Method 366.0 Determination of Dissolved Silicate in Estuarine and Coastal Waters by Gas Segmented Continuous Flow Colorimetric Analysis Jia-Zhong Zhang, Cooperative Institute for Marine and Atmospheric Studies, Rosenstiel School of Marine and Atmospheric Science, Atlantic Oceanographic and Meteorological Laboratory, National Oceanic and Atmospheric Administration, University of Miami, Miami, FL 33149 George A. Berberian, National Oceanic and Atmospheric Administration, Atlantic Oceanographic and Meteorological Laboratory, Ocean Chemistry Division, Miami, FL 33149 Project Officer Elizabeth J. Arar Version 1.0 September 1997 National Exposure Research Laboratory Office of Research and Development U.S. Environmental Protection Agency Cincinnati, Ohio 45268 366.0-1 Method 366.0 Determination of Dissolved Silicate in Estuarine and Coastal Waters by Gas Segmented Continuous Flow Colorimetric Analysis 1.0 Scope and Application contained in the sample with molybdate in acidic solution. The $-molybdosilicic acid is then reduced by ascorbic 1.1 This method provides a procedure for the acid to form molybdenum blue. The absorbance of the determination of dissolved silicate concentration in molybdenum blue, measured at 660 nm, is linearly estuarine and coastal waters. The dissolved silicate is proportional to the concentration of silicate in the sample. mainly in the form of silicic acid, H4SiO4, in estuarine and A small positive error caused by differences in the coastal waters. All soluble silicate, including colloidal refractive index of seawater and reagent water, and silicic acid, can be determined by this method. Long chain negative error caused by the effect of salt on the color polymers containing three or more silicic acid units do not formation, are corrected prior to data reporting. react at any appreciable rate1, but no significant amount of these large polymers exists in estuarine and coastal 3.0 Definitions waters.2,3 This method is based upon the method of 4 Koroleff, adapted to automated gas segmented 3.1 Calibration Standard (CAL) -- A solution 5-7 continuous flow analysis. prepared from the primary dilution standard solution or stock standard solution containing analytes. The CAL Chemical Abstracts Service solutions are used to calibrate the instrument response Analyte Registry Numbers (CASRN) with respect to analyte concentration. _______________________________________ Silicate 12627-13-3 3.2 Dissolved Analyte (DA) -- The concentration of _______________________________________ analyte in an aqueous sample that will pass through a 0.45 Fm membrane filter assembly prior to sample 1.2 A statistically determined method detection limit acidification or other processing. (MDL) of 0.0012 mg Si/L has been determined by one 8 laboratory in seawaters of three different salinities. The 3.3 Laboratory Fortified Blank (LFB) -- An aliquot method is linear to 6.0 mg Si/L using a Flow Solution of reagent water to which known quantities of the method System (Perstorp Analytical Inc., Silver Spring, MD). analytes are added in the laboratory. The LFB is analyzed exactly like a sample, and its purpose is to determine 1.3 Approximately 60 samples per hour can be whether method performance is within acceptable control analyzed. limits, and whether the laboratory is capable of making accurate and precise measurements. 1.4 This method should be used by analysts experienced in the use of automated gas segmented This is basically a standard prepared in reagent water that continuous flow colorimetric analyses, and familiar with is analyzed as a sample. matrix interferences and procedures for their correction. A minimum of 6-months experience under supervision is 3.4 Laboratory Fortified Sample Matrix (LFM) -- An recommended. aliquot of an environmental sample to which known quantities of the method analytes are added in the 2.0 Summary of Method laboratory. The LFM is analyzed exactly like a sample, and its purpose is to determine whether the sample matrix 2.1 An automated gas segmented continuous flow contributes bias to the analytical results. The background colorimetric method for the analysis of dissolved silicate concentrations of the analytes in the sample matrix must concentration is described. In the method, $- be determined in a separate aliquot and the measured molybdosilicic acid is formed by reaction of the silicate values in the LFM corrected for background concentrations. Version 1.0 September 1997 366.0-2 3.5 Laboratory Reagent Blank (LRB) -- An aliquot 4.0 Interferences of reagent water that is treated exactly as a sample including exposure to all labware, equipment, and 4.1 Interferences caused by hydrogen sulfide, such reagents that are used with other samples. The LRB is as occur in samples taken from deep anoxic basins can used to determine if method analytes or other be eliminated by oxidation with bromine or stripping with interferences are present in the laboratory environment, nitrogen gas after