2.0 Definition
The silicate concentration of seawater is given in units of mmol kg ^-1 in seawater.

3.0 Principle of Analysis
The determination of reactive silicate is based on the method of Strickland and Parson (1968). A seawater sample is allowed to react with ammonium molybdate under conditions which result in the formation of silicomolybdate, phosphomolybdate and arsenomolybdate complexes. A reducing agent of metol and oxalic acid is added and silicomolybdate is reduced to a silicomolybdous acid with a blue color, the absorbance of
which is measured spectrophotometrically.

4.0 Apparatus
Spectrophotometer

5.0 Reagents
5.1 Molybdate reagent: Dissolve 4.0 g of reagent quality ammonium paramolybdate, (NH₄)₆Mo₇O₂₄ * 4H₂O in 300 ml of deionized water. Slowly add 12.0 ml concentrated HCl and mix. Make up volume to 500 ml with deionized water. This solution is stable for many months if stored refrigerated in a polyethylene bottle. Discard if a white precipitant forms.

5.2 Metolsulphite solution: Dissolve 6g of anhydrous sodium sulphite, Na₂SO₄ , in 500 ml of deionized water and then add 10 g metol. When the metol has dissolved filter the solution through a No. 1 Whatman filter paper and store it in a glass bottle. This solution should be replaced monthly.

5.3 Oxalic acid solution: Shake 50 g of reagent grade oxalic acid dihydrate with 500 ml of deionized water. Store the solution in a glass bottle and decant the solution from the crystals for use.

5.4 Sulfuric acid solution: Dilute 250 ml of concentrated sulfuric acid to 500 ml using deionized water. Cool the solution and store in a glass bottle.

5.5 Reducing reagent: Mix 100 ml of metol-sulphite solution with 60 ml of oxalic acid solution. Slowly add 60 ml of the 50% sulfuric acid solution and make the solution to a volume of 300 ml with deionized water. This solution should be prepared for immediate use.

6.0 Sampling
6.1 Samples are collected on deeper casts after the oxygen, CO₂ and salinity samples have been drawn. Shallow samples (upper 250 m) are collected on the gases cast after oxygen, CO₂ , DOC and salinity samples are drawn. An in-line filter (0.8 mm Nuclepore filter) is connected to the Niskin bottle. The spigot is opened and three sets of samples are collected from the water that passes through the filter by gravity filtration. Each bottle is rinsed three times and then filled just below the shoulder. Care must be taken to avoid overfilling of samples to be frozen. These bottles are transferred to a freezer (- 20°C) and kept frozen until analysed.

6.2 Contamination is a major problem with nutrient samples, especially near the surface where the ambient concentrations are low. All the nutrient bottles are rigorously cleaned before use. The cleaning begins by a wash with a detergent (Aquet) followed by a rinse with 10% HCl, three rinses with deionized water and a final rinse with deionized water.

6.3 Prolonged storage of samples is not advised, even if frozen. Sufficient water should be sampled for washing purposes.

7.0 Procedures
7.1 Sample analysis

7.1.1 All glassware should initially be washed in chromic-sulfuric acid and rinsed well with deionized before and after each subsequent use.

7.1.2 Sample solutions should be stored at a temperature between 18° and 25°.

7.1.3 Add 10 ml of the molybdate reagent to a dry 50 ml measuring cylinder fitted with a stopper.

7.1.4 Pipette 25 ml of the sample into the cylinder, stopper, mix the solutions and allow the mixture to stand for 10 minutes.

7.1.5 Add the reducing reagent rapidly to make the volume 50 ml and mix immediately.

7.1.6 Allow the solution to stand for 2–3 hours then measure the extinction at 810 nm.

7.2 Reagent blank determination

7.2.1 The reagent blank is determined using open ocean surface seawater as a sample. Follow the exact procedure outlined in section 7.1. Repeat.

7.2.2 A reagent blank should not exceed 0.01 on a 1 cm cell or 0.1 on a 10 cm cell and should be determined for each batch of samples.

7.3 Standardization

7.3.1 Primary silicate standard: Dissolve 0.9403g dried Na₂SiF₆ in deionized water and make up to 1 liter with DIW. 1 ml = 5 mmol. Store in a dark polypropylene bottle. As with nitrite, ultra pure sodium fluosilicate is difficult to obtain. It may be advisable to compensate for these impurities.

7.3.2 Working standards of concentrations of 50, 25, 10 and 5 mM are prepared by diluting 10 ml, 5 ml, 2 ml and 1 ml of primary silicate standard respectively to 1000 ml in open ocean surface seawater. Carry out the exact procedure as described in section

7.2 These solutions should be stored in plastic beakers and used within a few hours.

8.0 Calculation and expression of results
8.1 Standardization Factor, F

8.1.1 Subtract the absorbance of the reagent blanks from the absorbance values of the standards. Perform a linear regression of the silicate concentration and the corrected extinction values. The slope of the line is the standardization factor, F. The value of F is typically 100. If a 10 cm cell is used, the F factor
may be assumed to be equal to 0.1 x F _{(1 cm)} .

8.1.2 F is a function of the salinity of the seawater samples. Between salinities of 25 and 35, the variation may be neglected. The factor Fs at a salinity of S is related to F by:
        Fs = F * ( 1 + 0.003S)/ 1.08
This correction should be used when the salinity varies more than 10 from a value of 28.

8.2 Concentration of reactive silicate (mmol/1) = F  * corrected extinction
where:
        Corrected absorbance= sample absorbance - blank absorbance
        F = standardization factor

9.0 Notes
The silicate and molybdate must combine before the reducing agent is added. Ten minutes is allowed for this reaction. The reducing solution must be added within 30 minutes or else changes in the isomeric form of the silicomolybdate complex will occur. The sample should be added to the acid molybdate solution instead of the reverse. The prevents unwanted isomeric forms of the silicomolybdate complex. The time required for the full color development varies with the amount of silicate present in the sample. With a concentration of less than 50 mM, 1 hour is sufficient. For amounts exceeding 75 mM, at least 3 hours should be allowed.

10.0 References
Strickland, J.D.H., and Parsons, T.R. (1968). Determination of reactive silicate. In: A Practical Handbook of Seawater Analysis. Fisheries Research Board of Canada, Bulletin 167, 65–70.