flask in refrigerator.
 
Ascorbic Acid:
 
Ascorbic acid                                                             8.8 g
Acetone                                                                     25 ml
Milli-Q water                                                             500 ml
Aerosol 22                                                              0.25 ml
 
Dissolve ascorbic acid in 250 ml. Milli-Q water containing 25 mls acetone and dilute to 500 ml. with Milli-Q. Then add 0.25 mL of Aerosol 22. Store in an amber plastic container at 4° C. 
 
Oxalic Acid:
 
Oxalic acid                                                                   25 g
Milli-Q water                                                            500 ml
 
Dissolve oxalic acid in Milli-Q water and dilute to 500 mls. Store in a plastic container at 4° C.
 
 
 
Standards:
 
The concentrations of silica in the Northern Temperate Lakes LTER site vary greatly from lake to lake. Thus, three ranges of standards are run for the lakes. 
 
                                    Low range 2-70 ppb(Stds. 5-8 & blank)
                                    Medium range 70-1000 ppb (Stds.1-5, & blank)
                                    High range 1000-20,000 ppb (Stds.11-14,1 & blank)
 
Big Muskellunge, Crystal, Crystal Bog and Trout Bog are run as medium range. Then any sample that has a peak height lower than Std 5 (70 ppb) must be reanalyzed on the low range. Sparkling, Trout, Allequash samples and the bottom depth samples of Trout Bog are diluted 1:10 (1 ml sample plus 9 ml. milli-Q water) and are run as the high range along with the high range standards diluted 1:10. The acid blanks from Trout Lake station are treated in the same manner as the samples and analyzed on medium and low level ranges.

 

Silica Stock Solution:
 
Sodium silicofluoride, Na2SiF6                                 0.9719 g.
Milli-Q water                                                            1000 mls.
 
Dissolve 0.9719 g. of sodium silicofluoride in Milli-Q water and dilute to 1 liter. Store in a dark polyethylene bottle at 4° C. Stability: Indefinite.
 
 
Working Standards:
Diluent: 1%(v/v) Ultrex HCl in Milli-Q. Use a polyethylene volumetric flask to prepare the diluent! All volumes are recorded as weights by preparing the standards on the AE200 Mettler balance. See the labbook for details of the standard preparation procedure.
 
          High Range: (made from Silica Stock)
          Std. 10: 10 mls. Silica Stock, dilute to 100 mls with diluent.       Std 11: 6 mls. Silica stock, dilute to 100 mls. with diluent.
          Std. 12: 4 mls. Silica Stock, dilute to 100 mls. with diluent.
          Std. 13 2 mls. Silica Stock, dilute to 100 mls. with diluent.
 
          Medium Range: (made from Std. 10)
          Std 1: 6 mls. Std. 10, dilute to 100 mls. with diluent.
          Std. 2: 4 mls. Std 10, dilute to 100 mls. with diluent.
          Std. 3: 2 mls. Std.10, dilute to 100 mls. with diluent.
          Std. 4: 1 mls. Std. 10, dilute to 100 mls. with diluent.
 
          Low Range:(made from Std 2)
          Std. 5: 12 mls. Std 2, dilute to 100mls. with diluent.
          Std. 6: 7 mls. Std. 2, dilute to 100 mls. with diluent.
          Std. 7: 4 mls. Std. 2, dilute to 100 mls.with diluent.
          Std. 8: 1 ml. Std. 2, dilute to 100 mls. with diluent.
 
          A standard blank is prepared from the 1% Ultrex HCl used to
          dilute the standards.
 
 
Run Set-up
 
1. Fill up tray with flat-bottomed cups.
2. Medium range is run first:
                   Positions 1-5 are Standards 1-5
                   Position 6 is standard blank
                   Positions 8-10 are TLBL’s (if necessary)
3. High level is run second:
                   Positions 1-6 are Standards 10-14 & 1
                   Position 7 is Blank
4. Low level is run third:
                   Positions 1-4 are Standards 5-8
                   Position 5 is Blank
 
 
Instrument conditions:
The Silica module is installed after the pump and the 660 nm. filters are inserted into the colorimeter. The standard calibration setting on the colorimeter is typically around 9.7 for DRSi analysis.
The Houston Omniscribe settings are as follows:
                             chart speed: 5 cm/min
                             toggle switch: ÷10
                             range: .01 V for medium and high samples
                                               .001 V for low range samples

 

Sample Preparation:

There is no elaborate procedure for DRSi sample preparation such as that of the BRSi analysis. The filtered samples are poured directly into the AutoAnalyzer II cups and analyzed.

 
Last revised: 8/24/97 by James Thoyre
Protocol Format
Process
Protocol ID
silica1
Protocol Type
laboratory