acidification. Interferences of the reagents, or apparatus. phosphate at concentrations larger than 0.15 mg P/L is eliminated by the use of oxalic acid in the color 3.6 Linear Dynamic Range (LDR) -- The absolute development step of this method. Interferences of fluoride quantity or concentration range over which the instrument at concentrations greater than 50 mg F/L can be reduced response to an analyte is linear. by complexing the fluoride with boric acid.4 3.7 Method Detection Limit (MDL) -- The minimum 4.2 Glassware made of borosilicate glass should be concentration of an analyte that can be identified, avoided for use in silicate analysis. Plastic labware such measured and reported with 99% confidence that the as polyethylene volumetric flasks and plastic sample analyte concentration is greater than zero.8 vials, should be used. 4.3 Sample turbidity and particles are removed by 3.8 Reagent Water (RW) -- Type 1 reagent grade filtration through a 0.45 µm non-glass membrane filters water equal to or exceeding standards established by after sample collection. American Society for Testing and Materials (ASTM). Reverse osmosis systems or distilling units followed by 4.4 This method corrects for refractive index and salt Super-Q Plus Water System that produce water with 18 error interferences which occur if sampler wash solution megohm resistance are examples of acceptable water and calibration standards are not matched with samples sources. in salinity. 3.9 Refractive Index (RI) -- The ratio of velocity of 4.5 Frozen samples should be filled about 3/4 full in light in a vacuum to that in a given medium. The relative the sample bottles. The expansion of water on freezing refractive index is the ratio of the velocity of light in two will squeeze some of the brine out of the bottle if the different media, such as estuarine or sea water versus bottle was overfilled. The overfill of the sample bottle reagent water. The correction for this difference is during freezing will drastically alter the nutrient referred to as refractive index correction in this method. concentrations in the sample that remains. 3.10 Stock Standard Solution (SSS) -- A concentrated solution of method analyte prepared in the 5.0 Safety laboratory using assayed reference compounds or purchased from a reputable commercial source. 5.1 Water samples collected from the estuarine and coastal environment are generally not hazardous. 3.11 Quality Control Sample (QCS) - A solution of However, the individual who collects samples should use method analyte of known concentrations which is used to proper technique. fortify an aliquot of LRB or sample matrix. The QCS is obtained from a source external to the laboratory and 5.2 Good laboratory technique should be used when different from the source of calibration standards. It is preparing reagents. A lab coat, safety goggles, and used to check laboratory performance with externally gloves should be worn when preparing the sulfuric acid prepared test materials. reagent. 3.12 SYNC Peak Solution - A colored solution used to produce a synchronization peak in the refractive index 6.0 Equipment and Supplies measurement. A synchronization peak is required by most data acquisition programs to initialize the peak 6.1 Gas Segmented Continuous Flow finding parameters. The first cup in every run must always Autoanalyzer Consisting of: be identified as a SYNC sample. The SYNC sample is usually a high standard, but can be any sample that 6.1.1 Autosampler. generates a peak at least 25% of full scale. 366.0-3 Version 1.0 September 1997 6.1.2 Analytical cartridge with reaction coils for silicate plate using a Teflon-coated stirring bar. Complete analysis. dissolution usually takes 2-24 hours. Dilute the solution to 1000 mL in polyethylene volumetric flask with reagent 6.1.3 Proportioning pump. water. Store the stock solution in a plastic bottle. This solution is stable for one year if care is taken to prevent 6.1.4 Monochromator or spectrophotometer equipped contamination and evaporation. with a tungsten lamp ( 380-800 nm) and a low refractive index flowcell. 7.1.4 Low Nutrient Sea Water (LNSW) - Obtain natural low nutrient seawater from surface seawater in the Gulf 6.1.5 Strip chart recorder or computer based data Stream or Sargasso Sea (salinity 36 %o, < 0.03 mg Si/L) acquisition system. and filter through 0.45 µm pore size non-glass membrane filters. In addition, commercially available low nutrient sea 6.2 Glassware and Supplies water ( < 0.03 mg Si/L) with salinity of 35 %o (Ocean Scientific International, Wormley, U.K. ) can be used. 6.2.1 All labware used in the analysis must be low in residual silicate to avoid sample or reagent 7.2 Working Reagents contamination. Soaking with lab grade detergent, rinsing with tap water, followed by rinsing with 10% HCl (v/v) and 7.2.1 Dowfax Start-up Solution - Add 2 mL of Dowfax thoroughly rinsing with reagent water was found to be 2A1 surfactant (Dow Chemical Company) to 1000 mL effective.