Spring 2008
Determination of Silicate
Purpose:
To determine the concentration of silicate (SiO3-2) in various water samples.
Materials:
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polyethylene bottles
pipettes (various sizes)
graduated cylinders (various sizes)
volumetric flasks
9 cuvettes
Spectrophotometer
Access to the following reagents:
Reagents: These reagents will be mixed for you. The recipes are included for your
benefit.
1. Molybdate solution
Dissolve 4.0 grams of reagent grade ammonium paramolybdate,
(NH4)6Mo7O24·4H2O, in about 300 mL of distilled water. Add 12 mL
concentrated hydrochloric acid, mix and volumetrically dilute to 500 mL. Store
in plastic in the dark.
2. Metol-sulfite solution
Dissolve 6 grams of anhydrous sodium sulfite, Na2SO3, in 500 mL of distilled
water. Add 10 grams of metol. After all is dissolved, filter the solution through a
No. 1 Whatman filter paper. Store in glass, replace monthly
3. Oxalic acid solution
Shake to mix 50 grams of oxalic acid dihydrate, (COOH)2 ·2H2O , in 500 mL of
distilled water. Store in glass or plastic, stable for many months. Decant desired
volumes (leave crystals).
4. Sulfuric acid solution (50% v/v)
While stirring slowly add 250 mL of concentrated sulfuric acid, H2SO4 , to 250
mL of distilled water. Use a water bath if necessary. Allow the solution to cool
to room temperature. If needed make the volume up to 500 mL with more DIW.
Methods:
1. Make a Reducing Reagent solution
a. Mix together in this order: 100 mL of the metol-sulfite solution and 60 mL of
the oxalic acid solution. While mixing slowly add 60 mL of the 50% sulfuric
acid solution. Make the mixture up to 300 mL with distilled water. LABEL
it.
2. Make a Primary Standard solution
a. Dissolve 0.960 grams of sodium silicofluoride, Na2SiF6 , in distilled water
and volumetrically dilute to 1 L. LABEL it.
3. Make Run Standard solution
a. Using distilled water volumetrically dilute 10 mL of the Primary Standard to
500 mL. LABEL it.
Spring 2008
b. Fill 3 graduated cylinders with 25 mL of Run Standard. LABEL them.
4. Make 2 Reagent Blanks
a. Fill 2 graduated cylinders with 25 mL of DIW. LABEL them.
5. Run Standard and Reagent Blank Analyses
a. Add 10 mL of Molybdate solution to each Run Standard and Reagent Blank.
Stopper and mix well. Let stand for at least 10 minutes but not more than 30
minutes.
b. Add 15 mL of Reducing Agent to make a 50 mL solution and mix
immediately.
c. After 1 hour measure the Absorbance at 810 nm in a 1cm cell or cuvet using a
spectrophotometer that has been blanked with distilled water.
d. Determine the average absorbance of the 3 Run Standards (ABSstnd) and 2
Reagent Blanks (ABSrb) for the next step.
e. F factor determination (F1cm should be about 100):
i. F1cm = ______100_______
(ABSstnd – ABSrb)
Before the water samples are run, a "F" or Calibration Factor is obtained
by running Standards and Reagent Blanks as a check on the reagents. You
will be graded on the accuracy of your F factor. Record your absorbances
and calculations on your data sheet.
6. Water Sample Analyses
a. Fill 4 graduated cylinders with 25 mL of each water sample. LABEL them.
b. Add 10 mL of Molybdate solution to each. Stopper and mix well. Let stand
for at least 10 minutes but not more than 30 minutes.
c. Add 15 mL of Reducing Agent to make a 50 mL solution and mix
immediately.
Each sample is allowed to react with the molybdate solution. If silicate is
present a complex is formed that is reduced to a blue complex.
d. After 1 hour measure the Absorbance at 810 nm in a 1cm cuvet using a
spectrophotometer that has been blanked with distilled water.
e. Do a Turbidity Blank determination for each water sample (read the
Absorbance of untreated water sample at 810 nm in a 1cm cuvet). These
cuvettes will be next to the spectrophotometer. Record the values on your
data sheet.
7. Calculate the sample's silicate concentration in microgram atoms of silicate per liter
of sample:
g-at Si/L = Corrected Absorbance x F
Corrected Absorbance = ABSsample - ABSrb - ABSturbidity