Method of Analysis for Colloidal Silicon Dioxide

1. Description

Light, fine, white amorphous powder. It has a particle size of about 15 nm.

2. Solubility

Practically insoluble in water and in mineral acid with exception of hydrofluoric acid Dissolves in hot solutions of alkali hydroxides. When 1 g is shaken vigorously with 20 ml of carbon tetrachloride for 3 minutes, a transparent gel is produced

3. Identification

Reaction of Silicates

Reagent required
Sodium fluoride
Sulphuric acid
Procedure: Weight about 20 mg. of the sample and transfer into a platinum crucible. Add 10 mg. of sodium fluoride and few drops of sulphuric acid. Mix by means of a copper wire to obtain the thin slurry. Cover the crucible with a thin transparent plate of plastic under which a drop of water is suspended. Warm the crucible gently, within a short time; a white ring is formed around the drop of water.

4. pH

Limit: Between 3.5 and 5.5
Procedure: Weigh 1.0 g. the sample in a 100 ml beaker, add 30 ml of water and disperse the substance by shaking the solution well. Measure the pH of the suspension.

5. Chloride

Limit: Not more than 250 ppm
Reagent required
Nitric acid
0.1M Silver nitrate
Chloride standard solution (25 ppm Cl)
Standard solution: Transfer 10 ml of chloride standard solution (25 ppm Cl) into a clean and dried Nessler cylinder, add 5 ml of water. Add 10 ml dilute nitric acid and make the volume to 50 ml with water.
Sample solution: Weigh 1.0 g sample into a 50 ml volumetric flask, add a mixture of 20 ml of 2M HNO3 and 30 ml of water. Heat the flask on a water bath for 15 minutes with frequent shaking. Cool the flask to room temperature. Make up the volume to 50 ml with water, if necessary, filter and cool. Transfer the filtrate into a clean and dried Nessler cylinder.
Procedure: Add 1 ml of 0.1 M silver nitrate, mix and allow standing for 5 minutes protected from light. When viewing transversely against a black background the opalescence if any, produced in the sample solution is less than that produced in the standard solution.

6. Arsenic

Limit: Not more than 8 ppm
Reagent required
Lead acetate cotton
3M hydrochloric acid
Arsenic standard solution (10 ppm As)
1M potassium iodide
Zinc granules
Mercuric chloride paper
Hydrochloric acid
Sample Preparation: Weigh 2.5 g. of the sample and transfer it to a 250 ml round bottom flask, add 50 ml of 3M hydrochloric acid and reflux for 30 minutes using water condenser. Cool, filter with the aid of suction, and transfer the filtrate to a 100 ml volumetric flask. Wash the filter and flask with several portions of hot water and add the washings to the flask. Cool, dilute to volume with water and mix. To 50 ml of the solution, add 3 ml of hydrochloric acid.
Standard Preparation: Transfer 1 ml of arsenic standard solution (10 ppm as) into arsenic test apparatus bottle and dilute to 50 ml with water.
Procedure: In each of the arsenic test apparatus bottles, add 5 ml of 1M potassium iodide and 10 g. Zinc granules. Immediately assemble the apparatus and immerse the bottles in a water-bath at a temperature such that a uniform evolution of gas is maintained. After 40 minutes, any stain produced on the mercuric chloride paper with the sample preparation is not more intense than that is obtained by standard preparation.

7. Heavy metals

Limit: Not more than 25 ppm
Reagent required
Strong ammonia
2M ammonia
Thioacetamide reagent
Acetate buffer pH 3.5
Lead standard solution (1 ppm Pb)
2M Hydrochloric acid
1M Hydrochloric acid
Phenolphthalein solution
Sample Preparation: Weigh 2.5 g. of the sample in a 100 ml beaker; add sufficient water to produce a semi-fluid slurry. Dry at 140°c. When the dried substance is white, break up the mass using a glass rod. Add 25 ml of 1M HCl, boil gently for 5 minutes, stirring frequently with the glass rod and transfer the solution into the centrifuge tube. Centrifuge for 20 minutes and filter the supernatant liquid through the membrane filter. To the residue in the centrifuge tube, add 3 ml of 2M hydrochloric acid and 9 ml of water. Boil, Centrifuge for 20 minutes and filter the supernatant liquid through the same membrane filter. Wash the residue with small quantities of water. Collect the combined filtrates and washings and dilute to 50 ml with water. Pipette out 20 ml of the solution and transfer it into 25 ml volumetric flask. Add 50 mg. of L-ascorbic acid and 1 ml of strong ammonia solution, neutralize with 2M ammonia and dilute to 25 ml with water. Pipette out 12 ml of above solution and transfer it into 50 ml Nessler cylinder.
Standard Solution: Take 10 ml of Lead standard solution (1 ppm Pb) into 50 ml Nessler cylinder and dilute to 10 ml with water. Add 2 ml of the test solution and mix.
Procedure: To both the cylinders add 2 ml of acetate buffer pH 3.5, mix and add 1.2 ml of thioacetamide reagent, mix, allow to stand for 2 minutes. Any color produced in the sample solution is not more intense than that produced in the standard solution.

8. Loss on ignition

Limit: Not more than 5.0%
Procedure: Ignite a clean platinum crucible at 900°c for 30 minutes, cool and weigh the crucible. Transfer about 200 mg. of the sample into the crucible and record the weight. Ignite the crucible along with the sample at 900°c for 2 hours. Cool the crucible to room temperature in a desiccator and record the weight.
Calculation
                                   W2 – W3
%Loss on ignition = --------------- X 100
                                   W2 – W1
Where:
W1 = Weight of empty platinum crucible
W2 = Weight of crucible + sample

9. Assay

Limit: Not less than 99.0% and Not more than 100.5% of SiO2, calculated with reference to the ignited substance.
Reagent required
Hydrofluoric acid
Sulphuric acid
Ethanol (95%)
Procedure: To the residue obtained in the test “Loss on ignition”, add 0.2 ml of sulphuric acid and sufficient ethanol (95%) to moisten the residue completely, add 6 ml of hydrofluoric acid. Evaporate to dryness in a well-ventilated hood on a hot plate at 95°c to 105°c, avoiding loss from sputtering. Wash the sides of the dish with 6 ml of hydrofluoric acid, evaporate to dryness in a well-ventilated hood, ignite at 1000°c, allow to cool in a desiccator and weigh the crucible (W4). The difference between the weight of the final residue and that of the residue obtained in the test for loss on ignition represents the amount of SiO2 in the amount of the sample taken for the test for Loss on ignition.
Calculation
                      (W3 – W1) – (W4 – W1)
% Assay = ---------------------------------- x 100
                              (W3 – W1)

Where,
W1 = Weight of empty platinum crucible.
W3 = Weight of crucible + sample. (After ignite)
W4 = Weight of crucible + sample. (After treatment of hydrofluoric acid)