Si Extraction Steps, May 10, 2009

Extraction Steps 10 May 2009

Lab equipment
*Water bath, 85˚C
*Spectrophotometer, 810 nm
*4-place scale
*Digital pipettes, 100-5000 µl (2)
*Acid bath, 10% HCl
*Glass Erlenmeyer flasks, 250, 500, 1000 ml
*Glass volumetric flasks (class A), 25, 100 ml
*Glass volumetric flask (class A or B), 1000 ml
*Plastic volumetric flask, (class A or B), 500 ml

Chemicals
*H2O
*H2SO4, 100%
*HCl, 32%
*Ammonium molybdate tetrahydrate
*Sodium sulfite
*1-amino-2-naphthol-4-sulfonic acid
*Sodium bisulfite
*Sodium hydroxide pellets

Notes
*All references to H2O assume Milli-Q deionized H2O unless otherwise specified.
*All glass labware should be rinsed 3x in H2O before using (even if it has been previously cleaned).
*Reagents and other liquids should not be stored in glass containers. Exposure to glass should be minimized.
*NaOH solution should never come into contact with glass unless diluted.
*When using pipettes, use the 2x aspirate-dispense procedure.
*Label all reagents!

Reagents
*2 M H2SO4, 1000 ml
-Add 800 ml H2O to 1000 ml volumetric flask.
-Add 108.6 ml 100% H2SO4.
-Fill to 1000 ml mark with H2O. Mix thoroughly.
-Dispense using 5 ml plastic repipettor. 5 ml dispensing volume.
-Reagent typically lasts indefinitely, but do not store in an open container.
*Molybdate, 1000 ml
-Add 27 g ammonium molybdate to 1000 ml flask
-Fill to 900 ml with H2O. Mix until dissolved.
-Add 6.178 ml 100% H2SO4. Fill to 1000 ml with H2O. Mix thoroughly.
-Dispense using 5 ml plastic repipettor. 10 ml dispensing volume.
-Store in opaque container. Do not use after 24 hr.
*Tartaric acid, 20%, 500 ml
-Add 100 g tartaric acid powder to 500 ml flask.
-Fill to 500 ml with H2O. Mix until dissolved.
-Dispense using 5 ml plastic repipettor. 2.5 ml dispensing volume.
-Reagent typically lasts indefinitely, but do not store in an open container.
*Reducing solution, 250 ml
-Add 2 g sodium sulfite and 0.4 g 1-amino-2-naphthol-4-sulfonic acid to 25 ml beaker. -Add 25-30 ml H2O, mix thoroughly using stir button under fume hood. Be careful of the fumes.
-Add 20 g sodium bisulfite to 250 ml flask. Fill to 200 ml with H2O. Mix until dissolved.
-Add sodium sulfite/naphthol mixture to sodium sulfite mixture. Fill to 250 ml with H2O. Mix thoroughly.
-Dispense using 1 ml repipettor. 1 ml dispensing volume.
-Store in refrigerator. Reagent typically lasts a few days. Do not use if precipitate is visible in the solution.
*0.5 M NaOH, 500 ml
-Add 450 ml H2O to 500 ml plastic volumetric flask.
-Add 20 g NaOH pellets. Fill to mark with H2O.
-Mix until dissolved. Fill to mark once again, mix.
-500 ml volume added to soil sample.
-Reagent typically lasts indefinitely, but do not store in an open container.
-Sample preparation and weighing
-Crush soil sample gently with a rubber pistil
-Dry in desiccator at least overnight.
-Weigh 1.3500 g of sample onto 50 ml scintillation bottle cap.
-Transfer weighed sample to 500 ml plastic bottle. Be sure no sample is stuck to the cap.
-Record sample weight and bottle number in notebook.
Dissolution procedure
*Add 500 ml 0.5 M NaOH (preheated to 85˚C) to soil sample. Mix thoroughly. Place in water bath. Record time of NaOH addition.
*Approximately 10 minutes before aliquot extraction, add 10 ml molybdate reagent to 25 ml flask. Add 5 ml 2 M H2SO4 to 100 ml flask, fill approximately ¾ full with H2O. Set aside.
*At specified time, remove sample from water bath. Mix thoroughly. Allow to sit for exactly 1 minute.
*Using the digital pipette, aspirate the appropriate aliquot volume. Dispense back into bottle, then aspirate again. Dispense aliquot into 100 ml volumetric flask.
*Fill flask to mark, mix thoroughly.
*Aspirate appropriate aliquot from 100 ml flask. Dispense into 25 ml flask with molybdate reagent. Mix thoroughly; allow to sit at least 2 minutes, but no more than 10.
*Add 2.5 ml tartaric acid reagent followed immediately by 1 ml reducing solution. Swirl. Fill to mark with H2O. Mix thoroughly.
*Allow to sit for at least ½ hour, but no more than 2.
*Measure Si-reagent mixture using spectrophotometer set to 810 nm. Be sure to thoroughly rinse cuvette with Si-reagent solution before measuring. Record values in notebook.
Dissolution procedure details
*The dissolution procedure typically lasts between 5 and 7 hours.
*Extraction begins at 3 hr and ends between hours 5 and 7.
*Extraction intervals are typically 20 or 30 minutes.
*Aliquot volumes are typically 2 ml for both 1st and 2nd aliquots. Aliquot volumes must be held constant throughout the procedure. Aliquot volumes must be the same as those used in the standard curve.
*Redundant aliquots are usually taken at each extraction (2-3 aliquots per extraction).
*Total ASi measurements though the dissolution procedure, n=20-30, but never less than 20.
*Linear regression is used to extrapolate total ASi. Total error associated with the y-intercept of the linear regression curve should be checked and kept to a minimum.
Standard Curve
*Follow reagent addition procedure outlined above. Si standards are located in cabinet below spectrophotometer.
*Use same aliquot volume as in dissolution procedure.