So THAT'S How You Dissolve Silica!

My current Si dissolution procedure:

Extraction Steps 12 February 2008
Items outlined in red have not been attempted yet

Items needed
50 ml Nalgene bottles
100 ml Nalgene bottles
Chemicals outlined by Jones & Dreher
pH meter
5N NaOH
0.5 M H2SO4
Digital pipette
Cuvettes

Reagents: follow the procedures outlines by Jones & Dreher.

Dissolution procedure
Weigh and record a dry 50 ml Nalgene bottle (cap included).

Place approximately 0.38 g soil in the bottle. Weigh and record.

Allow the soil sample to dry at least 2 hours at 70˚C, then weigh and record. Set aside.

Determine plant available Si
Add approximately 50 ml Academic water to another 50 ml Nalgene bottle. Record the time.

Place in the pre-heated 85˚C water bath for at least 0.5 h.

Using the digital pipette, add 48 ml of the heated water to the soil sample bottle. Swirl the mixture gently. Tighten the cap and place it in the water bath for 1 min.

Remove a 1 ml aliquot using the digital pipette. Place the aliquot in a 25 ml (class A) volumetric flask which is pre-filled with 5 ml 0.5 M H2SO4. Flush the pipette tip into the flask with Academic water at least 2x to remove any residual Si. Record the time. Set aside. To determine the absorbance, go to the spectrophotometric procedures section.

Add 2 ml of 5N NaOH to the soil sample bottle. Replace and tighten the cap, and swirl the mixture. Loosen the cap approximately ¼ turn from tight. Place the bottle in the water bath, and turn the agitation speed to 4.5. Record the time.

Remove 1 ml aliquots at pre-determined times (10 min, 0.5 h, and so on up to 5 h). Record the extraction times. Depending on the extraction time, the concentration of Si in the aliquot will be up to 1000 µg. Since this is too high for the spectrophotometric procedures, the aliquot must be diluted.Place the aliquot in a 25 ml (class A) volumetric flask which is pre-filled with 2.5 ml 0.5M H2SO4. Flush the pipette tip into the flask with Academic water at least 2x to remove any residual Si. Bring the solution to mark (meniscus bottom should be at the line). Shake the flask to mix the solution. Transfer the diluted Si solution to a dry 50 ml Nalgene bottle.

At this point, the Si aliquot is diluted 25x. For example, if the original aliquot holds 500 µg Si, its concentration is 500 µg Si ml-1 H2O. When diluted, the flask still holds 500 µg Si, but now its concentration is 20 µg Si ml-1 H2O. However, this is still too high.

Extract 2.5 ml of the diluted Si solution with the digital pipette.

Place the diluted aliquot in another 25 ml (class A) volumetric flask which is pre-filled with 5.0 ml 0.5 M H2SO4. Flush the pipette tip into the flask with Academic water at least 2x to remove any residual Si.

At this point the Si solution is diluted another 10x. Using the scenario outlined above, the 2.5 ml second aliquot (which has a concentration of 20 µg Si ml-1 H2O) will hold 30 µg Si total. When this aliquot is diluted in the second volumetric flask to 25 ml, its new concentration will be 0.2 µg Si ml-1 H2O, well within the range of the spectrophotometric procedures.

Proceed to the spectrophotometric procedures section

Spectrophotometric procedures
Set the spectrophotometer λ to 810 nm. Set the background to 0.200. Allow the machine to warm up for at least 0.5 h.

Add 5 ml of the molybdate reagent to the reaction vessel (the 25 ml volumetric which holds the 2.5 ml Si aliquot along with 5 ml 0.5 M H2SO4). Swirl the mixture. Record the time. Allow the reaction to continue for 5 min.

Add 2.5 ml 20% tartaric acid. Swirl the mixture. Record the time. Allow the reaction to continue for 5 min.

Add 1 ml of the reducing solution. Bring to mark. Place the cap on the reaction vessel and shake the mixture gently. Transfer to a dry 50 ml Nalgene bottle. Record the time. Allow the reaction to continue for 15 min.

Fill a clean cuvette with Academic water and record its absorbance in the spectrophotometer. Empty the same cuvette and fill with the blue solution from the reaction vessel. Empty the solution from the cuvette and refill. Record the absorbance.